Cobalamin vitamin structure

In cobalamin, vitamin Bu, one of the six ligands forming an octahedral structure around a cobalt atom is an organic molecule attached through a carbon-cobalt bond (red). The bond is weak and easily broken. 

The structures of the biologically active forms of B12 were solved relatively recently (1961) (78) and were shown to contain a cobalt atom surrounded by a corrin ring as shown in Fig. 16 (80). The crystal structure also showed a cobalt-carbon a bond which was quite surprising since the few compounds with cobalt-carbon a- bonds known at that time were quite unstable (79). The corrin ring is similar to the porphyrin ring, but its greater saturation imports less rigidity than the porphyrin. Corrinoids with the axial 5,6-dimethylbenzimidazole substituent are called cobalamins. Vitamin B12 with Co(III) and CN in the top axial position is... 

Figure 28-2. The structure of vitamin B12 (cobalamin). Vitamin B12 is composed of a planar corrin ring containing a cobalt atom at the center. The corrin ring system is composed of four pyrrole rings and is similar to porphyrin in heme. The cobalt atom is coordinated to the nitrogens of each pyrrole ring and to a dimethylbenzimidazole group. The R group attached to the sixth coordination site of the cobalt atom in vitamin B12 can be a -CN, -OH, -CH3, or adenosyl group.

Cobalt has no confirmed nutritional role in mammalian organisms aside from its participation in the corrin ring structure of cobalamins (vitamin B12). Nonetheless, inorganic cobalt is absorbed by the intestine. That this absorption pathway was shared with iron was first suggested by the observation of a mineral-mineral competition 42). The use of radioisotopes of iron in diagnostic tests of absorption for characterizing iron nutrition in human subjects has been advanced (43-45). An excellent correlation between absorption of radioiron and radiocobalt has been reported (43-45). 

Figure 42-1. The structure of cobalamin, vitamin X = deoxyadenosine in deoxyadenyosylcobalamin X = CHj in methylcobalamin X = CN in cyanocobalamin, the commercial form found in vitamin tablets.

Cobalamin vitamin Bn) has by far the most complicated structure of all the vitamins (cf. Chapt. VI-6). The cyanide ion of cyanocobalamin may be replaced by other ions (OH, N02 ) the corresponding substances are then called hydroxyco-balamin, nitritocobalamin, etc. Some analogs have a purine (adenine or hypoxan-... 

Figure 15.8 (a) Structure and (b) alternative conformations of cobalamine found in B12-dependent enzymes. The functional group R is deoxyadenosine in AdoCbl, methyl in MeCbl and -CN in vitamin B12. (From Bannerjee and Ragsdale, 2003. Reprinted with permission from Annual Reviews.)... 

Like many vitamins, cobalamin is functionally active as a derived coenzyme, coenzyme B12. Structurally, this is composed of a corrin ring a haem-like porphyrin ring containing cobalt (Co3+) at the centre held by four coordination bonds. The fifth... 

The structure of cobalamin is more complex than that of folic acid (Figure 15.2 and 15.3). At its heart is a porphyrin ring containing the metal ion cobalt at its centre. In catalytic reactions the cobalt ion forms a bond with the one-carbon group, which is then transferred from one compound to another. Vitamin B12 is the prosthetic group of only two enzymes, methylmalonyl-CoAmutase and methionine synthase. The latter enzyme is particularly important, as it is essential for the synthesis of nucleotides which indicates the importance of vitamin B12 in maintenance of good health. 

Vitamin B12 consists of a porphyrin-like ring structure, with an atom of Co chelated at its centre, linked to a nucleotide base, ribose and phosphoric acid (6.34). A number of different groups can be attached to the free ligand site on the cobalt. Cyanocobalamin has -CN at this position and is the commercial and therapeutic form of the vitamin, although the principal dietary forms of B12 are 5 -deoxyadenosylcobalamin (with 5 -deoxyadeno-sine at the R position), methylcobalamin (-CH3) and hydroxocobalamin (-OH). Vitamin B12 acts as a co-factor for methionine synthetase and methylmalonyl CoA mutase. The former enzyme catalyses the transfer of the methyl group of 5-methyl-H4 folate to cobalamin and thence to homocysteine, forming methionine. Methylmalonyl CoA mutase catalyses the conversion of methylmalonyl CoA to succinyl CoA in the mitochondrion. 

Structure of vitamin B12 (cyanocobalamin) and its coenzyme forms (methylcobalamin and 5 -deoxyadenosyl-cobalamin). 

Vitamin B12 (cobalamins) (Figure 2.29) are extremely complex structures based on a corrin ring, which, although similar to the porphyrin ring found in haem, chlorophyll, and cytochromes,... 

Figure 1 Schematic representation of the molecular structure, numbering of atoms, and designations of pyrrole rings of cobalamins. R = Me is methyl B12 R = Ado is adenosyl-cobalamin (coenzyme B12) X = CN is cyanocobalamin (vitamin B12). Five-membered rings are labeled A-D, and the amide side-chains are labeled a-g.

Some authors use the term adult onset pernicious anemia to distinguish this condition from rare disorder subdivisions of pernicious anemia due to congenital defects in IF secretion or structure or to various types of entero-cyte cobalamin malabsorption. In all other situations, the term vitamin B12 deficiency is used, and an associated anemia, if consequent on it, is called megaloblastic anemia, bearing in mind that identical appearances of the peripheral blood and the bone marrow may be... 

Vitamin B12, cobalamin or cyanocobaltamin, C63H88CoN14Oi4P, has the most complicated structure of the B vitamins. It contains one atom of cobalt in its molecule. It is required in minute amounts but without it the manufacture of proteins and red blood cells is affected. A diet which is deficient in liver, eggs, meat, fish or milk can lead to pernicious anaemia. Vegetarians are often advised to take vitamin B12 supplements. 

Gruber, K., Jogl, G., Klintschar, G., and Kratky, C., 1998, High-resolution crystal structures of cobalamins. In Vitamin B12 and B12-proteins (B. Krautler, D. Arigoni, and B. T. Golding, eds.), Wiley-VCH, Weinheim, pp. 3359348. 

Vitamin Bn (cobalamin) resembles the porphyrins in structure and>is synthesized in Porphy s Hemeland, but only by microorganisms. It has a cobalt ion in its center rather than iron (as in heme) or magnesium (as in chlorophyll). It is part of the coenzyme used for ... 

Cobalamins comprise a family of compounds which share a complex structure. Vitamin is known as cyanocobalamin because when originally isolated, an in-vitro artefact had placed a cyan group in the cobalt 3 position. Vitamin is an active cellular coenzyme essential for demethy-... 

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