J. Masuda et al., Radical production simulated by photoirradiation of the diol dehydratase-adeninylpentylcobalamin complex, J SYNCHROTR, 8, 2001, pp. 1182-1185
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.