Quantum catalysis in B 12 -dependent methylmalonyl-CoA mutase: experimental and computational insights

B 12 -dependent methylmalonyl-CoA mutase catalyses the interchange of a hydrogen atom and the carbonyl-CoA group on adjacent carbons of methylmalonyl-CoA to give the rearranged product, succinyl-CoA. The first step in this reaction involves the transient generation of cofactor radicals by homolytic rupture of the cobalt–carbon bond to generate the deoxyadenosyl radical and cob(II)alamin. This step exhibits a curious sensitivity to isotopic substitution in the substrate, methylmalonyl-CoA, which has been interpreted as evidence for kinetic coupling. The magnitude of the isotopic discrimination is large and a deuterium isotope effect ranging from 35.6 at 20 °C to 49.9 at 5 °C has been recorded. Arrhenius analysis of the temperature dependence of this isotope effect provides evidence for quantum tunnelling in this hydrogen transfer step. The mechanistic complexity of the observed rate constant for cobalt–carbon bond homolysis together with the spectroscopically silent nature of many of the component steps limits the insights that can be derived by experimental approaches alone. Computational studies using a newly developed geometry optimization scheme that allows determination of the transition state in the full quantum mechanical/molecular mechanical coordinate space have yielded novel insights into the strategy deployed for labilizing the cobalt–carbon bond and poising the resulting deoxyadenosyl radical for subsequent hydrogen atom abstraction.