ABSTRACT
Plants and microorganisms can synthesize all of the 20 standard amino acids. Prior to the metabolism of their carbon skeletons into a major metabolic intermediate, the a-amino group of the amino acid has first to be removed by a process known as transamination. The enzymes that catalyze these reactions are called transaminases (aminotransferases) and in mammals are found predominantly in the liver. The coenzyme (or prosthetic group) of all transaminases is pyridoxal phosphate, which is derived from pyridoxine (vitamin B6), and which is transiently converted into pyridoxamine phosphate during transamination. The amino acid is then hydrolyzed to form an a-keto acid and pyridoxamine phosphate, the a-amino group having been temporarily transferred from the amino acid substrate on to pyridoxal phosphate. Phenylalanine is first hydroxylated by phenylalanine hydroxylase to form another aromatic amino acid tyrosine.
