Structure cyanocobalamin

The structure of the first isolated vitamin cyanocobalamin [68-19-9] (la) is known to occur only sporadically, at best, in biological systems. Its isolation... 

FIGURE 18.28 The structure of cyanocobalamin (top) and simplified structures showing several coenzyme forms of vitamin Bi2- The Co—C bond of 5 -deoxyadenosylcobalamin is predominantly covalent (note the short bond length of 0.205 nm) but with some ionic character. Note that the convention of writing the cobalt atom as Co" " attributes the electrons of the Co—C and Co—N bonds to carbon and nitrogen, respectively. 

FIGURE 10.3 Structural formula of vitamin B[2 and partial structures of vitamin B[2 compounds. The partial structures of vitamin B,2 compounds show only those portions of the molecule that differ from vitamin B[2. 1 — 5 -deoxyadenosylcobalamin, 2 — methylcobal-amin, 3 — hydroxocobalamin, 4 — cyanocobalamin, 5 — benzimidazolyl cyanocobamide, 6 — pseudovitamin B[2, 7 — 5-hydroxybenzimidazolyl cyanocobamide, 8 — p-cresolyl cyanocobamide. 

Figure 15.3 Structural formula of deoxyadenosylcobalamin (coenzyme B, ). (a) A plan view of the corrin nucleus with substituents, (b) The position of the remaining two ligands of the cobalt atom. No attempt is made to show correct stereochemical relationships. Related compounds have different groups in place of the 5 -deoxyadenosyl group cyanocobalamin, (vitamin Bi2)-CN hydroxycobalamin, (vitamin Bi2)-0H methylcobalamin, (vitamin Bi2)-CH3.

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). 

Scheme 23 Incorporation of [2-l3C]ALA and [ CHjImethionine into cyanocobalamin by cells of P. shermanii. (64b) shows neo-cyanocobalamin (remainder of structure as 64)...

The Bj2 vitamers consist of a group of organometallic compounds that have a common cor-rinoid structure and vary in the substituent bound to the central cobalt atom (Fig. 7) (167,168). The principal naturally occurring Bl2 vitamers are hydroxocobalamin (HOCbl), methylcobal-amin (MeCbl), and adenosylcobalamin (AdoCbl). Cyanocobalamin (CNCbl) is the form commonly used for clinical, pharmaceutical, and food fortification purposes, due to its greater relative stability. 

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.

Various vitamin B12 derivatives are shown in Figure 6.2, where the R group is usually taken as CN-. This form of vitamin B12 is cyanocobalamin. In the active coenzyme, the CN" is replaced by a 5 -deoxyriboadenosyl residue or by -CH3. Vitamin B12 seems to be the only mammalian substance that contains cobalt. It also has a unique corrin ring structure, which is very similar to that of heme. Metabolic reactions requiring vitamin B12 are discussed in Chapters 19 and 20. 

Changes in the erythrocytes osmotic resistance were not observed. Adsorption of total plasma proteins on modified MC was lower than 12 %, but it was about 60 - 70 % on unmodified particles. Table 6 summarizes the results obtained of MC sorption efficiency to substances of different molecular mass in donor plasma. The sorption mechanism of low and middle molecular weight substances (phenobarbital and cyanocobalamin) on iron-carbon and restored-iron MC is apparently connected with absorption of molecules into the sorbent s pores. Iron-carbon composites have a more porous structure than restored-iron, therefore the... 

Moore FH, O Connor BH, Willis BTM, Hodgkin DC (1984) X-ray and neutron diffraction studies of the crystal and molecular structure of the predominant monocarboxylic acid obtained by mild acid hydrolysis of cyanocobalamin. Part III. Neutron diffraction studies of wet crystals. Proc Ind Acad Soc (Chem Sci) 93 235-260... 

As the name suggests, cyanocobalamin (vitamin B12) is a compound of cobalt (Co(lll)). With a complex organic structure, this essential water-soluble vitamin is obtained from dietary animal sources and is required for deoxyribonucleic acid (DNA) synthesis, where enzymes that use vitamin B12 are involved in the transfer of one-carbon units. The absorption of this vitamin from the gastrointestinal tract only occurs when intrinsic factor glycoprotein is present. While the body can store up to a 12-month supply of vitamin B12, rapid growth or conditions causing rapid cell turnover can increase the body s requirement for this vitamin. 

FIGURE 13. The structure of diol dehydrase-cyanocobalamin complex. The top figure shows the complete a(3y trimer the lower figures show each subunit separately and its spatial relationship to the coenzyme, substrate and essential potassium ion. Reprinted from Shibata et ah, 1999, with permission from Elsevier Science. 

Figure 1 In the above structure, R = CN denotes cyanocobalamin (CN-Cbl), whilst R = OH is hydroxocobalamin (OH-Cbl) R = 5 -deoxyadenosyl is coenzyme B12 (adenosylcobalamin, AdoCbl) and R = Me is methylcobalamin (MeCbl). By definition all cobalamins contain 5,6-dimethylbenzimidazole, which is the so-called 6th ligand to cobalt in the above structure. Substances containing the corrin ligand, i.e. the planar 14 electron p-system embracing cobalt in the above structure, are also called corrinoids.

Figure 22.12. Structure of Coenzyme B 2 (5 -deoxyadenosylcobalamin). Substitution of cyano and methyl groups create cyanocobalamin and methylcobalamin, respectively.

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-... 

The structure of vitamin appears in Figure 9,21. The X indicates the point of attachment of the cyanide group in cyanocobalamin, the hydroxyj group in hy drostocobalamin, and the methyl and deoxyadenosyl groups in the cofactor forms of the vitamin. 

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