Sant'Anna Carlos Mauricio R, Viana Andrea dos Santos, do Nascimento Junior Nailton Monteiro
Departamento de Química, ICE, Universidade Federal Rural do Rio de Janeiro, RJ 23851-970, Seropédica, Brazil.
Bioorg Chem. 2006 Apr;34(2):77-89. doi: 10.1016/j.bioorg.2006.01.002. Epub 2006 Mar 15.
The enzymatic mechanism of acetylcholine hydrolysis was evaluated by semiempirical molecular orbital calculations with a model constructed with the coordinates of sixteen amino acids and four water molecules from the crystallographic structure of Drosophila melanogaster acetylcholinesterase (AChE, entry 1QO9 in the Protein Data Bank). Nine proposed reaction points for the hydrolysis mechanism were obtained, including four for the acylation step and five for the deacylation step. Our results indicate that in the Michaelis complex of the acylation step, a looser interaction between the substrate and the oxyanion hole may result from an amino acid change in the acyl pocket observed in insect as compared to the vertebrate enzyme. Detailed descriptions of the reaction profile for the formation of both acylation and deacylation tetrahedral intermediates were obtained. The results indicate the occurrence of partially concerted mechanisms, with deprotonation of the nucleophiles (Ser238 in the acylation step and a water molecule in the deacylation step) by His480 facilitating the nucleophilic additions. Both processes were completed by enthalpically favorable steps, formation of choline in the acylation step and of acetic acid in the deacylation step.
利用来自果蝇乙酰胆碱酯酶（AChE，蛋白质数据库中条目1QO9）晶体结构的16个氨基酸和4个水分子的坐标构建模型，通过半经验分子轨道计算评估了乙酰胆碱水解的酶促机制。获得了9个水解机制的反应位点，其中4个用于酰化步骤，5个用于脱酰化步骤。我们的结果表明，在酰化步骤的米氏复合物中，与脊椎动物酶相比，昆虫中观察到的酰基口袋中的氨基酸变化可能导致底物与氧阴离子洞之间的相互作用更松散。获得了酰化和脱酰化四面体中间体形成反应过程的详细描述。结果表明存在部分协同机制，His480使亲核试剂（酰化步骤中的Ser238和脱酰化步骤中的水分子）去质子化，促进亲核加成。两个过程均通过焓有利步骤完成，酰化步骤中形成胆碱，脱酰化步骤中形成乙酸。
