Cobalt corrinoids synthesis

Lenhert and Hodgkin (15) revealed with X-ray diffraction techniques that 5 -deoxyadenosylcobalamin (Bi2-coenzyme) contained a cobalt-carbon o-bond (Fig. 3). The discovery of this stable Co—C-tr-bond interested coordination chemists, and the search for methods of synthesizing coen-zyme-Bi2 together with analogous alkyl-cobalt corrinoids from Vitamin B12 was started. In short order the partial chemical synthesis of 5 -de-oxyadenosylcobalamin was worked out in Smith s laboratory (22), and the chemical synthesis of methylcobalamin provided a second B 12-coenzyme which was found to be active in methyl-transfer enzymes (23). A general reaction for the synthesis of alkylcorrinoids is shown in Fig. 4. 

Vitamin Bn-deficient cells contained about 30-45% less DNA than cells with physiological levels of the vitamin (Vorobjeva and Iordan, 1976 Iordan, 1992). The DNA content in these cells increased by up to 80% when AdoCbl was added to cultures growing in cobalt-ffee medium (Iordan et al., 1983) (Table 5.1). Strains with a potential capacity for high corrinoid synthesis demonstrated a more significant stimulation by exogenous AdoCbl than strains with low synthetic capacity (Fig. 5.7). However, P. acnes represented an exception to this rule it responded weakly to the addition of AdoCbl, despite having a high potential for vitamin Bn synthesis. 

Fig. 4. Summary of reactions of methyl-cobalt-corrinoids involved in the formation of methane and the synthesis of acetate from COi. This figure is diagrammatic only. The CHa-Co-corrinoid-enzyme complexes involved in methane formation and acetate synthesis are difierent from one another.

Scheme 1 outlines the retrosynthetic analysis of the Woodward-Eschenmoser A-B variant of the vitamin B12 (1) synthesis. The analysis begins with cobyric acid (4) because it was demonstrated in 1960 that this compound can be smoothly converted to vitamin B12.5 In two exploratory corrin model syntheses to both approaches to the synthesis of cobyric acid,6 the ability of secocorrinoid structures (e. g. 5) to bind metal atoms was found to be central to the success of the macrocyclization reaction to give intact corrinoid structures. In the Woodward-Eschenmoser synthesis of cobyric acid, the cobalt atom situated in the center of intermediate 5 organizes the structure of the secocorrin, and promotes the cyclization... 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

Menon, S., and Ragsdale, S. W., 1998, Role of the [4Ee64S] cluster in reductive activation of the cobalt center of the corrinoid iron-sulfur protein from Clostridium thermoaceticum during acetyl-CoA synthesis Biochem. 37(16) 5689n5698. 

Vitamin B12 (cyanocobalamin) 3 is, in fact, not a natural product as the cyanide ligand to the cobalt ion is added during the isolation procedure. Coenzyme B12 (adenosylcobalamin) 4 and methylcobalamin 5 are the true final products of the biosynthetic pathway. Coenzyme 0,2 is the cofactor for a number of enzymic rearrangement reactions, such as that catalysed by methylmalonyl CoA mutase, and methylcobalamin is the cofactor for certain methyl transfer reactions, including the synthesis of methionine. A number of anaerobic bacteria produce related corrinoids in which the dimethylbenzimidazole moiety of the cobalamins (3 - 5) is replaced by other groups which may or may not act as ligands to the cobalt ion, such as adenine orp-cresol [12]. 

The corrin ring and the nucleotide moiety are synthesized only by bacteria and some algae, but the cobalt-binding p-ligands can be introduced by some enzymatic systems in animals and humans. The synthesis of precursors in adequate amounts is often impaired, that is why microorganisms, in addition to the biologically active forms, also contain incomplete forms of corrinoids, mainly those in which the nucleotide moiety is lacking cobyric acid, cobinamide, cobinamide-P and cobinamide-GDP. Under conditions unfavorable for the synthesis of the nucleotide moiety incomplete forms may predominate, especially cobinamide. The content of complete forms increases in old (or aerated) cultures at the expense of incomplete forms. 

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