THE PRIMARY STRUCTURE OF SHEEP LIVER CYTOSOLIC SERINE HYDROXYMETHYLTRANSFERASE AND AN ANALYSIS OF THE EVOLUTIONARY RELATIONSHIPS AMONG SERINE HYDROXYMETHYLTRANSFERASES

The complete amino-acid sequence of sheep liver cytosolic serine hydro xymethyltransferase was determined from an analysis of tryptic, chymot ryptic, CNBr and hydroxylamine peptides. Each subunit of sheep liver s erine hydroxymethyltransferase consisted of 483 amino-acid residues. A comparison of this sequence with 8 other serine hydroxymethyltransfer ases revealed that a possible gene duplication event could have occurr ed after the divergence of animals and fungi. This analysis also showe d independent duplication of SHMT genes in Neurospora crassa. At the s econdary structural level, all the serine hydroxymethyltransferases be long to the alpha/beta category of proteins. The predicted secondary s tructure of sheep liver serine hydroxymethyltransferase was similar to that of the observed structure of tryptophan synthase, another pyrido xal 5'-phosphate containing enzyme, suggesting that sheep liver serine hydroxymethyltransferase might have a similar pyridoxal 5'-phosphate binding domain. In addition, a conserved glycine rich region, G L Q G G P, was identified in all the serine hydroxymethyltransferases and co uld be important in pyridoxal 5'-phosphate binding. A comparison of th e cytosolic serine hydroxymethyltransferases from rabbit and sheep liv er with other proteins sequenced from both these sources showed that s erine hydroxymethyltransferase was a highly conserved protein. It was slightly less conserved than cytochrome c but better conserved than my oglobin, both of which are well known evolutionary markers. C67 and C2 03 were specifically protected by pyridoxal 5'-phosphate against modif ication with [C-14]iodoacetic acid, while C247 and C261 were buried in the native serine hydroxymethyltransferase. However, the cysteines ar e not conserved among the various serine hydroxymethyltransferases. Th e exact role of the cysteines in the reaction catalyzed by serine hydr oxymethyltransferase remains to be elucidated.