Role of Tyrosine Residue in the Activation of Co−C Bond in Coenzyme B12-Dependent Enzymes: Another Case of Proton-Coupled Electron Transfer?

Abstract

X-ray structural data along with density functional theory-based computations have been used to probe the role of tyrosine residue in the activation of Co-C bond in adenosylcobalamin (AdoCbl) -dependent enzymes. DFT computations have been carried out for tyrosine being in the immediate vicinity of AdoCbl using the structural mimics of the active sites of methylmalonyl CoA mutase and glutamate mutase enzymes. The calculations indicate the diradical nature of the deprotonated tyrosine-cofactor complex implying the possibility of electron transfer from tyrosine to the AdoCbl. Thus, the tyrosine residue may serve like an internal redox center to transfer an electron to the AdoCbl cofactor that can be critical for the activation of Co-C bond in B(12)-dependent enzymes. The local environment around the active site reveals that deprotonation of tyrosine motif may take place upon substrate binding, implying the possibility of proton-coupled electron transfer (PCET) in AdoCbl-dependent enzymes. Thus, it is proposed that PCET can have implications in the activation of Co-C bond in AdoCb1-dependent enzymes as the electron transfer from tyrosine to AdoCbl helps remarkably in cleaving the Co-C bond.