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Amino acids are
organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, and syntheses of organic compounds compr ...
s that contain
amino In organic chemistry Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemi ...
(–NH2) and
carboxyl A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
(–COOH)
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
s, along with a
side chain In organic chemistry Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemis ...
(R group) specific to each amino acid. The key elements of an amino acid are
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
(C),
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
(H),
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
(O), and
nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

nitrogen
(N), although other elements are found in the side chains of certain amino acids. About 500 naturally occurring amino acids are known as of 1983 (though only 20 appear in the
genetic code The genetic code is the set of rules used by living to information encoded within genetic material ( or sequences of nucleotide triplets, or codons) into s. Translation is accomplished by the , which links s in an order specified by (mRNA), u ...

genetic code
) and can be classified in many ways. They can be classified according to the core structural functional groups' locations as Alpha and beta carbon, alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to Chemical polarity, polarity, pH level, and side chain group type (
aliphatic In organic chemistry, hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
, Open-chain compound, acyclic, aromatic, containing Hydroxy group, hydroxyl or
sulfur Sulfur (in nontechnical British English: sulphur) is a with the  S and  16. It is , and lic. Under , sulfur atoms form cyclic octatomic molecules with a chemical formula . Elemental sulfur is a bright yellow, line solid at . Sul ...

sulfur
, etc.). In the form of
protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins differ from one another primarily ...

protein
s, amino acid residues form the second-largest component (
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...

water
is the largest) of human
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as
neurotransmitter A neurotransmitter is a secreted by a to affect another cell across a . The cell receiving the signal, or target cell, may be another neuron, but could also be a or . Neurotransmitters are released from into the where they are able to int ...
transport and
biosynthesis Biosynthesis is a multi-step, enzyme-Catalysis, catalyzed process where substrate (chemistry), substrates are converted into more complex Product (chemistry), products in living organisms. In biosynthesis, simple Chemical compound, compounds are mod ...

biosynthesis
. In
biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . Over the last decades of the 20th century, biochemistry has beco ...

biochemistry
, amino acids which have the amine group attached to the (alpha-) carbon atom next to the carboxyl group have particular importance. They are known as 2-, alpha-, or α-amino acids (generic
formula In , a formula is a concise way of expressing information symbolically, as in a mathematical formula or a . The informal use of the term ''formula'' in science refers to the . The plural of ''formula'' can be either ''formulas'' (from the mos ...
H2NCHRCOOH in most cases, where R is an
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or ...
substituent A substituent is one or a group of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can ...
known as a "
side chain In organic chemistry Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemis ...
"); often the term "amino acid" is used to refer specifically to these. They include the 22
proteinogenic Proteinogenic amino acids are amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, side chain (R group) specific to each amino acid. The k ...
("protein-building") amino acids, which combine into
peptide Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnikḗ'') is an independent branch of the Indo-European languages, Indo-European family of languages, native to Greece, Cyprus, Albania, ...
chains ("polypeptides") to form the building blocks of a vast array of
protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins differ from one another primarily ...

protein
s. These are all L-
stereoisomers In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is ...
("
left-handed In human biology Human biology is an interdisciplinary area of academic study that examines humans through the influences and interplay of many diverse fields such as human genetics, genetics, human evolution, evolution, human physiology, physi ...
"
isomer In , isomers are s or s with identical s — that is, same number of of each — but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Isomers do not necessarily share similar or . Two main forms of ...

isomer
s), although a few D-amino acids ("right-handed") occur in bacterial envelopes, as a
neuromodulator Neuromodulation is the physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, includi ...
(D-
serine Serine (symbol Ser or S) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, ...

serine
), and in some
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
s. Twenty of the proteinogenic amino acids are encoded directly by triplet
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
s in the
genetic code The genetic code is the set of rules used by living to information encoded within genetic material ( or sequences of nucleotide triplets, or codons) into s. Translation is accomplished by the , which links s in an order specified by (mRNA), u ...

genetic code
and are known as "standard" amino acids. The other two ("nonstandard" or "non-canonical") are
selenocysteine Selenocysteine (symbol Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. ...

selenocysteine
(present in many
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
s as well as most
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
s, but not coded directly by
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
), and
pyrrolysine Pyrrolysine (symbol Pyl or O; encoded by the 'amber' stop codon The genetic code is the set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or Messenger RNA, mR ...

