PURIFICATION AND PROPERTIES OF SERINE HYDROXYMETHYLTRANSFERASE FROM SULFOLOBUS-SOLFATARICUS

Serine hydroxymethyltransferase (SHMT) catalyzes the reversible cleava ge of serine to glycine with the transfer of the one-carbon group to t etrahydrofolate to form 5,10-methylenetetrahydrofolate. No SHMT has be en purified from a nonmethanogenic Archaea strain, in part because thi s group of organisms uses modified folates as the one-carbon acceptor. These modified folates are not readily available for use in assays fo r SHMT activity. This report describes the purification and characteri zation of SHMT from the thermophilic organism Sulfolobus solfataricus. The exchange of the alpha-proton of glycine with solvent protons in t he absence of the modified folate was used as the activity assay. The purified protein catalyzes the synthesis of serine from glycine and a synthetic derivative of a fragment of the natural modified folate foun d in S. solfataricus, Replacement of the modified folate with tetrahyd rofolate did not support serine synthesis. In addition, this SHMT also catalyzed the cleavage of both allo-threonine and beta-phenylserine i n the absence of the modified folate. The cleavage of these two amino acids in the absence of tetrahydrofolate is a property of other charac terized SHMTs. The enzyme contains covalently bound pyridoxal phosphat e. Sequences of three peptides showed significant similarity with thos e of peptides of SHMTs from two methanogens.