Department of Biochemistry, Albert Einstein College of Medicine of Yeshiva University, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
Biochemistry. 2012 Aug 28;51(34):6715-7. doi: 10.1021/bi300914q. Epub 2012 Aug 13.
The transition state for the Trypanosoma cruzi uridine phosphorylase (TcUP) reaction has an expanded S(N)2 character. We used binding isotope effects (BIE's) to probe uridine distortion in the complex with TcUP and sulfate to mimic the Michaelis complex. Inverse 1'-(3)H and 5'-(3)H BIE's indicate a constrained bonding environment of these groups in the complex. Quantum chemical modeling identified a uridine conformer whose calculated BIE's match the experimental values. This conformer differs in sugar pucker and uracil orientation from the unbound conformer and the transition-state structure. These results support ground-state stabilization in the Michaelis complex.
克氏锥虫尿苷磷酸化酶（TcUP）反应的过渡态具有扩展的 S（N）2 特征。我们使用结合同位素效应（BIE）来探测 TcUP 和硫酸盐复合物中尿苷的扭曲，以模拟米氏复合物。反位 1'-(3)H 和 5'-(3)H BIE 表明这些基团在复合物中的键合环境受到限制。量子化学建模确定了一种尿苷构象，其计算的 BIE 与实验值相匹配。该构象在糖环构象和嘧啶环取向方面与未结合构象和过渡态结构不同。这些结果支持米氏复合物中基态的稳定化。
