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

Coenzymes: Specialized Natural Reagents
Coenzyme Forms of Vitamin B12
Three Reactions of Non-Enzymatic Cleavage of Vitamin B12 Coenzyme Forms

The 5'-deoxyadenosyl coenzyme readily decomposes under METABOLISM/18.html">The Influence of various agents. Irradiation under anaerobic conditions yields mainly vitamin B12 and cyclic 5',8-deoxyadenosine (whereas exposure to air produces a variety of different compounds). The latter appears to be formed via an intermediate radical:

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Acid Hydrolysis of deoxyadenosylcobalamin (1 N HCl, 100°C, 90 min) affords hydroxycobalamin, adenine, and an unsaturated sugar:

The initiation step of the reaction is believed to be the protonation of the ribose ring oxygen. A similar Cleavage by an alkaline cyanide solution can be viewed as a nucleophilic substitution of the deoxyadenosyl anion by cyanide. The final reaction product is dicyanocobalamin, in which the loosely bound nucleotide containing dimethylbenzimidazole is displaced by a second cyanide ion. Methyl- and other alkylcobalamins are stable toward alkaline cyanide solutions.

Table 8-6 Coenzyme B12-dependent reactions, including type 10 hydrogen transfer reactions



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