Cobalamin Coenzymes and Vitamin B12

Abstract

Cobalamins are derivatives of vitamin B12 that serve as cofactors in various enzymatic reactions involving either radicals or supernucleophilic intermediates.

Keywords: cobalt–carbon bond; 5′‐desoxyadenosyl radical; carbon skeleton rearrangement; amino mutase; elimination; methyl transfer

Figure 1.

Structure of cobalamins.

Figure 2.

General reaction mechanism postulated for AdoCbl‐dependent mutases; R• = 5′‐deoxyadenosyl radical or Ado‐CH2•, X = migrating group. The mechanism of step III is only solved in glutamate mutase. In case of the eliminases an additional step V is necessary to eliminate water or ammonia. In ribonucleotide reductase R• generates a thiyl radical, which abstracts the 3′‐hydrogen atom (step II). From the resulting substrate radical water is eliminated followed by a two‐electron reduction and final redonation of the initially abstracted hydrogen atom, whereby R• is regenerated.

Figure 3.

Postulated reaction mechanism for methionine synthase.

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Buckel, Wolfgang(Sep 2007) Cobalamin Coenzymes and Vitamin B12. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000666.pub2]