Serine transhydroxymethylase: evidence for a sequential random mechanism

Abstract

Initial velocity patterns in the presence of product and dead-end inhibitors suggest that in reaction 1: the addition of substrates and release of products occur by a sequential random mechanism. L-serine + tetrahydrofolate .dblarw. glycine + 5,10-methylenetetrahydrofolate [catalyzed by rabbit liver serine transhydroxymethylase]. This interpretation is supported by equilibrium isotope-exchange studies. The relative maximum rates of exchange of L-serine .dblarw. glycine and L-serine .dblarw. 5,10-methylenetetrahydrofolate in this reaction were not inhibited by high levels of substrates. The relative rates of these 2 exchange reactions were similar but were not identical. The catalytic interconversion and dissociation of substrates may be of the same order of magnitude. Reaction 1 represents the transfer of a 1-C group from the 3rd C of L-serine to tetrahydrofolate. Inhibition studies showed that abortive enzyme ternary complexes are formed with L-serine and tetrahydrofolate compounds, which also contain a 1-C group, e.g., 5-methyltetrahydrofolate and 5,10-methylenetetrahydrofolate. The 1-C binding site may be able to accommodate two 1-C groups simultaneously without serious steric hindrance.