pyrrolysine
(found only in some
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...
and one
bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sma ...
). Pyrrolysine and selenocysteine are encoded via variant codons; for example, selenocysteine is encoded by
stop codon Stop may refer to: Places *Stop, Kentucky Stop is an unincorporated area, unincorporated community located in Wayne County, Kentucky, United States. The origin of the name "Stop" is obscure. References Unincorporated communities in Wayn ...
and SECIS element.
''N''-formylmethionine
''N''-formylmethionine
(which is often the initial amino acid of proteins in bacteria,
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
, and
chloroplast A chloroplast is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and ...

chloroplast
s) is generally considered as a form of
methionine Methionine (symbol Met or M) () is an essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come ...

methionine
rather than as a separate proteinogenic amino acid. Codon–
tRNA Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an e ...
combinations not found in nature can also be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating
non-proteinogenic amino acid In biochemistry, non-coded or non-proteinogenic amino acids are distinct from the 22 proteinogenic amino acids (21 in eukaryotesplus formylmethionine in eukaryotes with prokaryote organelles like mitochondria) which are naturally encoded in the ge ...
s. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the
human brain The human brain is the central organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

human brain
, glutamate (standard
glutamic acid Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compound ...
) and
gamma-aminobutyric acid ''gamma-''Aminobutyric acid, or γ-aminobutyric acid , or GABA , is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervou ...
("GABA", nonstandard gamma-amino acid) are, respectively, the main excitatory and inhibitory neurotransmitters.
Hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5H9 O3 N), is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, side chain ...

Hydroxyproline
, a major component of the
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consum ...
collagen Collagen () is the main structural in the found in the body's various s. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of ...

collagen
, is synthesised from
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
.
Glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

Glycine
is a biosynthetic precursor to
porphyrin Porphyrins ( ) are a group of heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). H ...

porphyrin
s used in
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek Greek may refer to: Greece Anything of, from ...

red blood cell
s.
Carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports Fatty acid, long-chain fatty acids into Mitochondrion, mitochondria to be Redox, o ...

Carnitine
is used in lipid transport. Nine proteinogenic amino acids are called " essential" for humans because they cannot be produced from other
compounds Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive structu ...
by the human body and so must be taken in as food. Others may be conditionally essential for certain ages or medical conditions. Essential amino acids may also differ between
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
. Because of their biological significance, amino acids are important in nutrition and are commonly used in
nutritional supplement A dietary supplement is a manufactured product intended to supplement one's diet by taking a pill, capsule, tablet, powder or liquid. A supplement can provide nutrients either extracted from food sources or that are synthetic in order to ...
s,
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

fertilizer
s, feed, and food technology. Industrial uses include the production of
drugs A drug is any that causes a change in an organism's or when consumed. Drugs are typically distinguished from and substances that provide nutritional support. Consumption of drugs can be via , , , , via a on the skin, , or . In , a drug ...
,
biodegradable plastic Biodegradable plastics are plastic Plastics are a wide range of synthetic polymers, synthetic or semi-synthetic materials that use polymers as a main ingredient. Their Plasticity (physics), plasticity makes it possible for plastics to be Inject ...
s, and chiral catalysts.


History

The first few amino acids were discovered in the early 19th century. In 1806, French chemists
Louis-Nicolas Vauquelin
Louis-Nicolas Vauquelin
and Pierre Jean Robiquet isolated a compound in
asparagus Asparagus, or garden asparagus, folk name sparrow grass, scientific name ''Asparagus officinalis'', is a perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom ...

asparagus
that was subsequently named
asparagine Asparagine (symbol Asn or N), is an α-amino acid that is Proteinogenic amino acid, used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid gr ...

asparagine
, the first amino acid to be discovered.
Cystine Cystine is the oxidized dimer form of the amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemica ...
was discovered in 1810, although its monomer,
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
, remained undiscovered until 1884.
Glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

Glycine
and
leucine Leucine (symbol Leu or L) is an essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional gro ...

leucine
were discovered in 1820. The last of the 20 common amino acids to be discovered was
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an Amine, α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated ...

threonine
in 1935 by
William Cumming Rose William Cumming Rose (April 4, 1887 – September 25, 1985) was an American biochemist Biochemists are scientists who are trained in biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific ...
, who also determined the
essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms ...
s and established the minimum daily requirements of all amino acids for optimal growth. The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. The first use of the term "amino acid" in the English language dates from 1898, while the German term, ''Aminosäure'', was used earlier. Proteins were found to yield amino acids after enzymatic digestion or acid
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
. In 1902,
Emil Fischer Hermann Emil Louis Fischer FRS FRSE FCS (c; ; 9 October 185215 July 1919) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person ...

