HO OH Acetyl-coenzyme A Figure 6 Pantothenic acid in the structure of coenzyme A.
coenzyme known to have a transition metal ion (cobalt) coordinated to its structure. The metal allows some usual chemistry (see 00058). The vitamin is present in a variety of foods but is almost totally lacking in plants. Although the vitamin can be synthesized de novo by intestinal flora, the absorption site anatomically is prior to the synthesis site in the gut, which means little benefit is derived from endogenous synthesis. Isolating the active form of the vitamin meant developing an in vitro assay for 'pernicious anemia,' one of the deficiency symptoms. In 1950, Shive introduced an assay in which homocysteine was converted to methionine in a B12 -dependent reaction. A second assay showed that the derivative 5-deoxyadenosyl cobalamin was essential for the interconversion of L-glutamate and ^-methyl aspartate. The latter discovery led to the isolation of 5' adenosylcobalamine, the principal active form of the vitamin.
Reactivity The core of the vitamin consists of a corrin ring with a central cobalt atom. Corrin contains four pyrrole rings linked together, which vaguely resembles structurally the porphyin ring in heme (Figure 7). An inactive form of the vitamin contains a displaceable CN group bound to the cobalt; hence the early name cyanocobalamin for one of its more familiar forms (Figure 7). The cobalt atom in the ring can have a +1, +2, or +3 oxidation state. The fifth valence (below the ring plane) has a dimethylbenzimidazole attached to the cobalt and the six can be either a methyl group, an -OH
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