THE ELECTROCHEMISTRY OF 5-HYDROXYBENZIMIDAZOLYLCOBAMIDE

Methanogenic archaea typically contain 5-hydroxybenzimidazolylcobamide (cba-HBI) as the prosthetic group of a number of methyltransferases i nvolved in their central metabolic pathways, In this paper the (acidic ) dissociation constants and standard oxidation-reduction potentials o f the Co3+/Co2+ and Co2+/Co1+ couples of isolated aquo-cba-HBI were me asured. Comparison of the data to those established for 5,6-dimethylbe nzimidazolylcobamide (cobalamin) showed that the 5-hydroxybenzimidazol yl (HBI) nucleotidic base hardly affected the redox potentials. HBI, h owever, proved to be the weaker ligand, thus favoring ''base-off'' for mation. The implications for the functioning of cba-HBI in biochemical methyl group transfer reactions are discussed. (C) 1995 Academic Pres s, Inc.