Langel Iu L, Speek M A, Iarv Ia L
Biokhimiia. 1980 Dec;45(12):2130-8.
The effect of pH on the kinetic constants of cholinesterase-catalyzed hydrolysis of isoamylacetate and acetylthiocholine was investigated. For the first substrate the rate-limiting step is the enzyme acetylation reaction, while the overall hydrolysis rate of the second substrate is limited by the deacetylation reaction. It was concluded that a basic group is involved in the deacetylation reaction and in the non-covalent binding step of both ionic and non-ionic substrates. An acidic group participates in the both reaction steps. It was suggested that protonation of the basic group leads to a conformational transition of the free enzyme and can also participate in the catalytic mechanism. The second basic group characterized by Ka3 can be the functional group of the enzyme anionic site. Taken together, the influence of pH on cholinesterase catalysis can be generalized by the overall reaction scheme.
研究了pH对胆碱酯酶催化水解异戊酸乙酯和乙酰硫代胆碱动力学常数的影响。对于第一种底物，限速步骤是酶的乙酰化反应，而第二种底物的总水解速率受脱乙酰化反应限制。得出的结论是，一个碱性基团参与脱乙酰化反应以及离子型和非离子型底物的非共价结合步骤。一个酸性基团参与这两个反应步骤。有人提出，碱性基团的质子化导致游离酶的构象转变，并且也可能参与催化机制。以Ka3表征的第二个碱性基团可能是酶阴离子位点的官能团。总之，pH对胆碱酯酶催化作用的影响可以通过总体反应方案来概括。
