Enzymatic Characterization and Elucidation of the Catalytic Mechanism of a Recombinant Bovine Glycine<i>N</i>-Acyltransferase
作者:Christoffel P. S. Badenhorst、Maritza Jooste、Alberdina A. van Dijk
DOI:10.1124/dmd.111.041657
日期:2012.2
Glycine conjugation, a phase II detoxification process, is catalyzed by glycine N -acyltransferase (GLYAT; E.C. 2.3.1.13). GLYAT detoxifies various xenobiotics, such as benzoic acid, and endogenous organic acids, such as isovaleric acid, which makes GLYAT important in the management of organic acidemias in humans. We cloned the open reading frame encoding the bovine ortholog of GLYAT from bovine liver mRNA into the bacterial expression vector pColdIII. The recombinant enzyme was expressed, partially purified, and enzymatically characterized. Protein modeling was used to predict Glu226 of bovine GLYAT to be catalytically important. This was assessed by constructing an E226Q mutant and comparing its enzyme kinetics to that of the wild-type recombinant bovine GLYAT. The Michaelis constants for benzoyl-CoA and glycine were determined and were similar for wild-type recombinant GLYAT, E226Q recombinant GLYAT, and GLYAT present in bovine liver. At pH 8.0, the E226Q mutant GLYAT had decreased activity, which could be compensated for by increasing the reaction pH. This suggested a catalytic mechanism in which Glu226 functions to deprotonate glycine, facilitating nucleophilic attack on the acyl-CoA. The recombinant bovine GLYAT enzyme, combined with this new understanding of its active site and reaction mechanism, could be a powerful tool to investigate the functional significance of GLYAT sequence variations. Eventually, this should facilitate investigations into the impact of known and novel sequence variations in the human GLYAT gene.
甘氨酸缀合是 II 期解毒过程,由甘氨酸 N-酰基转移酶 (GLYAT;E.C. 2.3.1.13) 催化。 GLYAT 可以解毒各种外源性物质(例如苯甲酸)和内源性有机酸(例如异戊酸),这使得 GLYAT 在人类有机酸血症的治疗中发挥着重要作用。我们将牛肝脏 mRNA 中编码 GLYAT 的牛直系同源物的开放阅读框克隆到细菌表达载体 pColdIII 中。表达、部分纯化重组酶并进行酶学表征。使用蛋白质模型来预测牛 GLYAT 的 Glu226 具有重要的催化作用。这是通过构建 E226Q 突变体并将其酶动力学与野生型重组牛 GLYAT 的酶动力学进行比较来评估的。测定了苯甲酰辅酶A和甘氨酸的米氏常数,并且与野生型重组GLYAT、E226Q重组GLYAT和牛肝脏中存在的GLYAT相似。在 pH 8.0 时,E226Q 突变体 GLYAT 的活性降低,这可以通过提高反应 pH 值来补偿。这表明 Glu226 具有使甘氨酸去质子化的催化机制,促进对酰基辅酶 A 的亲核攻击。重组牛 GLYAT 酶,结合对其活性位点和反应机制的新认识,可能成为研究 GLYAT 序列变异功能意义的有力工具。最终,这将有助于研究人类 GLYAT 基因中已知和新的序列变异的影响。