The stereochemistry of pyridoxal phosphate enzymes. The absolute stereochemistry of cofactor C' 4 protonation in the transamination of holoserine hydroxymethylase by D-alanine.

Serine hydroxymethylase catalyzes the reversible cleavage of several P-hydroxy-a-amino acids to glycine and the appropriate aldehyde. With D-alanine as substrate, the a-hydrogen is labilized and a slow transamination to pyruvate and the pyridoxamine phosphate enzyme occurs. The absolute stereochemistry of this transamination has been determined by using pyridoxamine pyruvate transaminase to analyze the pyridoxamine phosphate product of the serine hydroxymethylase transamination. These two enzymes were found to protonate the cofactor C’4 from the same face. Thus, all five pyridoxal phosphate enzymes so far examined show the same absolute stereochemistry of cofactor protonation, adding the proton from the si face of C’4, so that the labile proton occupies the pro-S position of the pyridoxamine methylene group.