1981 年 39 巻 11 号 p. 1039-1052
Recent advances in studies on the mechanism of action of vitamin B12 coenzymes and on the biosynthesis of vitamin B12 are reviewed. Adenosyl-B12 is the first known naturally occurring organometallic compound containing a stable Co-C sigma bond. This coenzyme participates in the eleven seemingly different enzymatic rearrangement and reduction reactions, and has been well established to act as an intermediate hydrogen carrier. It is generally accepted that the early event in the reactions is the cleavage of the Co-C bond of the enzyme-bound coenzyme, which leads to generation of the catalytic center. A possible role of the apoprotein in activation of the Co-C bond of the coenzyme, which is essential for manifestation of functions of the coenzyme, is discussed on the bases of the structure-function relationship of B12 coenzymes in the diol dehydrase reaction and of the apoenzyme-coenzyme interactions. The biosynthetic route for vitamin B12 is also briefly summarized here with emphasis on the ring contraction step in formation of the corrin nucleus.