[Effect of pH on the cholinesterase hydrolysis of different substrates]

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.