carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
s. The
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
of many pentoses is , and their
molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
is 150.13 g/mol.-Ribose . PubChem compound webpage, accessed on 2010-02-06.
Pentoses are very important in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
.
Ribose
Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compo ...
is a constituent of
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, and the related molecule,
deoxyribose
Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. D ...
Phosphorylated
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
pentoses are important products of the
pentose phosphate pathway
The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-pho ...
, most importantly
ribose 5-phosphate
Ribose 5-phosphate (R5P) is both a product and an intermediate of the pentose phosphate pathway. The last step of the oxidative reactions in the pentose phosphate pathway is the production of ribulose 5-phosphate. Depending on the body's state, r ...
(R5P), which is used in the synthesis of
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
erythrose 4-phosphate
Erythrose 4-phosphate is a phosphate of the simple sugar erythrose. It is an intermediate in the pentose phosphate pathway and the Calvin cycle.
In addition, it serves as a precursor in the biosynthesis of the aromatic amino acids tyrosine, pheny ...
(E4P), which is used in the synthesis of aromatic amino acids.
Like some other monosaccharides, pentoses exist in two forms, open-chain (linear) or closed-chain (cyclic), that easily convert into each other in water solutions. The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons. Four of these carbons have one
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
(–OH) each, connected by a single
bond
Bond or bonds may refer to:
Common meanings
* Bond (finance), a type of debt security
* Bail bond, a commercial third-party guarantor of surety bonds in the United States
* Chemical bond, the attraction of atoms, ions or molecules to form chemica ...
, and one has an oxygen atom connected by a double bond (=O), forming a
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
group (C=O). The remaining bonds of the carbon atoms are satisfied by six
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms. Thus the structure of the linear form is H–(CHOH)''x''–C(=O)–(CHOH)4-''x''–H, where ''x'' is 0, 1, or 2.
The term "pentose" sometimes is assumed to include deoxypentoses, such as
deoxyribose
Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. D ...
: compounds with general formula that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.
Classification
The aldopentoses are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1, forming an
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
derivative with structure H–C(=O)–(CHOH)4–H. The most important example is
ribose
Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compo ...
. The
ketopentose
In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.ketone derivative with structure H–CHOH–C(=O)–(CHOH)3–H (2-ketopentose) or H–(CHOH)2–C(=O)–(CHOH)2–H (3-ketopentose). The latter is not known to occur in nature and are difficult to synthesize.
In the open form, there are eight aldopentoses and four 2-ketopentoses,
stereoisomer
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
s that differ in the spatial position of the hydroxyl groups. These forms occur in pairs of
optical isomer
In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are d ...
s, generally labelled "" or "" by conventional rules (independently of their optical activity).
Aldopentoses
The aldopentoses have three
chiral center
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups ...
s; therefore, eight (23) different
stereoisomer
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
s are possible.
Ribose
Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compo ...
is a constituent of
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, and the related molecule,
deoxyribose
Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. D ...
, is a constituent of DNA. Phosphorylated pentoses are important products of the
pentose phosphate pathway
The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-pho ...
, most importantly
ribose 5-phosphate
Ribose 5-phosphate (R5P) is both a product and an intermediate of the pentose phosphate pathway. The last step of the oxidative reactions in the pentose phosphate pathway is the production of ribulose 5-phosphate. Depending on the body's state, r ...
(R5P), which is used in the synthesis of
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
s and nucleic acids, and
erythrose 4-phosphate
Erythrose 4-phosphate is a phosphate of the simple sugar erythrose. It is an intermediate in the pentose phosphate pathway and the Calvin cycle.
In addition, it serves as a precursor in the biosynthesis of the aromatic amino acids tyrosine, pheny ...
(E4P), which is used in the synthesis of
aromatic amino acid
An aromatic amino acid is an amino acid that includes an aromatic ring.
Among the 20 standard amino acids, the following are classically considered aromatic: phenylalanine, tryptophan and tyrosine. Although histidine contains an aromatic ring, ...
s.
Ketopentoses
The 2-ketopentoses have two chiral centers; therefore, four (22) different stereoisomers are possible. The 3-ketopentoses are rare.
Cyclic form
The closed or cyclic form of a pentose is created when the carbonyl group interacts with a hydroxyl in another carbon, turning the carbonyl into a hydroxyl and creating an ether bridge –O– between the two carbons. This
intramolecular reaction Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.
Examples
* intramolecular hydride transfer (transfer of a hy ...
yields a
cyclic
Cycle, cycles, or cyclic may refer to:
Anthropology and social sciences
* Cyclic history, a theory of history
* Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr.
* Social cycle, various cycles in s ...
molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then called
furanose
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle furan, bu ...
s, for having the same rings as the
cyclic ether
Cycle, cycles, or cyclic may refer to:
Anthropology and social sciences
* Cyclic history, a theory of history
* Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr.
* Social cycle, various cycles in so ...
tetrahydrofuran
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
. Library of Congress catalog 66-25695
The closure turns the carboxyl carbon into a
chiral center
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups ...
, which may have any of two configurations, depending on the position of the new hydroxyl. Therefore, each linear form can produce two distinct closed forms, identified by prefixes "α" and "β".
Deoxypentoses
The one deoxypentose has two total stereoisomers.
Properties
In the cell, pentoses have a higher metabolic stability than
hexose
In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol.
Hexoses exist in two forms, open-chain or cyclic, that easily convert ...
s.
A
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
composed of pentose sugars is called a
pentosan Pentosans are polymers composed of pentoses. In contrast to cellulose, which is composed of hexose (glucose) monomers, pentosans are derived from five-carbon sugars such as xylose. Pentosan-rich biomass is the precursor to furfural.
The pentosan co ...
.
Tests for pentoses
The most important tests for pentoses rely on converting the pentose to furfural, which then reacts with a chromophore. In Tollens’ test for pentoses (not to be confused with Tollens' silver-mirror test for
reducing sugar
A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...
phloroglucinol
Phloroglucinol is an organic compound with the formula C6H3(OH)3. It is a colorless solid. It is used in the synthesis of pharmaceuticals and explosives. Phloroglucinol is one of three isomeric benzenetriols. The other two isomers are hydroxyqu ...
to produce a colored compound; in the
aniline acetate test
The aniline acetate test is a chemical test for the presence of certain carbohydrates, in which they are converted to furfural with hydrochloric acid, which reacts with aniline acetate to produce a bright pink color. Pentoses give a strong reactio ...
with aniline acetate; and in
Bial's test
Bial's test is a chemical test for the presence of pentoses. It is named after Manfred Bial, a German physician. The components include orcinol, hydrochloric acid, and ferric chloride. A pentose, if present, will be dehydrated to form furfural ...
, with
orcinol
Orcinol is an organic compound with the formula CH3C6H3(OH)2. It occurs in many species of lichens including '' Roccella tinctoria'' and ''Lecanora''. Orcinol has been detected in the "toxic glue" of the ant species ''Camponotus saundersi''. It is ...
. In each of these tests, pentoses react much more strongly and quickly than hexoses.