Acyl-CoA dehydrogenases (ACADs) are a class of

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

that function to catalyze the initial step in each cycle of fatty acid

β-oxidation In biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cyc ...

in the

mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. Their action results in the introduction of a

trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Arts, entertainment, and media * Trans (festival), a former festival in Belfast, Northern Ireland, United Kingdom * ''Trans'' (fil ...

double-bond between C2 (α) and C3 (β) of the acyl-CoA thioester substrate.

Flavin adenine dinucleotide Flavin may refer to: Placename * Flavin, Aveyron, a commune in southern France Surname * Adrian Flavin (born 1979), a professional rugby player * Christopher Flavin, president of the Worldwatch Institute * Dan Flavin (1933–1996), a minimalis ...

(FAD) is a required co-factor in addition to the presence of an active site

glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can synt ...

in order for the enzyme to function. The following reaction is the

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

of the

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...

by FAD to afford an α,β-unsaturated fatty acid thioester of

Coenzyme A Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a subs ...

: ACADs can be categorized into three distinct groups based on their specificity for short-, medium-, or long-chain

acyl-CoA Acyl-CoA is a group of coenzymes that metabolize fatty acids. Acyl-CoA's are susceptible to beta oxidation, forming, ultimately, acetyl-CoA. The acetyl-CoA enters the citric acid cycle, eventually forming several equivalents of ATP. In this wa ...

substrates. While different dehydrogenases target

fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...

of varying chain length, all types of ACADs are mechanistically similar. Differences in the enzyme occur based on the location of the active site along the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

sequence. ACADs are an important class of

mammalian Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...

because of their role in metabolizing

present in ingested food materials. This enzyme's action represents the first step in

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...

(the process of breaking long chains of fatty acids into acetyl CoA molecules). Deficiencies in these

are linked to genetic disorders involving

(i.e. metabolic disorders). ACAD enzymes have been identified in animals (of which there are 9 major eukaryotic classes), as well as plants, nematodes, fungi, and bacteria. Five of these nine classes are involved in fatty acid β-oxidation (SCAD, MCAD, LCAD, VLCAD, and VLCAD2), and the other four are involved in branched chain amino acid metabolism (i3VD, i2VD, GD, and iBD). Most acyl-CoA dehydrogenases are α₄

homotetramer A tetrameric protein is a protein with a quaternary structure of four subunits (tetrameric). Homotetramers have four identical subunits (such as glutathione S-transferase), and heterotetramers are complexes of different subunits. A tetramer c ...

s, and in two cases (for very long chain fatty acid substrates) they are α₂ homodimers. An additional class of acyl-CoA dehydrogenase was discovered that catalyzes α,β-unsaturation reactions with steroid-CoA thioesters in certain types of bacteria. This class of ACAD was demonstrated to form α₂β₂ heterotetramers, rather than the usual α₄ homotetramer, a protein architecture that evolved in order to accommodate a much larger steroid-CoA substrate. ACADs are classified as .

Structure

The medium chain acyl-CoA dehydrogenase (MCAD) is the best known structure of all ACADs, and is the most commonly deficient

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

within the class that leads to metabolic disorders in animals. This protein is a homotetramer with each subunit containing roughly 400

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

and one equivalent of FAD per monomer. The tetramer is classified as a "dimer of dimers" with an overall diameter of approximately 90 Å. The interface between the two monomers of a single dimer of an ACAD contains the FAD binding sites and has extensive bonding interactions. In contrast, the interface between the two dimers has fewer interactions. There are a total of 4 active sites within the tetramer, each of which contains a single FAD molecule and an

substrate binding site. This gives a total of four FAD molecules and four

substrate binding sites per enzyme. FAD is bound between the three domains of the monomer, where only the nucleotide portion is accessible. FAD binding contributes significantly to overall

stability. The

substrate is bound completely within each monomer of the

. The active site is lined with the residues F252, T255, V259, T96, T99, A100, L103, Y375, Y375, and E376. The area of interest within the substrate becomes wedged between Glu 376 and FAD, lining up the molecules into an ideal position for the reaction. MCAD can bind to a rather broad range of chain-lengths in the

substrate, however studies show that its specificity tends to target

octanoyl-CoA Octanoyl-coenzyme A is the endpoint of beta oxidation in peroxisomes. It is produced alongside acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism ...

(C8-CoA). A novel ACAD enzyme architecture in some species of steroid-utilizing bacteria (''

Actinomycetota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to s ...

'' and ''

Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. Th ...

