Amino acid
Amino acids are organic compounds containing amine (-NH) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid. The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of certain amino acids. About 500 naturally occurring amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways. They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.
D-amino acid oxidase
While D-amino acid oxidase differs to some extent between various organisms, the structure is basically the same across most eukaryotes, excluding plants. This enzyme is a flavoprotein belonging to the FAD dependent oxidoreductase family, and acts on the CH-NH group of D-amino acid donors with oxygen as acceptor. It is also considered a peroxisomal enzyme containing FAD as a cofactor. Each DAO monomer has an FAD-binding domain (FBD) containing a Rossman fold, and a substrate-binding domain (SBD) that also forms an interface with the other monomer in the protein. DAO exists as a dimer, with each monomer containing both an FBD and SBD. Each monomer is composed of 347 amino acids in human DAO, though among other eukaryotes the protein can range from 345 to 368 amino acids long. In human DAO, the two monomers are connected in a head-to-head fashion. DAO of other organisms, such as yeast, can be present as head-to-tail dimers. The hDAAO gene is found on chromosome 12 and contains 11 exons.