Function of the active-site lysine in Escherichia coli serine hydroxymethyltransferase

Serine hydroxymethyltransferase has a conserved lysine residue (Lys-229) that forms the internal aldimine with pyridoxal 5'-phosphate. In other pyridoxal 5'-phosphate enzymes investigated so far, this conserved lysine residue also plays a catalytic role as a base that removes the alpha-proton from the amino acid substrate. Three mutant forms of Escherichia coli serine hydroxymethyltransferase (K229Q, K229R, and K229H) were constructed, expressed, and purified. The absorbance spectra, rapid reaction kinetics, and thermal denaturation of the mutant analogs were studied. Only the K229Q mutant serine hydroxymethyltransferase resembled the wild-type enzyme. The results indicate that Lys-229 plays a critical role in expelling the product by converting the external aldimine to an internal aldimine. In the absence of Lys-229, ammonia can also catalyze the same function at a much slower rate. However, Lys-229 apparently is not the base that removes the alpha-proton from the amino acid substrate. The K229Q mutant enzyme could catalyze one turnover of either serine to glycine or glycine to serine at rates approaching those of the wild-type enzyme. After one turnover, the mutant enzyme could not expel the product and bind new substrate. The K229Q mutant enzyme can also transaminate D-alanine, which, like the hydroxymethyltransferase activity, also requires removing the alpha-proton from the substrate. The absorbance spectra of the K229R and K229H serine hydroxymethyltransferases showed that their pyridoxal 5'-phosphate could not readily form an external aldimine with substrates, suggesting that Lys-229 in the wild-type enzyme may never bear a positive charge, further evidence that it is not the base that removes the alpha-proton.