Serine Transhydroxymethylase: Evidence for a Sequential Random Mechanism

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 ⇌ glycine + 5,10-methylenetetrahydrofolate. This interpretation is supported by equilibrium isotope-exchange studies. The relative maximum rates of exchange of L-serine ⇌ glycine and L-serine ⇌ 5,10-methylenetetrahydrofolate in reaction 1 were not inhibited by high levels of substrates. The relative rates of these two exchange reactions were similar but were not identical. These results suggest that the catalytic interconversion and dissociation of substrates are of the same order of magnitude. Reaction 1 represents the transfer of a one-carbon group from the third carbon 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 one-carbon group, e.g., 5-methyltetrahydrofolate and 5,10-methylenetetrahydrofolate. This suggests that the one-carbon binding site can accommodate two one-carbon groups simultaneously without serious steric hindrance.