Emil Fischer
and
Franz Hofmeister Franz Hofmeister (30 August 1850, Prague – 26 July 1922, Würzburg Würzburg (; Main-Franconian: ; bar, Wiazbuag or ) is a List of cities and towns in Germany, city in the traditional region of Franconia in the north of the Germany, German s ...
independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "
peptide Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnikḗ'') is an independent branch of the Indo-European languages, Indo-European family of languages, native to Greece, Cyprus, Albania, ...
".


General structure

In the structure shown at the top of the page, R represents a
side chain In organic chemistry Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemis ...
specific to each amino acid. The
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
atom next to the
carboxyl group A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...

carboxyl group
is called the α–carbon. Amino acids containing an
amino group In organic chemistry, amines (, ) are organic compound, compounds and functional groups that contain a base (chemistry), basic nitrogen atom with a lone pair. Amines are formally derivative (chemistry), derivatives of ammonia, wherein one or ...
bonded directly to the alpha carbon are referred to as ''alpha amino acids''. These include amino acids such as
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
which contain
secondary amine In organic chemistry, amines (, ) are compounds and functional group In organic chemistry, a functional group is a substituent or moiety in a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pent ...

secondary amine
s, which used to be often referred to as "imino acids".


Isomerism

Alpha-amino acids are the common natural forms of amino acids. With the exception of
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
, other natural amino acids adopt the L configuration. While L-amino acids represent all of the amino acids found in
protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins differ from one another primarily ...

protein
s during translation in the ribosome. The L and D convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of
glyceraldehyde Glyceraldehyde (glyceral) is a triose is an aldotriose because the carbonyl group is at the end of the chain Image:Dihydroxyacetone.png">150px, Dihydroxyacetone is a ketone, ketotriose because the carbonyl group is the center of the chain. A tr ...

glyceraldehyde
from which that amino acid can, in theory, be synthesized (D-glyceraldehyde is dextrorotatory; L-glyceraldehyde is levorotatory). In alternative fashion, the (''S'') and (''R'') designators are used to indicate the ''absolute configuration''. Almost all of the amino acids in proteins are (''S'') at the α carbon, with
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
being (''R'') and
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
non-
chiral Chirality is a property of important in several branches of science. The word ''chirality'' is derived from the (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from its ; that is, i ...

chiral
. Cysteine has its side chain in the same geometric location as the other amino acids, but the ''R''/''S'' terminology is reversed because
sulfur Sulfur (in nontechnical British English: sulphur) is a with the  S and  16. It is , and lic. Under , sulfur atoms form cyclic octatomic molecules with a chemical formula . Elemental sulfur is a bright yellow, line solid at . Sul ...

sulfur
has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the Cahn-Ingold-Prelog sequence rules, whereas the atoms in most other side chains give them lower priority compared to the carboxyl group. D-amino acid residues are found in some proteins, but they are rare.


Side chains

Amino acids are designated as α- when the nitrogen atom is attached to the carbon atom adjacent to the carboxyl group: in this case the compound contains the substructure N–C–CO2. Amino acids with the sub-structure N–C–C–CO2 are classified as β- amino acids. γ-Amino acids contain the substructure N–C–C–C–CO2, and so on. Amino acids are usually classified by the
properties Property (''latin: Res Privata'') in the abstract is what belongs to or with something, whether as an attribute or as a component of said thing. In the context of this article, it is one or more components (rather than attributes), whether phys ...
of their side chain into four groups. The side chain can make an amino acid a
weak acid Acid strength is the tendency of an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond wi ...

weak acid
or a
weak base A weak base is a base (chemistry), base that, upon dissolution in water, does not dissociate completely, so that the resulting aqueous solution contains only a small proportion of hydroxide ions and the concerned basic radical, and a large proportio ...
, and a
hydrophile A hydrophile is a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In phys ...

hydrophile
if the side chain is
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the clim ...
or a
hydrophobe In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of at ...
if it is
nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
. The phrase "
branched-chain amino acid A branched-chain amino acid (BCAA) is an amino acid having an aliphatic substituent, side-chain with a branch (a central carbon atom bound to three or more carbon atoms). Among the proteinogenic amino acids, there are three BCAAs: leucine, isoleuc ...
s" or BCAA refers to the amino acids having
aliphatic In organic chemistry, hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
side chains that are linear; these are
leucine Leucine (symbol Leu or L) is an essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional gro ...

leucine
,
isoleucine Isoleucine (symbol Ile or I) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bon ...

isoleucine
, and
valine Valine (symbol Val or V) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bo ...

valine
.
Proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

Proline
is the only
proteinogenic Proteinogenic amino acids are amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known ...
amino acid whose side-group links to the α-amino group and, thus, is also the only proteinogenic amino acid containing a secondary amine at this position. In chemical terms, proline is, therefore, an imino acid, since it lacks a amine, primary amino group, although it is still classed as an amino acid in the current biochemical nomenclature and may also be called an "''N''-alkylated alpha-amino acid".


