γ-Glutamylcysteine synthetase (γGCS), a rate-limiting enzyme in glutathione biosynthesis, plays a central role in glutathione homeostasis and is a target for development of potential therapeutic agents against parasites and cancer. We have determined the crystal structures of
Escherichia coli
γGCS unliganded and complexed with a sulfoximine-based transition-state analog inhibitor at resolutions of 2.5 and 2.1 Å, respectively. In the crystal structure of the complex, the bound inhibitor is phosphorylated at the sulfoximido nitrogen and is coordinated to three Mg
2+
ions. The cysteine-binding site was identified; it is formed inductively at the transition state. In the unliganded structure, an open space exists around the representative cysteine-binding site and is probably responsible for the competitive binding of glutathione. Upon inhibitor binding, the side chains of Tyr-241 and Tyr-300 turn, forming a hydrogen-bonding triad with the carboxyl group of the inhibitor's cysteine moiety, allowing this moiety to fit tightly into the cysteine-binding site with concomitant accommodation of its side chain into a shallow pocket. This movement is caused by a conformational change of a switch loop (residues 240–249). Based on this crystal structure, the cysteine-binding sites of mammalian and parasitic γGCSs were predicted by multiple sequence alignment, although no significant sequence identity exists between the
E. coli
γGCS and its eukaryotic homologues. The identification of this cysteine-binding site provides important information for the rational design of novel γGCS inhibitors.
γ-谷氨酰半胱氨酸合成酶(γGCS)是谷胱甘肽合成中的限速酶,对谷胱甘肽稳态起着中心作用,是开发潜在治疗寄生虫和癌症的药物靶点。我们已经确定了大肠杆菌γGCS未配体和与基于磺酰亚胺的过渡态类似物抑制剂配合的晶体结构,分别在2.5和2.1 Å的分辨率下。在复合物的晶体结构中,结合的抑制剂在磺酰亚胺氮上磷酸化,并与三个Mg2+离子配合。半胱氨酸结合位点被确定;它在过渡态时感应形成。在未配体的结构中,代表性半胱氨酸结合位点周围存在一个开放空间,可能负责竞争性结合谷胱甘肽。在抑制剂结合时,酪氨酸-241和酪氨酸-300的侧链转动,与抑制剂的半胱氨酸基团的羧基形成氢键三连,使得该基团紧密地适配到半胱氨酸结合位点中,其侧链同时适应到一个浅口袋中。这种运动是由一个开关环(残基240-249)的构象变化引起的。基于这个晶体结构,哺乳动物和寄生虫γGCS的半胱氨酸结合位点通过多序列比对进行预测,尽管大肠杆菌γGCS和其真核同源物之间没有显著的序列同源性。这个半胱氨酸结合位点的确定为新型γGCS抑制剂的合理设计提供了重要信息。