Cobamides

Cobinic acid and cobinamide, which aie hnked to libose 3-phosphate, are known as cobamic acid and cobamide (5), respectively. 

NAD and NADP and FMN and FAD, respectively. Pantothenic acid is a component of the acyl group carrier coenzyme A. As its pyrophosphate, thiamin participates in decarboxylation of a-keto acids and folic acid and cobamide coenzymes function in one-carbon metabolism. 

Ethanolamine ammonia lyase, L-/ -lysine mutase, D-a-lysine mutase and ornithine mutase are representative of cobamide enzymes in which transfer of hydrogen occurs with cleavage of the C—N bond (Fig. 16). 

Microbiological assay should stress accuracy over precision. Standardization of an assay method should include comparisons with at least one other organism having a different nutritional pattern and specificity toward the compound being assayed. Such a comparison was made for cyanocobalamin (vitamin Bi2 ) content of human blood and serum, using four microorganisms differing in their cobamide requirements and metabolism (B9). 

B23. Barker, H., Structure and function of cobamide coenzymes. Federation Proc. 20, 956-961 (1961). 

Vitamin Bj2 (cyanocobalamine, a-[5,6-dimethylbenzimidazolyl]cyano cobamide) [68-19-9] M 1355.4, dark at 210-220° and does not melt below 300°, [a] g 6 -59° (H2O). Crystd from de-ionized H2O, solubility in H2O is lg/80g and dried under vacuum over MgfClO. The dry red crystals are hygroscopic and can absorb -12% of H2O. A soln at pH 4.5-5 can be autoclaved for 20min at 120° without dec. Aqueous solns are stabilised by addition of (NH SO [Golding Comprehensive Organic Chem vol 5 (ed Haslam Pergamon Press, NY, 1979) pp549-584]. 

In view of work by Barker et al. (1960A,B) and Weissbach et al. (1961), it appears that the natural cobamide produced in bacterial cultures is not vitamin Bi2 but rather coenzyme B12. Berry and Bullerman (1966) and Bullerman and Berry (1966A.B) described a two-stage proc-... 

Barker, H. A., Smyth, R. D., Weissbach, H., Toohey, J. I., Ladd, J. N. and Volcani, B. E. 1960B. Isolation and properties of crystalline cobamide coenzymes containing benzimidazole or 5,6-dimethylbenzimidazole. J. Biol. Chem. 235, 480-488. 

Cobamides consist of a cobalt corrinoid with a pendant nucleotide, which together occupy five of the six positions of an octahedral Co. The most important... 

In contrast to coenzyme Bi2, where the alkyl moiety serves purely in a catalytic role, the alkyl group of methyl cobamides (MeCba s) is utilized as a reagent by MeCba-dependent enzymes it is only the cobamide portion of the coenzyme that is catalytic. The cobamide-dependent methyl transferases have been reviewed [11,24-27,165], Three cobamide-dependent methyl transferases have been studied in some cases, more than one protein is required. The Bi2 proteins include methionine synthase (officially called 5-methyltetrahydrofolate-L-homocysteine-S-methyltransferase [HCM] EC 2.1.1.13) MeCba-dependent enzyme from Meth-anosarcina barkeri (MT 0 and the corrinoid/Fe-S protein from Clostridium ther-moaceticum. 

Incorporation of solvent hydrogen into the substrate does not rule out its transfer as hydride if the source of hydride is dithiol, as in the ribonucleotide reductases. In the most understood enzyme of this type, from Lactobacillus leichmannii, the function of the dithiol (45) is to reduce the cobamide coenzyme (46) and provide a source... 

Abeles, R. H., and Beck, W. S., 1967, The mechanism of action of cobamide coenz>me in the ribonucleotide reductase reaction. J. Biol. Chem. 242 3589fi3593. 

Hogenkamp, H. P. C., Ghambeer, R. K., Brownson, C., Blakley, R. L., and Vitols, E., 1968, Cobamides and ribonucleotide reduction. VI. Enzyme-catalyzed hydrogen exchange between water and deoxyadenosylcobalamin. J. Biol. Chem. 243 7999808. 

Leutbecher, U., Bocher, R., Linder, D., and Buckel, W., 1992, Glutamate mutase from Clostridium cochlearium. Purification, cobamide content and stereospecific inhibitors, Eur. J. Biochem. 205 7599765. 

Stupperich, E., Eisinger, H. J., and Albracht, S. P. J., 1990, Evidence for a super-reduced cobamide as the major corrinoid fraction in vivo and a histidine residue as a cobalt ligand of the para-cresolyl cobamide in the acetogenic bacterium Sporomusa-ovata, Eur. J. Biochem. 193 1050109. 

Complete corrin a cobamide with an appended nucleotide base... 

X= NHCH2CH(CH3)0H diaqua-Co "-cobinamide X = NHCH2CH(CH3)0P02 (D O ribofuranosyl) diaqua-Co" -cobamid... 

Figure 1 Representative structural formulae and corresponding symbols of cobalamins([5, 6 -dimethylbenzunidazolyl]cobamides, (a)) and of diaquo-Co forms of incomplete corrins (b)...

The major biosynthetic paths for the formation of adeno-sylcobamides (such as (3)) and of methylcobamides (such as (4)) proceed stereospecificaUy by alkylation (via nucleophilic attack) of enzyme-bound Co -cobamides with adenosine-5 -triphosphate (ATP) and with A -methyltetra-hydropterins or 5-adenosyhnethionine (SAM), respectively. ... 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

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