• coenzyme B12;
  • adenosylcobalamin;
  • B12 enzyme;
  • diol dehydratase;
  • adenine-anchored radicals.

In the course of structural studies of diol dehydratase–cobalamin complexes, it was found that the electron density corresponding to the cyano group of the enzyme-bound cyanocobalamin is almost not observable at room temperature and very low even at cryogenic temperatures, suggesting its dissociation from the Co atom upon X-ray irradiation. On the contrary, the adenine moiety of the enzyme-bound adeninylpentylcobalamin was clearly located in the electron density map. When the enzyme–adeninylpentylcobalamin complex was illuminated 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 indicates a Co—C bond cleavage and that the adenine moiety remains held by hydrogen bonds with some residues in the enzyme. Thus, the formation of an adenine-anchored radical upon illumination was demonstrated crystallographically with this complex. These observations clearly indicate that homolysis of the Co—C bond of alkylcobalamin takes place upon illumination with visible light but is not readily cleaved during X-ray irradiation.