enzyme-bound preQ0 as covalent thioimide (1.63 s-1) and are thus mainly rate-limiting for the enzyme's kcat (=0.12 s-1). Kinetic studies combined with simulation reveal a large primary deuterium kinetic isotope effect of 3.3 on the covalent thioimide reduction and a smaller kinetic isotope effect of 1.8 on the imine reduction to preQ1 7-Formyl-7-deazaguanine, a carbonyl analogue of the imine intermediate, was
酶QueF催化腈基四电子还原成胺,这是
生物学上已知的唯一反应。实际上,QueF将7-
氰基-7-脱氮
鸟嘌呤(preQ0)转化为7-
氨基甲基-7-脱氮
鸟嘌呤(preQ1),用于
生物合成tRNA插入的核苷queuosine。拟议的QueF机制涉及preQ0和酶中的半胱
氨酸亲核体之间的共价
硫酰亚胺加成物,该加成物随后在两个
NADPH依赖性还原步骤中转化为preQ1。在这里,我们表明大肠杆菌QueF在强烈放热过程中结合preQ0(DeltaH = -80.3 kJ / mol; -TDeltaS = 37.9 kJ / mol,Kd = 39 nm),从而
硫代
酰亚胺加成物形成了一半的在同二聚酶中的位点反应性。preQ0还原的两个步骤都涉及从
NADPH转移4-pro-R-氢。它们的捕获过程比捕获与酶结合的preQ0为共价
硫酰亚胺(1.63 s-1)的过程慢约4-7倍,因此主要限制了酶的kcat(= 0