Zwitterions

In aqueous solution amino acids exist in two forms (as illustrated at the right), the molecular form and the zwitterion form in equilibrium with each other. The two forms coexist over the pH range to , which for glycine is pH0–12. The ratio of the concentrations of the two isomers is independent of pH. The value of this ratio cannot be determined experimentally. Because all amino acids contain amine and carboxylic acid functional groups, they are Amphoterism, amphiprotic. At (approximately 2.2) there will be equal concentration of the species and and at (approximately 10) there will be equal concentration of the species and . It follows that the neutral molecule and the zwitterion are effectively the only species present at biological pH. It is generally assumed that the concentration of the zwitterion is much greater than the concentration of the neutral molecule on the basis of comparisons with the known p''K'' values of amines and carboxylic acids.


Isoelectric point

At pH values between the two p''K''a values, the zwitterion predominates, but coexists in dynamic equilibrium with small amounts of net negative and net positive ions. At the exact midpoint between the two p''K''a values, the trace amount of net negative and trace of net positive ions exactly balance, so that average net charge of all forms present is zero. This pH is known as the isoelectric point p''I'', so p''I'' = (p''K''a1 + p''K''a2). For amino acids with charged side chains, the p''K''a of the side chain is involved. Thus for aspartate or glutamate with negative side chains, p''I'' = (p''K''a1 + p''K''a(R)), where p''K''a(R) is the side chain p''K''a. Cysteine also has potentially negative side chain with p''K''a(R) = 8.14, so p''I'' should be calculated as for aspartate and glutamate, even though the side chain is not significantly charged at physiological pH. For histidine, lysine, and arginine with positive side chains, p''I'' = (p''K''a(R) + p''K''a2). Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point.


Occurrence and functions in biochemistry


Proteinogenic amino acids

Amino acids are the structural units (monomers) that make up proteins. They join together to form short polymer chains called
peptide Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnikḗ'') is an independent branch of the Indo-European languages, Indo-European family of languages, native to Greece, Cyprus, Albania, ...
s or longer chains called either polypeptides or
protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins differ from one another primarily ...

protein
s. These chains are linear and unbranched, with each amino acid residue within the chain attached to two neighboring amino acids. The process of making proteins encoded by DNA/RNA genetic material is called ''translation (biology), translation'' and involves the step-by-step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome. The order in which the amino acids are added is read through the
genetic code The genetic code is the set of rules used by living to information encoded within genetic material ( or sequences of nucleotide triplets, or codons) into s. Translation is accomplished by the , which links s in an order specified by (mRNA), u ...

genetic code
from an Messenger RNA, mRNA template, which is an RNA copy of one of the organism's genes. Twenty-two amino acids are naturally incorporated into polypeptides and are called
proteinogenic Proteinogenic amino acids are amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known ...
or natural amino acids. Of these, 20 are encoded by the universal
genetic code The genetic code is the set of rules used by living to information encoded within genetic material ( or sequences of nucleotide triplets, or codons) into s. Translation is accomplished by the , which links s in an order specified by (mRNA), u ...

genetic code
. The remaining 2,
selenocysteine Selenocysteine (symbol Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. ...

selenocysteine
and
pyrrolysine Pyrrolysine (symbol Pyl or O; encoded by the 'amber' stop codon The genetic code is the set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or Messenger RNA, mR ...

pyrrolysine
, are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a
stop codon Stop may refer to: Places *Stop, Kentucky Stop is an unincorporated area, unincorporated community located in Wayne County, Kentucky, United States. The origin of the name "Stop" is obscure. References Unincorporated communities in Wayn ...
. Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG, which is normally a stop codon in other organisms. This UAG codon is followed by a PYLIS downstream sequence. Several independent evolutionary studies, using different types of data, have suggested that Gly, Ala, Asp, Val, Ser, Pro, Glu, Leu, Thr (i.e. G, A, D, V, S, P, E, L, T) may belong to a group of amino acids that constituted the early genetic code, whereas Cys, Met, Tyr, Trp, His, Phe (i.e. C, M, Y, W, H, F) may belong to a group of amino acids that constituted later additions of the genetic code.


