Binding of substrate to ornithine 4,5-aminomutase (OAM) and methylmalonyl-CoA mutase (MCM) leads to the formation of an electrostatic interaction between a conserved glutamate side chain and the adenosyl ribose of the adenosylcobalamin (AdoCbl) cofactor. The contribution of this residue (Glu338 in OAM from Clostridium sticklandii and Glu392 in human MCM) to AdoCbl Co-C bond labilization and catalysis was evaluated by substituting the residue with a glutamine, aspartate, or alanine. The OAM variants, E338Q, E338D, and E338A, showed 90-, 380-, and 670-fold reductions in catalytic turnover and 20-, 60-, and 220-fold reductions in k(cat)/K(m), respectively. Likewise, the MCM variants, E392Q, E392D, and E392A, showed 16-, 330-, and 12-fold reductions in k(cat), respectively. Binding of substrate to OAM is unaffected by the single-amino acid mutation as stopped-flow absorbance spectroscopy showed that the rates of external aldimine formation in the OAM variants were similar to that of the native enzyme. The decrease in the level of catalysis is instead linked to impaired Co-C bond rupture, as UV-visible spectroscopy did not show detectable AdoCbl homolysis upon binding of the physiological substrate, d-ornithine. AdoCbl homolysis was also not detected in the MCM mutants, as it was for the native enzyme. We conclude from these results that a gradual weakening of the electrostatic energy between the protein and the ribose leads to a progressive increase in the activation energy barrier for Co-C bond homolysis, thereby pointing to a key role for the conserved polar glutamate residue in controlling the initial generation of radical species.