Buckel Wolfgang, Kratky Christoph, Golding Bernard T
Fachbereich Biologie, Philipps-Universität, 35032 Marburg, Germany.
Chemistry. 2005 Dec 23;12(2):352-62. doi: 10.1002/chem.200501074.
Coenzyme B12 initiates radical chemistry in two types of enzymatic reactions, the irreversible eliminases (e.g., diol dehydratases) and the reversible mutases (e.g., methylmalonyl-CoA mutase). Whereas eliminases that use radical generators other than coenzyme B12 are known, no alternative coenzyme B12 independent mutases have been detected for substrates in which a methyl group is reversibly converted to a methylene radical. We predict that such mutases do not exist. However, coenzyme B12 independent pathways have been detected that circumvent the need for glutamate, beta-lysine or methylmalonyl-CoA mutases by proceeding via different intermediates. In humans the methylcitrate cycle, which is ostensibly an alternative to the coenzyme B12 dependent methylmalonyl-CoA pathway for propionate oxidation, is not used because it would interfere with the Krebs cycle and thereby compromise the high-energy requirement of the nervous system. In the diol dehydratases the 5'-deoxyadenosyl radical generated by homolysis of the carbon-cobalt bond of coenzyme B12 moves about 10 A away from the cobalt atom in cob(II)alamin. The substrate and product radicals are generated at a similar distance from cob(II)alamin, which acts solely as spectator of the catalysis. In glutamate and methylmalonyl-CoA mutases the 5'-deoxyadenosyl radical remains within 3-4 A of the cobalt atom, with the substrate and product radicals approximately 3 A further away. It is suggested that cob(II)alamin acts as a conductor by stabilising both the 5'-deoxyadenosyl radical and the product-related methylene radicals.
辅酶B12在两类酶促反应中引发自由基化学反应，即不可逆消除酶（如二醇脱水酶）和可逆变位酶（如甲基丙二酰辅酶A变位酶）。虽然已知存在使用除辅酶B12之外的自由基生成剂的消除酶，但对于甲基可逆转化为亚甲基自由基的底物，尚未检测到替代的不依赖辅酶B12的变位酶。我们预测此类变位酶不存在。然而，已检测到不依赖辅酶B12的途径，该途径通过不同中间体进行，从而无需谷氨酸、β-赖氨酸或甲基丙二酰辅酶A变位酶。在人类中，柠檬酸甲酯循环表面上是丙酸氧化中依赖辅酶B12的甲基丙二酰辅酶A途径的替代途径，但未被使用，因为它会干扰三羧酸循环，从而损害神经系统的高能量需求。在二醇脱水酶中，辅酶B12碳-钴键均裂产生的5'-脱氧腺苷自由基从钴胺素（II）中的钴原子移开约10埃。底物和产物自由基在距钴胺素（II）相似的距离处产生，钴胺素（II）仅作为催化的旁观者。在谷氨酸和甲基丙二酰辅酶A变位酶中，5'-脱氧腺苷自由基保留在距钴原子3 - 4埃内，底物和产物自由基再远约3埃。有人提出，钴胺素（II）通过稳定5'-脱氧腺苷自由基和与产物相关的亚甲基自由基而起到导体的作用。