Non-proteinogenic amino acids

Aside from the 22 proteinogenic amino acids, many ''non-proteinogenic'' amino acids are known. Those either are not found in proteins (for example carnitine, Gamma-aminobutyric acid, GABA, levothyroxine) or are not produced directly and in isolation by standard cellular machinery (for example, hydroxyproline and selenomethionine). Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification, which is modification after translation during protein synthesis. These modifications are often essential for the function or regulation of a protein. For example, the carboxylation of glutamate allows for better binding of calcium in biology, calcium cations, and
collagen Collagen () is the main structural in the found in the body's various s. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of ...

collagen
contains hydroxyproline, generated by hydroxylation of
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
. Another example is the formation of hypusine in the Eukaryotic initiation factor, translation initiation factor EIF5A, through modification of a lysine residue. Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane. Some non-proteinogenic amino acids are not found in proteins. Examples include 2-aminoisobutyric acid and the neurotransmitter
gamma-aminobutyric acid ''gamma-''Aminobutyric acid, or γ-aminobutyric acid , or GABA , is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervou ...
. Non-proteinogenic amino acids often occur as intermediates in the metabolic pathways for standard amino acids – for example, ornithine and citrulline occur in the urea cycle, part of amino acid catabolism (see below). A rare exception to the dominance of α-amino acids in biology is the β-amino acid beta alanine (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B5), a component of coenzyme A.


Nonstandard amino acids

The 20 amino acids that are encoded directly by the codons of the universal
genetic code The genetic code is the set of rules used by living to information encoded within genetic material ( or sequences of nucleotide triplets, or codons) into s. Translation is accomplished by the , which links s in an order specified by (mRNA), u ...

genetic code
are called ''standard'' or ''canonical'' amino acids. A modified form of methionine () is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and chloroplasts. Other amino acids are called ''nonstandard'' or ''non-canonical''. Most of the nonstandard amino acids are also non-proteinogenic (i.e. they cannot be incorporated into proteins during translation), but two of them are proteinogenic, as they can be incorporated translationally into proteins by exploiting information not encoded in the universal genetic code. The two nonstandard proteinogenic amino acids are
selenocysteine Selenocysteine (symbol Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. ...

selenocysteine
(present in many non-eukaryotes as well as most eukaryotes, but not coded directly by DNA) and
pyrrolysine Pyrrolysine (symbol Pyl or O; encoded by the 'amber' stop codon The genetic code is the set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or Messenger RNA, mR ...

pyrrolysine
(found only in some archaea and at least one
bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sma ...
). The incorporation of these nonstandard amino acids is rare. For example, 25 human proteins include selenocysteine in their primary structure, and the structurally characterized enzymes (selenoenzymes) employ selenocysteine as the catalytic moiety (chemistry), moiety in their active sites. Pyrrolysine and selenocysteine are encoded via variant codons. For example, selenocysteine is encoded by
stop codon Stop may refer to: Places *Stop, Kentucky Stop is an unincorporated area, unincorporated community located in Wayne County, Kentucky, United States. The origin of the name "Stop" is obscure. References Unincorporated communities in Wayn ...
and SECIS element.


In human nutrition

When taken up into the human body from the diet, the 20 standard amino acids either are used to synthesize proteins, other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis. Of the 20 standard amino acids, nine (Histidine, His, Isoleucine, Ile, Leucine, Leu, Lysine, Lys, Methionine, Met, Phenylalanine, Phe, Threonine, Thr, Tryptophan, Trp and Valine, Val) are called
essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms ...
s because the human body cannot biosynthesis, synthesize them from other
compounds Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive structu ...
at the level needed for normal growth, so they must be obtained from food. In addition,
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
, tyrosine, and arginine are considered semiessential amino acids, and taurine a semiessential aminosulfonic acid in children. The metabolic pathways that synthesize these monomers are not fully developed. The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids. Dietary exposure to the nonstandard amino acid beta-Methylamino-L-alanine, BMAA has been linked to human neurodegenerative diseases, including ALS.


