<unk> s (ACADs) are a class of enzymes that function to catalyze the initial step in each cycle of fatty acid β-oxidation in the mitochondria of cells. Their action results in the introduction of a trans double-bond between C2 (α) and C3 (β) of the<unk> -CoA thioester substrate. Flavin adenine dinucleotide (FAD) is a required co-factor in addition to the presence of an active site glutamate in order for the enzyme to function. The following reaction is the oxidation of the fatty acid by FAD to afford an α,β-unsaturated fatty acid thioester of Coenzyme A: center ACADs can be categorized into three distinct groups based on their specificity for short-, medium-, or long-chain fatty acid<unk> -CoA substrates. While different<unk> s target fatty acids 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 sequence. ACADs are an important class of enzymes in mammalian cells because of their role in metabolizing fatty acids present in ingested food materials. This enzyme's action represents the first step in fatty acid metabolism (the process of breaking long chains of fatty acids into acetyl<unk>  molecules). Deficiencies in these enzymes are linked to genetic disorders involving fatty acid oxidation (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<unk> s are α4 homotetramers, and in two cases (for very long chain fatty acid substrates) they are α2 homodimers. An additional class of<unk>  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 .
Acyl-CoA dehydrogenase
Pyruvate carboxylase
Phenylalanine hydroxylase
Aldehyde dehydrogenase
Glutamate dehydrogenase
Butyrate esterase
Glycogen phosphorylase isoenzyme BB
Monoamine oxidase B