The electrochemistry of 5-hydroxybenzimidazolylcobamide

Methanogenic archaea typically contain 5-hydroxybenzimidazolylcobamide (cba-HBI) as the prosthetic group of a number of methyltransferases involved in their central metabolic pathways. In this paper the (acidic) dissociation constants and standard oxidation-reduction potentials of the Co3+/Co2+ and Co2+/Co1+ couples of isolated aquo-cba-HBI were measured. Comparison of the data to those established for 5,6-dimethylbenzimidazolylcobamide (cobalamin) showed that the 5-hydroxybenzimidazolyl (HBI) nucleotidic base hardly affected the redox potentials. HBI, however, proved to be the weaker ligand, thus favoring "base-off" formation. The implications for the functioning of cba-HBI in biochemical methyl group transfer reactions are discussed.