Non-protein functions

In humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the
neurotransmitter A neurotransmitter is a secreted by a to affect another cell across a . The cell receiving the signal, or target cell, may be another neuron, but could also be a or . Neurotransmitters are released from into the where they are able to int ...
gamma-aminobutyric acid ''gamma-''Aminobutyric acid, or γ-aminobutyric acid , or GABA , is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervou ...
(GABA). Many amino acids are used to synthesize other molecules, for example: * Tryptophan is a precursor of the neurotransmitter serotonin. * Tyrosine (and its precursor phenylalanine) are precursors of the catecholamine
neurotransmitter A neurotransmitter is a secreted by a to affect another cell across a . The cell receiving the signal, or target cell, may be another neuron, but could also be a or . Neurotransmitters are released from into the where they are able to int ...
s dopamine, epinephrine and norepinephrine and various trace amines. * Phenylalanine is a precursor of phenethylamine and tyrosine in humans. In plants, it is a precursor of various phenylpropanoids, which are important in plant metabolism. *
Glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

Glycine
is a precursor of
porphyrin Porphyrins ( ) are a group of heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). H ...

porphyrin
s such as heme. * Arginine is a precursor of nitric oxide. * Ornithine and S-Adenosyl methionine, ''S''-adenosylmethionine are precursors of polyamines. * Aspartate,
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
, and glutamine are precursors of nucleotides. However, not all of the functions of other abundant nonstandard amino acids are known. Some nonstandard amino acids are used as Plant defense against herbivory, defenses against herbivores in plants. For example, canavanine is an analogue of arginine that is found in many legumes, and in particularly large amounts in ''Canavalia gladiata'' (sword bean). This amino acid protects the plants from predators such as insects and can cause illness in people if some types of legumes are eaten without processing. The non-protein amino acid mimosine is found in other species of legume, in particular ''Leucaena leucocephala''. This compound is an analogue of tyrosine and can poison animals that graze on these plants.


Uses in industry

Amino acids are used for a variety of applications in industry, but their main use is as additives to Compound feed, animal feed. This is necessary, since many of the bulk components of these feeds, such as soybeans, either have low levels or lack some of the
essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms ...
s: lysine, methionine, threonine, and tryptophan are most important in the production of these feeds. In this industry, amino acids are also used to chelate metal cations in order to improve the absorption of minerals from supplements, which may be required to improve the health or production of these animals. The food industry is also a major consumer of amino acids, in particular,
glutamic acid Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compound ...
, which is used as a flavor enhancer, and aspartame (aspartylphenylalanine 1-methyl ester) as a low-calorie artificial sweetener. Similar technology to that used for animal nutrition is employed in the human nutrition industry to alleviate symptoms of mineral deficiencies, such as anemia, by improving mineral absorption and reducing negative side effects from inorganic mineral supplementation. The chelating ability of amino acids has been used in fertilizers for agriculture to facilitate the delivery of minerals to plants in order to correct mineral deficiencies, such as iron chlorosis. These fertilizers are also used to prevent deficiencies from occurring and improving the overall health of the plants. The remaining production of amino acids is used in the synthesis of
drugs A drug is any that causes a change in an organism's or when consumed. Drugs are typically distinguished from and substances that provide nutritional support. Consumption of drugs can be via , , , , via a on the skin, , or . In , a drug ...
and cosmetics. Similarly, some amino acids derivatives are used in pharmaceutical industry. They include 5-HTP (5-hydroxytryptophan) used for experimental treatment of depression, L-DOPA, L-DOPA (L-dihydroxyphenylalanine) for Parkinson's treatment, and eflornithine drug that inhibits ornithine decarboxylase and used in the treatment of African trypanosomiasis, sleeping sickness.


Expanded genetic code

Since 2001, 40 non-natural amino acids have been added into protein by creating a unique codon (recoding) and a corresponding transfer-RNA:aminoacyl – tRNA-synthetase pair to encode it with diverse physicochemical and biological properties in order to be used as a tool to exploring protein structure and function or to create novel or enhanced proteins.


Nullomers

Nullomers are codons that in theory code for an amino acid, however in nature there is a selective bias against using this codon in favor of another, for example bacteria prefer to use CGA instead of AGA to code for arginine. This creates some sequences that do not appear in the genome. This characteristic can be taken advantage of and used to create new selective cancer-fighting drugs and to prevent cross-contamination of DNA samples from crime-scene investigations.


Chemical building blocks

Amino acids are important as low-cost feedstocks. These compounds are used in chiral pool synthesis as enantiomer, enantiomerically pure building blocks. Amino acids have been investigated as precursors chiral catalysts, such as for asymmetric hydrogenation reactions, although no commercial applications exist.


