Radical production simulated by photoirradiation of the diol dehydratase-adeninylpentylcobalamin complex

In the course of structural studies of diol dehydratase-cobalamin complexes , it was found that the electron density corresponding to the cyano group o f the enzyme-bound cyanocobalamin is almost not observable at room temperat ure and very low even at cryogenic temperatures, suggesting its dissociatio n from the Co atom upon X-ray irradiation. On the contrary, the adenine moi ety of the enzyme-bound adeninylpentylcobalamin was clearly located in the electron density map. When the enzyme-adeninylpentylcobalamin complex was i lluminated with visible light, the electron density between the C5' and Co atoms disappeared, and the temperature factors of the atoms comprising the pentamethylene group became much larger than those in the dark. This indica tes a Co-C bond cleavage and that the adenine moiety remains held by hydrog en bonds with some residues in the enzyme. Thus, the formation of an adenin e-anchored radical upon illumination was demonstrated crystallographically with this complex. These observations clearly indicate that homolysis of th e Co-C bond of alkylcobalamin takes place upon illumination with visible li ght but is not readily cleaved during X-ray irradiation.