Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Coenzymes are specialized natural reagents
Coenzyme forms of vitamin B12
Reduction of cyanocobalamin and synthesis of alkylcobalamins

Cyanocobalamin can be reduced in two stages, with one electron being added at each stage [163, 167].

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The cyanide ion is lost in The First stage (stage a), which can be carried out using chromium(II) acetate at pH 5 or via catalytic hydrogenation. The reaction product at this stage is a brown paramagnetic compound, B_12r, which is a tetragonal low-spin cobalt(II) complex. In the second stage (stage b), another electron is added—for instance, from sodium borohydride or chromium(II) acetate at pH 9.5—yielding grey-green, highly reactive B_12s. The latter is believed to be in equilibrium with a cobalt(III) hydride, as shown in the following scheme (stage a):

The hydride is unstable and slowly decomposes into H₂ and B_12r (stage b) [168].

Vitamin B₁₂ readily reacts with alkyl iodides (such as methyl iodide or the 5'-chloro derivative of adenosine) via a nucleophilic substitution mechanism to form alkylcobalt forms of vitamin B₁₂:

These reactions provide a convenient method for synthesizing isotope-labeled alkylcobalamins, including selectively ¹³C-enriched cobalamins of interest for NMR studies [169]. The Biosynthesis of 5'-deoxyadenosylcobalamin relies on the exact same type of reaction involving ATP as a substrate [170]. B₁₂ adenosyltransferase catalyzes nucleophilic Substitution at the 5'-carbon of ATP to yield the coenzyme and inorganic tripolyphosphate.