Sando G N, Blakley R L, Hogenkamp H P, Hoffmann P J
J Biol Chem. 1975 Nov 25;250(22):8774-9.
A series of 17 analogs of 5'-deoxy-5'-adenosylcobalamin(adenosylcobalamin) have been synthesized with modifications in the base or ribose moiety of the nucleoside ligand. These analogs have been examined for their effects on reactions catalyzed by the ribonucleotide reductase of Lactobacillus leichmannii. All the analogs are inhibitors of ATP reduction in the presence of adenosylcobalamin as coenzyme, and hence all are bound to the catalytic site. Only the 3-beta-D-ribofuranosyladenine analog (isoadenosylcobalamin) showed substantial activity as a coenzyme in ATP reduction, giving a rate of 59% of that obtained with the adenosylcobalamin. Lesser rates of reduction were obtained with nebularyl-, 2'-deoxyadenosyl-, tubercidyl-, isopropylideneadenosyl-, L-adenosyl-, and ara-adenosylcobalamin, coenzyme activity decreasing in that order. Other analogs showed no significant coenzyme activity. The rate of hydrogen exchange into water from the 5'-methylene group of the nucleoside ligand appeared to parallel the coenzyme activity in those analogs examined, but only the four cobalamins with highest coenzyme activity (adenosyl, isoadenosyl, nebularyl, 2'-deoxyadenosyl) gave detectable amounts of "active coenzyme B12," THe rapidly formed paramagnetic intermediate of ribonucleotide reduction. The enzyme system produced the slowly formed paramagnetic species characterized by a doublet EPR spectrum only with adenosyl- and isoadenosylcobalamin. By contrast the enzymic degradation of analogs to cob(II)alamin and 5'-deoxynucleoside occurred not only with those analogs active as coenzymes and in the exchange reaction but also with a number of coenzymically inactive analogs, and the rate of degradation was unrelated to the rate of ribonucleotide reduction for those analogs with coenzyme activity.
已合成了一系列17种5'-脱氧-5'-腺苷钴胺素（腺苷钴胺素）类似物，其核苷配体的碱基或核糖部分有所修饰。已研究了这些类似物对莱氏乳杆菌核糖核苷酸还原酶催化反应的影响。在腺苷钴胺素作为辅酶存在的情况下，所有类似物都是ATP还原的抑制剂，因此它们都与催化位点结合。只有3-β-D-呋喃核糖基腺嘌呤类似物（异腺苷钴胺素）在ATP还原中表现出作为辅酶的显著活性，其反应速率为腺苷钴胺素的59%。nebularyl-、2'-脱氧腺苷基-、结核菌素基-、异亚丙基腺苷基-、L-腺苷基-和阿拉伯糖腺苷钴胺素的还原速率较低，辅酶活性按此顺序降低。其他类似物未显示出显著的辅酶活性。在所研究的那些类似物中，核苷配体5'-亚甲基向水中的氢交换速率似乎与辅酶活性平行，但只有四种辅酶活性最高的钴胺素（腺苷基、异腺苷基、nebularyl、2'-脱氧腺苷基）产生了可检测量的“活性辅酶B12”，即核糖核苷酸还原的快速形成的顺磁性中间体。该酶系统仅与腺苷钴胺素和异腺苷钴胺素产生以双峰EPR光谱为特征的缓慢形成的顺磁性物质。相比之下，类似物向钴胺（II）素和5'-脱氧核苷的酶促降解不仅发生在那些作为辅酶有活性且参与交换反应的类似物中，也发生在许多无辅酶活性的类似物中，对于那些有辅酶活性的类似物，降解速率与核糖核苷酸还原速率无关。
