乙酰胆碱酯酶的晶体结构及功能位点

Crystal Structure and Functional Sites of Acetylcholinesterase

乙酰胆碱酯酶晶体结构研究表明 ,ACh E存在一个深而窄的谷 ,谷底部有一个活性位点 ,它是由催化三联体和所谓的阴离子亚位点组成。阴离子亚位点识别底物的四价铵基团 ;由 Glu32 7、His4 4 0和 Ser2 0 0构成的催化三联体负责底物乙酰胆碱的水解。在谷的入口处 ,有一个调节位点被称为外周阴离子位点 ,残基 Trp84和 Trp2 79分别在催化和外周的阴离子位点起关键作用。朝着谷底部方向存在一个强的静电偶极子 ,该偶极子对阳离子产物胆碱的清除是非常不利的 ,并与酶的高催化效率形成明显对照。位于催化口袋底部的后门开放为产物释放提供了另一种途径 ,但对后门学说的争论仍然十分激烈。X-衍射晶体学与化学修饰研究有助于阐明配基结合活性中心谷特异性的分子基础。位点特异性突变分析在确定氨基酸残基参与乙酰胆碱酯酶空间结构或催化功能中起着非常重要的作用。

The crystal structure of acetylcholinesterase revealed that the active site, constituted by a catalytic triad and a so called anionic subsite, is located at the bottom of a deep and narrow gorge. The anionic subsite recognizes the quaternary ammonium group of the substrate, while the catalytic triad, formed by Glu327, His440, and Ser200, is responsible for hydrolysis of the substrate. At the entrance of gorge, there is allosteric site called the peripheral anionic site. And residues Trp84 and Trp279 play key roles in catalytic and peripheral anionic sites, respectively. The presence of a strong electrostatic dipole directed toward the bottom of the gorge has been elucidated. Such a dipole should be unfavorable to the clearance of the cationic product choline, with an apparent contrast to the high catalytic rate of the enzyme. The opening of a back door at the bottom of the catalytic pocket is a likely alternative for release of products, but this hypothesis is still an object under great controversy. X ray crystallograpgy and chemical modification studies on the dimensions of the active center gorge will promote the elucidation for molecular basis of the specificity of binding ligands. And the site directed mutagenesis could play an instrumental role in identifying amino acid residues essential for steric structure and enzymic function.