The monovalent cation binding site and substrate specificity of glutamine synthetase

• Main Authors: Guo, Yi Jun, 郭奕君
• Other Authors: Liao, Shu Hui
• Format: Others
• Language: zh-TW
• Published: 1995
• Online Access: http://ndltd.ncl.edu.tw/handle/62527301474304121439

碩士 === 國立臺灣大學 === 分子醫學研究所 === 83 === Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of glutamate and ammonia to yield glutamine. The binding sites of the substrates glutamate and ATP, and the two divalent cations within the funnel-shaped active site have been determined by X-ray diffraction methods, but the site for the small substrate, ammonia or ammonium ion, has remained elusive (Liaw & Eisenberg, 1994). Recently, the binding site of the monovalent cations, thallium (an ammonium analogue) and cesium, discovered also by X-ray crystallography, was proposed to be the binding site of the ammonium ion (Liaw et al., 1995). Here, we report our enzyme kinetic measurements which suggest that thallium, cesium, ammonium, and lithium are competitive inhibitors to hydroxylamine in the transferase reaction. Thus, our kinetic studies support the proposal that the monovalent cations, thallium, and cesium, bind at the substrate ammonium ion binding site.