Energetic Feasibility of Hydrogen Abstraction and Recombination in Coenzyme B12-Dependent Diol Dehydratase Reaction.

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Coenzyme B12 serves as a cofactor for enzymatic radical reactions. The essential steps in all the coenzyme B12-dependent rearrangements are two hydrogen abstraction steps: hydrogen abstraction of the adenosyl radical from substrates, and hydrogen back-abstraction (recombination) of a product-derived radical from 5'-deoxyadenosine. The energetic feasibility of these hydrogen abstraction steps in the diol dehyratase reaction was examined by theoretical calculations with a protein-free, simplified model at the B3LYP/6-311G_??_ level of density functional theory. Activation energies for the hydrogen abstraction and recombination with 1, 2-propanediol as substrate are 9. 0 and 15. 1 kcal/ mol, respectively, and essentially not affected by coordination of the substrate and the radical intermediate to K+. Since these energies can be considered to be supplied by the substrate-binding energy, the computational results with this simplified model indicate that the hydrogen abstraction and recombination in the coenzyme B12-dependent diol dehydratase reaction are energetically feasible.
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