Serine hydroxymethyltransferase from soybean root nodules : purification and kinetic properties

Serine hydroxymethyltransferase has been purified 1,550-fold from the plant fraction of soybean (Glycine max L]. Merr. cv Williams) nodules. The pH optimum for the enzyme was at 8.5. The native molecular weight was 230,000 with a subunit molecular weight of 55,000 which suggested a tetramer of identical subunits. The enzyme kinetics for the enzyme were Michaelis-Menten; there was no evidence for cooperativity in the binding of either substrates or product inhibitors. There were two K_m values for serine at 1.5 and 40 millimolar. The K_m for l-tetrahydrofolate was 0.25 millimolar. l-Methyl-, l-methenyl-, and l-methylene-tetrahydrofolates were all noncompetitive inhibitors with l-tetrahydrofolate with K_i values of 1.8, 3.0, and 2.9 millimolar, respectively. Glycine was a competitive inhibitor with serine with a K_i value of 3.0 millimolar. The intersecting nature of the double reciprocal plots together with the product inhibition data suggested an ordered mechanism with serine the first substrate to bind and glycine the last product released. The enzyme was insensitive to a wide range of metabolites which have previously been reported to affect its activity. These results are discussed with respect to the roles of serine hydroxymethyltransferase and the one-carbon metabolite pool in control of the carbon flow to the purine biosynthetic pathway in ureide biogenesis.