Fachbereich Biologie , Philipps-Universität , 35032 Marburg , Germany.
Biochemistry. 2019 Dec 31;58(52):5221-5233. doi: 10.1021/acs.biochem.9b00171. Epub 2019 Apr 23.
Ketyls are radical anions with nucleophilic properties. Ketyls obtained by enzymatic one-electron reduction of thioesters were proposed as intermediates for the dehydration of ()-2-hydroxyacyl-CoA to ()-2-enoyl-CoA. This concept was extended to the Birch-like reduction of benzoyl-CoA to 1,5-cyclohexadienecarboxyl-CoA. Nature uses two methods to achieve the therefore required low reduction potentials of less than -600 mV, either by an ATP-driven electron transfer similar to that catalyzed by the iron protein of nitrogenase or by electron bifurcation. Ketyls formed by thiyl radical-initiated oxidation of alcohols followed by deprotonation are involved in coenzyme B-independent diol dehydratases, other glycyl radical enzymes mediating key reactions in the degradations of choline, taurine, and 4-hydroxyproline, and all three classes of ribonucleotide reductases. A special case is the dehydration of 4-hydroxybutyryl-CoA to crotonyl-CoA, which most likely proceeds via an oxidation to an allylic ketyl but requires neither a strong reductant nor an external radical generator.
酮基是具有亲核性质的自由基阴离子。通过硫酯的酶促单电子还原获得的酮基被提出为（）-2-羟基酰基辅酶 A 脱水为（）-2-烯酰基辅酶 A 的中间体。这一概念被扩展到类似于氮酶中铁蛋白催化的 Birch 还原，用于将苯甲酰辅酶 A 还原为 1,5-环己二烯羧酸辅酶 A。自然界使用两种方法来实现所需的低还原电位低于-600 mV，一种是通过类似于氮酶中铁蛋白催化的 ATP 驱动的电子转移，另一种是电子分叉。由硫自由基引发的醇氧化形成的酮基参与辅酶 B 非依赖性二醇脱水酶、介导胆碱、牛磺酸和 4-羟脯氨酸降解中关键反应的甘氨酰基自由基酶，以及所有三类核苷酸还原酶。一个特殊的例子是 4-羟基丁酰辅酶 A 脱水为巴豆酰辅酶 A，这很可能通过氧化为烯丙基酮基进行，但既不需要强还原剂也不需要外部自由基发生器。
