Mechanism of transfer of the methyl group from (6S)- methyltetrahydrofolate to the corrinoid/iron-sulfur protein catalyzed by the methyltransferase from Clostridium thermoaceticum: A key step in the Wood- Ljungdahl pathway of acetyl-CoA synthesis

The methyltetrahydrofolate:corrinoid/iron-sulfur protein methyltransferase (MeTr) from Clostridium thermoaceticum catalyzes transfer of the N⁵-methyl group from (6S)-methyltetrahydrofolate (CH₃-H₄folate) to the cobalt center of a corrinoid/iron-sulfur protein (CFeSP), forming methylcob(III)amide and H₄folate. This reaction initiates the unusual biological organometallic reaction sequence that constitutes the Wood- Ljungdahl or reductive acetyl-CoA pathway. The present paper describes the use of steady-state, product inhibition, single-turnover, and kinetic simulation experiments to elucidate the mechanism of the MeTr-catalyzed reaction. These experiments complement those presented in the companion paper in which binding and protonation of CH₃-H₄folate are studied by spectroscopic methods [Seravalli, J., Shoemaker, R. K., Sudbeck, M. J., and Ragsdale, S. W. (1999) Biochemistry 38, 57365745]. Our results indicate that a pH-dependent conformational change is required for methyl transfer in the forward and reverse directions; however, this step is not rate-limiting. CH₃-H₄folate and the CFeSP [in the cob(I)amide state] bind randomly and independently to form a ternary complex. Kinetic simulation studies indicate that CH₃-H₄folate binds to MeTr in the unprotonated form and then undergoes rapid protonation. This protonation enhances the electrophilicity of the methyl group, in agreement with a 10-fold increase in the pK(a) at N⁵ of CH₃-H₄folate. Next, the Co(I)-CFeSP attacks the methyl group in a rate- limiting S(N)2 reaction to form methylcob(III)amide. Finally, the products randomly dissociate. The following steady-state constants were obtained: k(cat) = 14.7 ± 1.7 s^-1, K(m) of the CFeSP = 12 ± 4 μM, and K(m) of (6S)-CH₃-H₄folate = 2.0 ± 0.3 μM. We assigned the rate constants for the elementary reaction steps by performing steady-state and pre-steady-state kinetic studies at different pH values and by kinetic simulations.