General reactions of amino acids

Some general reactions that involve degradation or interconversion of amino acids provide for the synthesis of nonessential amino acids from a-keto acid precursors derived from carbohydrate intermediates. 

Removal of the a-amino group is the first step in catabolism of amino acids. It may be accomplished oxidatively or nonoxidatively. 

Oxidative deamination is stereospecific and is catalyzed by L- or D-amino acid oxidase. The initial step is removal of two hydrogen atoms by the flavin coenzyme, with formation of an unstable a-amino acid intermediate. This intermediate undergoes decomposition by addition of water and forms ammonium ion and the corresponding a-keto acid L-Amino acid oxidase occurs in the liver 

High levels of D-amino acid oxidases are found in the liver and kidney. The enzyme contains flavin adenine dinucleotide (FAD) and deaminates many D-amino acids and 

D-Amino acid oxidase occurs in peroxisomes containing other enzymes that produce H2O2 (e.g., L-a-hydroxy acid oxidase, citrate dehydrogenase, and L-amino acid oxidase) and catalase and peroxidase, which destroy H2O2. In leukocytes, killing of bacteria involves hydrolases of lysosomes and production of H2O2 by NADPH oxidase (Chapter 15). Conversion of D-amino acids to the corresponding a-keto acids removes the asymmetry at the a-carbon atom. The keto acids may be aminated to L-amino acids. By this conversion from D- to L-amino acids, the body utilizes D-amino acids derived from the diet  

---

Chelation with various metal ions is a general reaction of amino acids. As the nonbonding electrons of the a-amino group are coordinated to the metal, only the side chain amino function is free to react with the acylating agent. The a-amino function may be freed by treatment with hydrogen sulfide. 

---