[ω-(Adenosin-5′-O-yl)alkyl)cobalamins Mimicking the Posthomolysis Intermediate of Coenzyme B12-Dependent Rearrangements: Kinetic Investigations on Methylmalonyl-CoA Mutase

Coenzyme-B12 analogues carrying oligomethylene chains (C3-C7) inserted between the central Co atom and the 5′-O atom of the adenosine moiety mimicking the putative posthomolysis intermediate in coenzyme B12-dependent rearrangements were synthesized and examined for their effects on methylmalonyl-CoA mutase from Propionibacterium shermanii. All analogues proved to be inhibitors of methylmalonyl-CoA mutase and in all cases competitive inhibition with respect to coenzyme B12 was found. Inhibition constants (Ki) were determined by two independent methods and showed in both cases the predicted trend: the Ki values versus chain length had minima at the C6 analogue in which the distance is about 10 Å between the central Co atom and the 5′ carbon of the adenosine, assuming a zig-zag chain conformation. This is the postulated distance between the Co and 5′-methylene paramagnetic centers generated in the methylmalonyl-CoA-coenzyme B12 complex after homolytic cleavage of the CoC bond.