Mechanism of Light Induced Radical Pair Formation in Coenzyme B₁₂-Dependent Ethanolamine Ammonia-Lyase

Abstract

Coenzyme B12 (adenosylcobalamin = AdoCbl)-dependent enzymes catalyze complex molecular transformations by employing radical chemistry. The initial step in the native catalytic cycle, upon substrate binding, involves homolytic cleavage of the Co–C bond of AdoCbl to form the Co(II)/Ado• radical pair (RP). Formation of Co(II)/Ado• is subsequently coupled with H atom abstraction from the substrate. Interestingly, these same RPs can be generated upon light absorption without presence of a substrate. Herein, the photochemistry associated with the mechanism of Co–C bond photocleavage inside the AdoCbl-dependent ethanolamine ammonia-lyase (EAL) was investigated using a combined time-dependent density functional theory and molecular mechanics (TD-DFT/MM) approach. Excited state potential energy surfaces (PESs), constructed as a function of axial bond lengths, were used to understand the photocleavage of the Co–C bond and to elucidate the mechanism of photodissociation for AdoCbl inside the enzyme. The S1 PES is characterized by two minima regions, namely, metal-to-ligand charge transfer (MLCT) and ligand field (LF) states, which are key minima regions along the reaction pathway. There are two possible routes for photolysis of AdoCbl inside EAL named Path A and Path B. Path B is slightly more energetically favorable than Path A and involves the elongation of the Co–Nax bond followed by the elongation of both axial bonds Co–C and Co–Nax. To further understand the effect of environment on the formation of RP, the photochemical data for AdoCbl-dependent EAL was also compared with base-on and base-off AdoCbl in solution.