Mechanistic Insights for Formation of an Organometallic Co–C Bond in the Methyl Transfer Reaction Catalyzed by Methionine Synthase

Abstract

Methionine synthase (MetH) catalyzes the transfer of a methyl group from methyltetrahydrofolate (CH₃–H₄Folate) to the cob­(I)­alamin intermediate to form an organometallic Co–C bond, a reaction similar to that of CH₃–H₄Folate:corrinoid/iron–sulfur protein (CFeSP) methyltransferase (MeTr). How precisely it is formed remains elusive because the displacement of a methyl group from the tertiary amine is not a facile reaction. To understand the electronic structure and mechanistic details of the MetH–cob­(I)­alamin:CH₃–H₄Folate reaction complex, we applied quantum mechanics/molecular mechanics (QM/MM) computations. The hybrid QM/MM calculations reveal the traditionally assumed S_N2 mechanism for formation the CH₃–cob­(III)­alamin resting state where the activation energy barrier for the S_N2 reaction was found to be ∼8–9 kcal/mol, which is comparable with respect to the determined experimental rate constant. However, the possibility of an electron transfer (ET) based radical mechanism consistent with the close-lying diradical states observed from triplet and open-shell singlet states has also been suggested as an alternative, where first an electron transfer from His-on cob­(I)­alamin to the pterin ring of the protonated CH₃–H₄Folate takes place, forming the Co^II(d⁷)–pterin radical (π*)¹ diradical state, followed by a methyl radical transfer. Although the predicted energy barrier for the ET-mediated radical reaction is comparable to that of the S_N2 pathway, the major advantage of ET is that a methyl radical can be transferred at a longer distance, which does not require the close proximity of two binding modules of MetH as does the S_N2 type. In addition, based on the energy barrier of the transition state (TS) in both the protonated (∼8–9 kcal/mol) and the unprotonated N5 (39 kcal/mol) species of the CH₃–H₄Folate, it can be inferred that the protonation event must takes place either prior to or during the methyl transfer reaction in a ternary complex. The results of the present study including mechanistic insights can have implications to a broad class of corrinoid–methyltransferases, which utilize a CH₃–H₄Folate substrate or its related analogues as methyl donor