Bacterial Biosynthesis of Vitamin B2

Vitamin B12 is synthesized only by bacteria and possibly some algae. There are no plant sources of the vitamin, and no plant enzymes are known to require vitamin B12 as a coenzyme. A number of reports have suggested that vitamin B12 occurs in some algae, but this may be the result of bacterial contamination of the water in which they were grown. Nori, made from the edible seaweed Porphyra tenera, has been reported to contain biologically active cobalamin when it is fresh; but, on drying, there is a considerable loss of the vitamin as a result of the formation of inactive corrinoids (Yamada et al., 1999).

The precursor for vitamin B12 synthesis is uroporphyrinogen III, the common precursor for all porphyrins, including heme and chlorophyll. Uropor-phyrinogen III is synthesized by condensation between succinyl coenzyme A (CoA) and glycine to yield S-aminolevulinic acid. Two molecules of <5-amino-levulinic acid then condense to form the pyrrole phorphobilinogen, and four molecules of porphobilinogen condense to yield uroporphobilinogen III.

Uroporphobilinogen III then undergoes the following sequence of reactions (Raux et al., 2000; Roessner et al., 2001):

1. successive methylations, in which S-adenosyl methionine is the methyl donor;

2. excision of C-20 to give the direct fused link between the A and D pyrrole rings;

3. insertion of the central cobalt atom;

4. attachment of 5 -deoxyadenosine to the cobalt atom, yielding cobyrinic acid;

5. amidation of the acidic side chains to yield cobinamide; and

6. attachment of the dimethylbenzimidazole nucleotide, which is either formed from riboflavin, or synthesized by a similar pathway to that of riboflavin synthesis (see Figure 7.3), yielding adenosylcobalamin.

10.8 METABOLIC FUNCTIONS OF VITAMIN Bt2

In microorganisms, vitamin B12 is involved in a variety of reactions, including methyl transfer; the reduction of carbon dioxide to methane, a number of isomerase, mutase, and aminomutase reactions (1,2-migration reactions), diol dehydrogenase, ethanolamine ammonia-lyase, and in some organisms ribonucleotide reductase, which catalyzes the formation of deoxyribonu-cleotides from ribonucleotides. In eukaryotes, this reaction is catalyzed by an iron-containing enzyme.

In mammals, there are only three vitamin B12 -dependent enzymes: methionine synthetase, methylmalonyl CoA mutase, and leucine aminomutase. The enzymes use different coenzymes: methionine synthetase uses methylcobal-amin, and cobalt undergoes oxidation during the reaction; methylmalonyl CoA mutase and leucine aminomutase use adenosylcobalamin and catalyze the formation of a 5-deoxyadenosyl radical as the catalytic intermediate.

In addition, vitamin B12 has a role in the metabolism of cyanide, forming cyanocobalamin. This prevents the binding of cyanide to cytochrome oxidase and permits (relatively slow) metabolism to yield thiocyanate.

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