These are the most complex group of vitamins consisting of a corrin core with a central cobalt atom with +1, +2, or +3 oxidation state. The corrin core has four pyrrol units, two of which are attached directly whereas the others are joined by methylene bridges. Out of six coordination bonds of cobalt, four are bonded to pyrrole nitrogens and the fifth is bound to one of the nitrogen atoms of the dimethylbenzimidazole derivative containing ribose 3-phosphate and aminoisopropanol linked through an amide bond to the side chain of the corrin core. The sixth substituent of the cobalt atom can be -CN, -OH, -CH3, or a 5'-deoxyadenosyl unit (Fig. 28.45). In hy-droxocobalamin, the cobalt atom is in the + 3 oxidation state and called vitamin B12b (Co3+), which is reduced by flavo-protein reductase to the vitamin B12r (Co2+) form and then to the vitamin B12s (Co+) form, which is converted to the methyl coenzyme form. Vitamin B12r (Co2+) is believed to be converted to the adenosylcoenzyme form by cob(I)yrinic acid a, c-diamide adenosyltransferase (EC using ATP.

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