Enzymology of Amino Acid Conjugation Reactions*

Abstract

Amino acid conjugation is a route of metabolism for both a variety of xenobiotic carboxylic acids and also bile acids. However, knowledge of the overall enzymology (including multiplicity of enzymes, substrate selectivity, catalytic mechanism, protein structure, polymorphisms, and gene regulation) is not as advanced in comparison to the cytochromes P450 and uridine 5′-diphosphate (UDP)-glucuronosyltransferases. Amino acid conjugation differs mechanistically from other conjugating reactions in that it is a coupled enzyme system involving any one of a number of adenosine triphosphate (ATP)-dependent acid: CoA ligases and acyl-CoA:amino acid N-acyltransferases. In relation to xenobiotic carboxylic acids, amino acid conjugation involves enzymes located in the mitochondrial matrix while conjugation of bile acids is extramitochondrial involving enzymes located in the endoplasmic reticulum and peroxisomes. The first step in both xenobiotic and bile acid conjugation involves activation to the corresponding acyl-CoA, which is then a substrate for amino acid conjugation. Within mitochondria, amino acid conjugation terminates the biological reactivity of xenobiotic acyl-CoAs, whereas conjugation of bile acids increases aqueous solubility and promotes bile acid flow and intestinal absorption. For both pathways the amino acid utilized is species dependent and in mammals excretion is predominantly as either glycine or taurine conjugates. Amino acid conjugation has a fundamentally important role both in health and disease. Manipulation of mitochondrial amino acid conjugation is used in the treatment of nonketotic hyperglycinemia and hyperammonemia as failure to amino acid conjugate bile acids can lead to cholestasis and loss of absorption of fat and fat-soluble vitamins.