γ-Glutamyl transpeptidase acylation with peptidic substrates: free energy relationships measured by an HPLC kinetic assay
作者:Mylène Morin、Caroline Rivard、Jeffrey W. Keillor
DOI:10.1039/b606914b
日期:——
γ-Glutamyl transpeptidase (GGT, EC 2.3.2.2) is a highly glycosylated heterodimeric enzyme linked to the external cellular membrane that catalyzes the hydrolysis of glutathione as well as the transfer of its γ-glutamyl group to amino acids and dipeptides in a transpeptidation reaction. The measurement of both the hydrolytic and transpeptidation activity of this important enzyme is a challenge, since its native substrates are not highly chromogenic. We have developed an HPLC-based method for the quantitative photometric detection of numerous enzyme substrates and products, after their pre-column derivation with dabsyl chloride. The broad applicability of this method was demonstrated in the kinetic investigation of transpeptidation reactions of rat kidney GGT with glutathione, its native substrate, as well as a series of pertinent glutathione analogues. The pH-rate profile constructed for glutathione confirmed the dependence on the ionisation state of at least two residues. Analysis of the free-energy relationships in the series of synthetic peptidic substrate analogues revealed the importance of enzymeâsubstrate interactions unrelated to amine leaving group basicity during the acylation step. These results are further interpreted in the context of the recently published structure for a similar GGT.
γ-谷氨酰转肽酶(GGT,EC 2.3.2.2)是一种高度糖基化的异二聚体酶,与细胞外膜相连,能够催化谷胱甘肽的水解以及在转肽反应中将其γ-谷氨酰基团转移到氨基酸和二肽上。由于其天然底物不具备高色性,因此测定这种重要酶的水解和转肽活性是一项挑战。我们开发了一种基于HPLC的方法,通过与丹磺酰氯的柱前衍生化,对多种酶底物和产物进行定量光度检测。该方法的广泛适用性在以谷胱甘肽及其一系列相关类似物为底物的鼠肾GGT转肽反应动力学研究中得到了证明。为谷胱甘肽构建的pH-速率曲线证实了其依赖于至少两种残基的离子化状态。对一系列合成肽底物类似物中的自由能关系的分析揭示了在酰化步骤中与胺离去基团碱性无关的酶-底物相互作用的重要性。这些结果进一步在最近发表的类似GGT结构的背景下进行了解释。