A number of enzymes that catalyze the same reactions as do pyridoxal phosphate-dependent enzymes contain a catalytic pyruvate residue at the amino terminal of the peptide chain. The catalytic mechanism is assumed to be the same as for pyridoxal phosphate-dependent enzymes, except that the proton donor is a glutamate residue rather than lysine.
The most studied enzyme is histidine decarboxylase from Lactobacillus 30a. There are pyruvate residues at the amino terminals of each of 5 of the 10 sub-units in this enzyme. When the organism is grown on [14C]serine, the specific radioactivity of the pyruvate is the same as that of serine incorporated into the protein and much greater than that of free lactate or pyruvate in the culture medium. This suggests that pyruvate arises by postsynthetic modification of a serine residue.
A mutant strain of the organism has been isolated that produces an inactive precursor of histidine decarboxylase with only five separable subunits and no pyruvate residues. Prolonged incubation of this zymogen leads to apparently autocatalytic activation. Each subunit gives rise to two subunits, with the formation of an amino terminal pyruvate from the serine residue adjacent to the point of cleavage. The precursor protein has two adjacent serine residues, and undergoes an autocatalytic nonhydrolytic cleavage between these two serines. Oxygen from the side chain of the distal serine becomes the second oxygen of what now becomes the carboxy terminal of one peptide, leaving an imine at the amino terminal of the other. This imine undergoes hydrolysis to release ammonia and yield the amino terminal pyruvate residue.
Other pyruvate-containing enzymes include aspartate f-decarboxylase from Escherichia coli, the enzyme that catalyzes the formation of f -alanine for the synthesis of pantothenic acid (Section 12.2.4); proline reductase from Clostridium sticklandii; phosphatidylserine decarboxylase from E. coli; and phenylalanine aminotransferase from Pseudomonas fluorescens. Phospho-pantetheinoyl cysteine decarboxylase, involved in the synthesis of coenzyme A (Section 12.2.1), and S-adenosylmethionine decarboxylase seem to be the only mammalian pyruvoyl enzymes (Snell, 1990).
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