Biodegradable plastics

Amino acids have been considered as components of biodegradable polymers, which have applications as environmentally friendly packaging and in medicine in drug delivery and the construction of prosthesis, prosthetic implants. An interesting example of such materials is Sodium poly(aspartate), polyaspartate, a water-soluble biodegradable polymer that may have applications in disposable diapers and agriculture. Due to its solubility and ability to chelation, chelate metal ions, polyaspartate is also being used as a biodegradeable antiFouling, scaling agent and a corrosion inhibitor. In addition, the aromatic amino acid tyrosine has been considered as a possible replacement for phenols such as bisphenol A in the manufacture of polycarbonates.


Synthesis


Chemical synthesis

The commercial production of amino acids usually relies on mutant bacteria that overproduce individual amino acids using glucose as a carbon source. Some amino acids are produced by enzymatic conversions of synthetic intermediates. 2-Aminothiazoline-4-carboxylic acid is an intermediate in one industrial synthesis of cysteine, L-cysteine for example. Aspartic acid is produced by the addition of ammonia to fumarate using a lyase.


Biosynthesis

In plants, nitrogen is first assimilated into organic compounds in the form of glutamate, formed from alpha-ketoglutarate and ammonia in the mitochondrion. For other amino acids, plants use transaminases to move the amino group from glutamate to another alpha-keto acid. For example, aspartate aminotransferase converts glutamate and oxaloacetate to alpha-ketoglutarate and aspartate. Other organisms use transaminases for amino acid synthesis, too. Nonstandard amino acids are usually formed through modifications to standard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosylmethionine, ''S''-adenosylmethionine, while hydroxyproline is made by a post translational modification of
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
. Microorganisms and plants synthesize many uncommon amino acids. For example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged derivative of alanine. Both of these amino acids are found in peptidic lantibiotics such as alamethicin. However, in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene#Ethylene as a plant hormone, ethylene.


Reactions

Amino acids undergo the reactions expected of the constituent functional groups.


Peptide bond formation

As both the amine and carboxylic acid groups of amino acids can react to form amide bonds, one amino acid molecule can react with another and become joined through an amide linkage. This polymerization of amino acids is what creates proteins. This condensation reaction yields the newly formed peptide bond and a molecule of water. In cells, this reaction does not occur directly; instead, the amino acid is first activated by attachment to a transfer RNA molecule through an ester bond. This aminoacyl-tRNA is produced in an Adenosine triphosphate, ATP-dependent reaction carried out by an aminoacyl tRNA synthetase. This aminoacyl-tRNA is then a substrate for the ribosome, which catalyzes the attack of the amino group of the elongating protein chain on the ester bond. As a result of this mechanism, all proteins made by ribosomes are synthesized starting at their ''N''-terminus and moving toward their ''C''-terminus. However, not all peptide bonds are formed in this way. In a few cases, peptides are synthesized by specific enzymes. For example, the tripeptide glutathione is an essential part of the defenses of cells against oxidative stress. This peptide is synthesized in two steps from free amino acids. In the first step, gamma-glutamylcysteine synthetase condenses
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
and
glutamic acid Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compound ...
through a peptide bond formed between the side chain carboxyl of the glutamate (the gamma carbon of this side chain) and the amino group of the cysteine. This dipeptide is then condensed with
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
by glutathione synthetase to form glutathione. In chemistry, peptides are synthesized by a variety of reactions. One of the most-used in peptide synthesis, solid-phase peptide synthesis uses the aromatic oxime derivatives of amino acids as activated units. These are added in sequence onto the growing peptide chain, which is attached to a solid resin support. Libraries of peptides are used in drug discovery through high-throughput screening. The combination of functional groups allow amino acids to be effective polydentate ligands for metal–amino acid chelates. The multiple side chains of amino acids can also undergo chemical reactions.


Catabolism

Amino acids must first pass out of organelles and cells into blood circulation via amino acid transporters, since the amine and carboxylic acid groups are typically ionized. Degradation of an amino acid, occurring in the liver and kidneys, often involves deamination by moving its amino group to alpha-ketoglutarate, forming glutamate. This process involves transaminases, often the same as those used in amination during synthesis. In many vertebrates, the amino group is then removed through the urea cycle and is excreted in the form of urea. However, amino acid degradation can produce uric acid or ammonia instead. For example, serine dehydratase converts serine to pyruvate and ammonia. After removal of one or more amino groups, the remainder of the molecule can sometimes be used to synthesize new amino acids, or it can be used for energy by entering glycolysis or the citric acid cycle, as detailed in image at right.


