Evidence for axial coordination of 5,6-dimethylbenzimidazole to the cobalt atom of adenosylcobalamin bound to diol dehydratase

It was demonstrated by electron paramagnetic resonance (EPR) spectroscopy that organic radical intermediates disappeared and cob(II)alamin accumulated upon suicide inactivation of diol dehydratase by 2-methyl-1,2- propanediol. The resulting EPR spectra showed that the eight hyperfine lines due to the divalent cobalt atom of cob(II)alamin further split into triplets by the superhyperfine coupling to the ¹⁴N nucleus. Essentially the same superhyperfine splitting of the octet into triplets was observed with [¹⁴N]-and [¹⁵N]apoenzyme. When the adenosyl form of [¹⁴N₂]- and [¹⁵N₂]imidazolyl analogues of the coenzyme [Toraya, T., and Ishida, A. (1991) J. Biol. Chem. 266, 5430-5437] was used with unlabeled apoenzyme, the octet showed superhyperfine splitting into triplets and doublets, respectively. Therefore, it was concluded that cobalamin is bound to this enzyme with 5,6-dimethylbenzimidazole coordinating to the cobalt atom. This conclusion is consistent with the fact that the consensus sequence forming part of a cobalamin-binding motif, conserved in methionine synthase and some of the other cobalamin enzymes, was not found in the deduced amino acid sequences of the subunits of diol dehydratase. Adenosylcobinamide methyl phosphate, a coenzyme analogue lacking the nucleotide moiety, underwent cleavage of the cobalt-carbon bond upon binding to the enzyme in the presence of substrate, forming a cob(II)-inamide derivative without nitrogenous base coordination, as judged by EPR and optical spectroscopy. Therefore, this analogue may be a useful probe for determining whether the replacement of the 5,6-dimethylbenzimidazole ligand by a histidine residue takes place upon binding of cobalamin to proteins.