Sacrificial Cobalt–Carbon Bond Homolysis in Coenzyme B12as a Cofactor Conservation Strategy

A sophisticated intracellular trafficking pathway in humans is used to tailor vitamin B12into its active cofactor forms, and to deliver it to two known B12-dependent enzymes. Herein, we report an unexpected strategy for cellular retention of B12, an essential and reactive cofactor. If methylmalonyl-CoA mutase is unavailable to accept the coenzyme B12product of adenosyltransferase, the latter catalyzes homolytic scission of the cobalt–carbon bond in an unconventional reversal of the nucleophilic displacement reaction that was used to make it. The resulting homolysis product binds more tightly to adenosyltransferase than does coenzyme B12, facilitating cofactor retention. We have trapped, and characterized spectroscopically, an intermediate in which the cobalt–carbon bond is weakened prior to being broken. The physiological relevance of this sacrificial catalytic activity for cofactor retention is supported by the significantly lower coenzyme B12concentration in patients with dysfunctional methylmalonyl-CoA mutase but normal adenosyltransferase activity.