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首页> 外文期刊>Chemistry: A European journal >Stabilisation of Methylene Radicals by Cob(II)alamin in Coenzyme B_(12) Dependent Mutases
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Stabilisation of Methylene Radicals by Cob(II)alamin in Coenzyme B_(12) Dependent Mutases

机译：辅酶B_（12）依赖性突变中Cob（II）alamin稳定亚甲基自由基

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摘要

Coenzyme B_(12) 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 B_(12) are known,no alternative coenzyme B_(12) independent mutases have been detected for substrates in which a methyl group is revers-ibly converted to a methylene radical.We predict that such mutases do not exist.However,coenzyme B_(12) 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 methyl-citrate cycle,which is ostensibly an alternative to the coenzyme B_(12) dependent methylmalonyl-CoA pathway for pro-pionate 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 de-hydratases the 5'-deoxyadenosyl radical generated by ho-molysis of the carbon-cobalt bond of coenzyme B_(12) 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.
机译：辅酶B_（12）在两种类型的酶促反应中引发自由基化学反应，即不可逆的消除酶（例如，二醇脱水酶）和可逆的突变酶（例如，甲基丙二酰-CoA突变酶）。而使用辅酶B_（12）以外的自由基产生剂的消除酶已知，对于甲基可逆转化为亚甲基的底物，未检测到替代的辅酶B_（12）独立突变酶。我们预测不存在此类突变酶。但是，辅酶B_（12）独立途径已检测到通过不同的中间体来避免对谷氨酸，β-赖氨酸或甲基丙二酰-CoA突变酶的需求。在人类中，柠檬酸甲酯循环显然是辅酶B_（12）依赖性甲基丙二酰-CoA途径的替代方法不使用丙酸酯氧化，因为它会干扰克雷布斯循环，从而损害神经系统的高能需求。在二醇中，5'-脱氧腺苷辐射会脱水辅酶B_（12）的碳-钴键的热解所产生的cal距离钴（II）阿拉明中的钴原子约10 A.底物和产物自由基的生成距离与cob（II）相似在谷氨酸和甲基丙二酰辅酶A突变酶中，5'-脱氧腺苷基保留在钴原子的3-4 A之内，底物和产物自由基约3A。 Cob（II）alamin通过稳定5'-脱氧腺苷基团和产物相关的亚甲基基团充当导体。

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