Acetylcholinesterase: reversible inhibitors, substrate inhibition.

Observations with acetylcholinesterase can be well explained by postulating that the active site contains two subsites: one, an anionic site which binds and orients substituted ammonium ions, and the other, an esteratic site where the hydrolytic process occurs and which contains a basic group which is acetylated and deacetylated during the process (4, 5). The formation of an acetyl enzyme suggests some interesting kinetic consequences, especially if the second step is slower or at least not very much more rapid than the first. In this case, simple reversible inhibitors such as substituted ammonium ions may act not only by competition with the substrate for the free enzyme but possibly by combining with the acetyl enzyme at the anionic site. It is easy to imagine that the deacetylation would be sterically blocked, especially if the water molecule had to approach from the direction of the anionic site. This restriction on the direction of the approaching water molecule is to be anticipated for the following reason. The enzyme must catalyze the synthesis of acetylcholine as well as its hydrolysis. In the process of synthesis, a choline molecule attacks the acetyl enzyme and this attack clearly comes from the direction of the anionic site. If, then, the hydrolysis of the acetyl enzyme can be inhibited by reversible inhibitors, the kinetics should show a noncompetitive component, i.e. a plot of the reciprocal velocity against the reciprocal substrate concentration should intercept the ordinate at a position displaced upward from the intercept obtained in the absence of the inhibitor in accordance with Equation 1 (6) :