Cobalamin catalysis

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

Two-Step Chemical Catalysis of the Reduction of Alkyl Halides by Low-Valent Cobalamins and Cobinamides... 

Banerjee, R. and Ragsdale, S.W. (2003) The many faces of vitamin B12 catalysis by cobalamin-dependent enzymes, Annu. Rev. Biochem., 72, 209-247. 

A fourth possibility, which arose from our collaborative structural work on both models and cobalamins, is mechanism (4), an alteration in the dmbim position or Co—N (dmbim) bond length [119], UV spectral data have been used to suggest that the Co-dmbim bond is broken during catalysis, giving the so-called base-off (benzimidazole-unhydronated) form of the coenzyme [163]. Furthermore, some unconfirmed evidence exists that axial base-free Ado-Cbi+ is still a partially active cofactor [76]. 

Cob(II)alamin was found to be a competitive inhibitor with respect to adenosylcobalamin of ATP reduction by the L. leichmannii enzyme. In the presence of 5 -deoxyadenosine the apparent Ki was decreased more than 10-fold adenosine was able to enhance inhibition by cob(II)-alamin to a lesser extent, but 2 -deoxyadenosine, 3 -deoxyadenosine, and 4, 5 -didehydro-5 -deoxyadenosine were much less effective. These studies indicate that cobalamin and nucleoside bind at sites that are occupied by adenosylcobalamin during catalysis and that the binding of the cobalamin moiety is greatly enhanced by the presence of a specific nucleoside (106). 

Scheme 13. Mechanism of RTPR-mediated catalysis using cobalamin R, triphosphate (49).

Two mechanistic aspects of the cobalamin-dependent methyltransferases distinguish them from adenosyl-cobalamin-dependent enzymes. No methylcobalamin synthetases are known. Methylcobalamin is produced at enzymatic sites as an intermediate in the primary overall reaction. Second, methylcobalamin does not function as a coenzyme in the sense that it assists in catalysis to date, it is instead a catalytic intermediate in its enzymatic reactions. 

Free radicals complexed to cobalt ions are formed in the same way by adding RX to reduced cobalamin, B12 and are also extensively studied as key intermediates in vitamin B12 catalysis [5]. 

Figure 2, Catalysis and reactivation of methionine synthase. Methionine for,motion occurs via two half reactions in which cobalamin serves as the intermediate methyl carrier. Reactivation is depicted in the right-hand portion of the diagram. An electron donor and AdoMet convert the inactive cob(II)alamin form of the enzyme to methylcob(III)alamin. In E. colU flavodoxin serves as the reductant for this priming reaction (7).

---