Corrinoid-dependent synthesis

Copper-sulfide cluster 884s Coproporphyrin III 843,845s Comified cell envelope 439 Corrin in transmethylation 592 Corrin ring 867, 868 Corrinoid-dependent synthesis of acetyl-CoA 876, 877 Cosmarium 22 COSY-NOESY diagram 143 Cotransport (symport) process 411,416,417 Coulomb 283... 

To summarize, corrinoids in propionic acid bacteria are involved not onh in fermentation, but also in such important anabolic processes as protein and DNA synthesis and DNA methylation. In this respect, corrinoids differ from other related tetrapyrrole compounds by their polyfunctionalit. The involvement of corrinoid-dependent enzymes in different metabolic processes in propionibacteria explains the propensity of anaerobic strains of the classical propionic acid bacteria to synthesize large amounts of corrinoids under suitable conditions. 

Whereas tetrahydrobiopterin is biosynthesized from GTP via just three enzyme-catalyzed steps (2), some coenzyme biosynthetic pathways are characterized by enormous complexity. Thus, the biosynthesis of vitamin B12 requires five enzymes for the biosynthesis of the precursor uroporhyrinogen III (16) from succinyl-CoA (10) and glycine (11) that is then converted into vitamin B12 via the sequential action of about 20 enzymes (3). Additional enzymes are involved in the synthesis of the building blocks aminopropanol and dimethylbenzimidazole (4, 5). Vitamin B12 from nutritional sources must then be converted to coenzyme B12 by mammalian enzymes. Ultimately, however, coenzyme B12 is used in humans by only two enzymes, albeit of vital importance, which are involved in fatty acid and amino acid metabolism (6). Notably, because plants do not generate corrinoids, animals depend on bacteria for their supply of vitamin B12 (which may be obtained in recycled form via nutrients such as milk and meat) (7). 

Another B12 coenzyme-dependent dehydrase converts glycerol to 3-hydroxypropionaldehyde. A B12-dependent ethanolamine deaminase catalyzes the irreversible conversion of ethanolamine to acetaldehyde and ammonia. A corrinoid protein participates in methyl transfer reactions and methane formation by bacteria producing methane. Corrinoid proteins are also involved in acetate synthesis from carbon dioxide. 

The competition of vitamin B12 synthesis for amino acids has an effect on protein synthesis, as shown by the rate of incorporation of labeled leucine in cellular proteins (Fig. 5.6). In cells containing corrinoids at levels that are either higher or lower than the optimal (physiological) level, protein synthesis is reduced (Iordan et al, 1984). Apparently, vitamin B is indirectly involved in protein synthesis, since the protein content in vitamin Bi2-deficient cells is 67% of the control cells. The decrease in protein content may be a consequence of the inhibition of synthesis and activity of a number of enzymes (first of all SH-dependent enzymes) in vitamin Bn-deficient cells. 

The observation that corrinoid-deficient cells of P. shermanii can grow and replicate normally pointed to the existence of alternative, corrinoid-independent, pathways of the above mentioned syntheses in propionibacteria. Indeed, as shown above, in corrinoid-deficient P. shermanii the AdoCbl-independent (probably metal-dependent) oxygen-requiring RNR enzyme functions, and DNA-methylation is catalyzed by the cobalamin-independent methylase which uses AdoMet as methyl donor. Conversely, in vitamin B -replete cells the AdoCbl-dependent RNR is more active than the metal-dependent RNR, which is inhibited by high concentrations of corrinoids. The discovery of two systems operating in DNA synthesis (ribose-to-deoxyribose conversion) is an example of the... 

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