Complexation

Amino acids are bidentate ligands, forming transition metal amino acid complexes. :


Physicochemical properties of amino acids

The ca. 20 canonical amino acids can be classified according to their properties. Important factors are charge, hydrophile, hydrophilicity or hydrophobe, hydrophobicity, size, and functional groups. These properties influence protein structure and protein–protein interactions. The water-soluble proteins tend to have their hydrophobic residues (leucine, Leu, isoleucine, Ile, valine, Val, phenylalanine, Phe, and tryptophan, Trp) buried in the middle of the protein, whereas hydrophilic side chains are exposed to the aqueous solvent. (Note that in
biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . Over the last decades of the 20th century, biochemistry has beco ...

biochemistry
, a residue refers to a specific monomer within the polymer, polymeric chain of a polysaccharide,
protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins differ from one another primarily ...

protein
or nucleic acid.) The integral membrane proteins tend to have outer rings of exposed hydrophobic amino acids that anchor them into the lipid bilayer. Some peripheral membrane proteins have a patch of hydrophobic amino acids on their surface that locks onto the membrane. In similar fashion, proteins that have to bind to positively charged molecules have surfaces rich with negatively charged amino acids like glutamate and aspartate, while proteins binding to negatively charged molecules have surfaces rich with positively charged chains like lysine and arginine. For example, lysine and arginine are highly enriched in Low complexity regions in proteins, low complexity regions of nucleic-acid binding proteins. There are various hydrophobicity scales of amino acid residues. Some amino acids have special properties such as
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
, that can form covalent disulfide bonds to other cysteine residues,
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
that forms cyclic compound, a cycle to the polypeptide backbone, and
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
that is more flexible than other amino acids. Furthermore, glycine and proline are highly enriched within Low complexity regions in proteins, low complexity regions of eukaryotic and prokaryotic proteins, whereas the opposite (under-represented) has been observed for highly reactive, or complex, or hydrophobic amino acids, such as cysteine, phenylalanine, tryptophane, methionine, valine, leucine, isoleucine. Many proteins undergo a range of posttranslational modifications, whereby additional chemical groups are attached to the amino acid side chains. Some modifications can produce hydrophobic lipoproteins, or hydrophilic glycoproteins. These type of modification allow the reversible targeting of a protein to a membrane. For example, the addition and removal of the fatty acid palmitic acid to cysteine residues in some signaling proteins causes the proteins to attach and then detach from cell membranes.


Table of standard amino acid abbreviations and properties

Two additional amino acids are in some species coded for by genetic code, codons that are usually interpreted as
stop codon Stop may refer to: Places *Stop, Kentucky Stop is an unincorporated area, unincorporated community located in Wayne County, Kentucky, United States. The origin of the name "Stop" is obscure. References Unincorporated communities in Wayn ...
s: In addition to the specific amino acid codes, placeholders are used in cases where Protein sequencing, chemical or X-ray crystallography, crystallographic analysis of a peptide or protein cannot conclusively determine the identity of a residue. They are also used to summarise Conserved sequence, conserved protein sequence motifs. The use of single letters to indicate sets of similar residues is similar to the use of Nucleic acid notation, abbreviation codes for degenerate bases. Unk is sometimes used instead of Xaa, but is less standard. In addition, many Non-proteinogenic amino acids, nonstandard amino acids have a specific code. For example, several peptide drugs, such as Bortezomib and MG132, are peptide synthesis, artificially synthesized and retain their protecting groups, which have specific codes. Bortezomib is pyrazinoic acid, Pyz–Phe–boroLeu, and MG132 is Carboxybenzyl, Z–Leu–Leu–Leu–al. To aid in the analysis of protein structure, photo-reactive amino acid analogs are available. These include photoleucine (pLeu) and photomethionine (pMet).


Chemical analysis

The total nitrogen content of organic matter is mainly formed by the amino groups in proteins. The Total Kjeldahl Nitrogen (TKN) is a measure of nitrogen widely used in the analysis of (waste) water, soil, food, feed and organic matter in general. As the name suggests, the Kjeldahl method is applied. More sensitive methods are available.


See also

* Amino acid dating * Beta-peptide * Degron * Erepsin * Homochirality * Hyperaminoacidemia * Leucines * Miller–Urey experiment * Nucleic acid sequence * RNA codon table


Notes


References


Further reading

* * * *


External links

* {{DEFAULTSORT:Amino Acid Amino acids, Nitrogen cycle Zwitterions