Halpern J
Science. 1985 Feb 22;227(4689):869-75. doi: 10.1126/science.2857503.
Coenzyme B12 serves as a cofactor in various enzymatic reactions in which a hydrogen atom is interchanged with a substituent on an adjacent carbon atom. Measurement of the dissociation energy of the coenzyme's cobalt-carbon bond and studies of the rearrangement of model free radicals related to those derived from methylmalonyl-coenzyme A suggest that these enzymatic reactions occur through homolytic dissociation of the coenzyme's cobalt-carbon bond, abstraction of a hydrogen atom from the substrate by the coenzyme-derived 5'-deoxyadenosyl radical, and rearrangement of the resulting substrate radical. The only role thus far identified for coenzyme B12 in these reactions--namely, that of a free radical precursor--reflects the weakness, and facile dissociation, of the cobalt-carbon bond.
辅酶B12在各种酶促反应中作为辅助因子，在这些反应中，一个氢原子与相邻碳原子上的取代基发生交换。对辅酶钴-碳键离解能的测量以及对与甲基丙二酰辅酶A衍生的自由基相关的模型自由基重排的研究表明，这些酶促反应是通过辅酶钴-碳键的均裂离解、辅酶衍生的5'-脱氧腺苷自由基从底物中提取氢原子以及所得底物自由基的重排而发生的。到目前为止，在这些反应中确定的辅酶B12的唯一作用——即作为自由基前体的作用——反映了钴-碳键的弱点和容易离解的特性。
