Enzymatic radical catalysis: coenzyme B12-dependent diol dehydratase

A peculiar function resides in a peculiar structure. Coenzyme B(12) or adenosylcobalamin, a naturally occurring organometallic compound, serves as a cofactor for enzymatic radical reactions. How do the enzymes form catalytic radicals at the active sites? How do the enzymes utilize and control the high reactivity of the radicals for catalysis? Recently, three-dimensional structures of several radical-containing or radical-forming enzymes including B(12) enzymes have been reported, enabling the analysis of the fine mechanisms of the action of these interesting enzymes. Our biochemical, mutational, and crystallographic studies as well as theoretical calculations on diol dehydratase, an adenosylcobalamin-dependent enzyme, revealed that its structure is adapted for its function-that is, activation of the Cobond;C bond toward homolysis, abstraction of a specific hydrogen atom from the substrate and its recombination to a particular product, and transition state stabilization in the hydroxyl group migration of a substrate-derived radical. The functions of K(+) and the active-site amino acid residues in enzyme catalysis are also investigated. Based on the results, the fine mechanism of the enzyme and the energetic feasibility of enzymatic radical catalysis are described here.