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BACKGROUND: Adenosylcobalamin (coenzyme B(12)) serves as a cofactor for enzymatic radical reactions. The adenosyl radical, a catalytic radical in these reactions, is formed by homolysis of the cobalt-carbon bond of the coenzyme, although the mechanism of cleavage of its organometallic bond remains unsolved. RESULTS: We determined the three-dimensional structures of diol dehydratase complexed with adeninylpentylcobalamin and with cyanocobalamin at 1.7 A and 1.9 A resolution, respectively, at cryogenic temperatures. In the adeninylpentylcobalamin complex, the adenine ring is bound parallel to the corrin ring as in the free form and methylmalonyl-CoA-mutase-bound coenzyme, but with the other side facing pyrrole ring C. All of its nitrogen atoms except for N(9) are hydrogen-bonded to mainchain amide oxygen and amide nitrogen atoms, a sidechain hydroxyl group, and a water molecule. As compared with the cyanocobalamin complex, the sidechain of Seralpha224 rotates by 120 degrees to hydrogen bond with N(3) of the adenine ring. CONCLUSIONS: The structure of the adenine-ring-binding site provides a molecular basis for the strict specificity of diol dehydratase for the coenzyme adenosyl group. The superimposition of the structure of the free coenzyme on that of enzyme-bound adeninylpentylcobalamin demonstrated that the tight enzyme-coenzyme interactions at both the cobalamin moiety and adenine ring of the adenosyl group would inevitably lead to cleavage of the cobalt-carbon bond. Rotation of the ribose moiety around the glycosidic linkage makes the 5'-carbon radical accessible to the hydrogen atom of the substrate to be abstracted.