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Cobamide coenzymes

Cobamide coenzymes are derivatives of the vitamin B12 (cobalamin) which was first isolated from liver.
The vitamin B12 is considered an important nutritional factor and its deficiency leads to pernicious anaemia. Murphy in 1926 demonstrated that feeding sufficient quantities of liver of anaemia patients could cure the disease. The vitamin is found in  animals and microorganisms. But never in plants.
    The structure of Cobamide coenzyme (coenzyme B12) is quite complex. The vitamin contains the base dimethylbenziminazole attached to ribose and phosphoric acid form a nucleotide structure. This nucleotide form is linked to a porphytin ring system having an atom of cobalt. A series of compounds are formed having vitamin –like prosperities when cobalt  ligands with CN­, OH­,CI­,etc. when the cobalt is linked to CN­, the compound is called cyanocobalamin, and when to OH­,the compound is called hydroxycobalamin , both the forms are active forms.
Cobamide coenzymes participate in many biochemical reactions and most of them have been described in microbial system. Some of these reactions are explained here.
 1.   The most important is the conversion of methylmalonyl-CoA to succinyl -CoA ~ SCoA component is transferred by the cobamide coenzyme.

the conversion of methylmalony -coa to succinyl -coa

2.    The coenzyme B12  can transfer C-1 fragments from tetrahydrofolic acid (THF) to homoserine for the synthesis of methionine. The reaction is quite complex.

3.    Cobamide coenzymes also participate in dismutation of diols through carbon –oxygen bond cleavage:

vitamin b12 cyanocobalamin

propane 1-2 diol and propionaldehyde

4.   An important reaction catalyzed by this coenzyme is reduction of ribonucleotides to deoxyribonucleotides. Ribonucleotide reductase carries this coenzyme, and the carbanion formed in the reaction is subsequently reduced.

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