'') was discovered, and is involved in the utilization of ubiquitous steroid substrates like cholesterol by pathogenic organisms like ''Mycobacterium tuberculosis''. Genetically, the structure is encoded by two separate genes ( open reading frames) that form an obligate α₂β₂ heterotetramic complex. The structure was most likely the result of an evolutionary event that caused

gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. ...

and partial loss of function, since half of the FAD cofactor binding residues are in each gene, and only make a complete binding site when expressed together as a complex. This probably allowed for the substrate binding site to open up considerably to accommodate much larger polycyclic-CoA substrates, rather than fatty acids of varying chain lengths.

Mechanism

The acyl-CoA dehydrogenase mechanism proceeds through an E2 elimination. This elimination is initiated by a

residue, which, while necessary for the mechanism, is not conserved. The residue appears in a wide range of locations within the different types of the

(it is Glu 376 in MCAD). The

residue deprotonates the pro-R

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-to ...

of the alpha

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 ma ...

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-to ...

bonding of the substrate's carbonyl oxygen to both the 2'-OH of the ribityl side-chain of FAD and to the main chain N-H of the previously mentioned

residue lowers the pKa of this

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

, allowing it to be readily removed by

. As the alpha

is being deprotonated, the pro-R

of the beta

leaves as a hydride to FAD in a concerted step. It adds to the Re face of FAD at the N-5 position, and the

holds FAD in place through

bonding with the

pyrimidine Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The othe ...

portion and

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

interactions with the dimethylbenzene portion. The substrate has now been transformed into an α,β unsaturated thioester. As FAD picks up the hydride, the carbonyl oxygen adjacent to the N-1

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

becomes negatively charged. These

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

are in resonance with the N-1

, distributing and stabilizing the resulting negative charge. The charge is also stabilized by hydrogen bonding between the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

and

of interest and various residues within the

Clinical significance

Deficiencies in acyl-CoA dehydrogenases result in decreased ability to oxidize

, thereby signifying metabolic dysfunction. Medium-chain acyl-CoA dehydrogenase deficiencies ( MCADD) are well known and characterized because they occur most commonly among acyl-CoA dehydrogenases, leading to

disorders and the potential of life-threatening metabolic

diseases A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

. Some symptoms of medium-chain acyl-CoA dehydrogenase deficiency include intolerance to

fasting Fasting is the abstention from eating and sometimes drinking. From a purely physiological context, "fasting" may refer to the metabolic status of a person who has not eaten overnight (see " Breakfast"), or to the metabolic state achieved after ...

hypoglycemia Hypoglycemia, also called low blood sugar, is a fall in blood sugar to levels below normal, typically below 70 mg/dL (3.9 mmol/L). Whipple's triad is used to properly identify hypoglycemic episodes. It is defined as blood glucose bel ...

, and

sudden infant death syndrome Sudden infant death syndrome (SIDS) is the sudden unexplained death of a child of less than one year of age. Diagnosis requires that the death remain unexplained even after a thorough autopsy and detailed death scene investigation. SIDS usuall ...

. These symptoms are seen as directly connected to the inability to metabolize fats. Intolerance to

and

result from the inability to gain

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

and make

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or do ...

from fat stores, which is how most of humans' excess energy is stored. Also,

s can begin to accumulate in the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...

, lowering the blood's pH and causing

acidosis Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma. The term ''acidemia'' describe ...

. MCAD is related to / has an association with sudden infant death. Approximately 90% of cases of MCAD are due to a single point

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

where the

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated − ...

at position 304 (Lys304)is replaced by a

residue and this prevents the

from properly functioning. It is reported that, every year, 1 in 20,000

infants An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used ...

is born with a deficiency in his/her medium-chain acyl-CoA dehydrogenases that is caused by a

. The

recessive In genetics, dominance is the phenomenon of one variant ( allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant an ...

, and often parents of

children A child ( : children) is a human being between the stages of birth and puberty, or between the developmental period of infancy and puberty. The legal definition of ''child'' generally refers to a minor, otherwise known as a person younger ...

who have the deficiency can be diagnosed afterward as carriers. In

humans" \n\n\n\n\nThe robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visi ...

the most common naturally occurring

in MCAD is located at

residue Lys-304. The altered residue occurs as a result of a single-point

in which the

side chain is replaced by a

. Lys-304 typically interacts with surrounding

residues by forming

hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

with Gln-342, Asp-300, and Asp-346. When a

causes

to take the place of

, an additional negative charge is introduced at that site, which disrupts the normally occurring H-bonding. Such a disruption alters the folding pattern of the

, ultimately compromising its stability and inhibiting its function in

. The efficiency of the mutated

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

is about 10 times lower than that of the natural

. This can lead to the symptoms of the deficiency listed above.

References

* "Molecular graphics images were produced using the UCSF Chimera package from the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIH P41 RR-01081). " * *

External links

* {{Portal bar, Biology, border=no EC 1.3.99

Structure

Mechanism

Clinical significance

See also

References

Further reading

External links