The most accepted theory of enzyme binding with the substrate is the induced fit theory. According to this theory, the enzyme molds itself in such a manner that the substrate binds to it easily.
It is not only the enzyme’s active site that molds to accommodate the substrate, but the substrate also alters itself to fit in the active site. There are a number of interactions that take place between the enzyme and the substrate.
Ionic, hydrogen, and van-der-waal interactions are some of the binding interactions taking place. Ionic interactions are rare as the active site mostly has a hydrophobic environment. In the case of acetylcholine, it has a positive charge, so a nucleophilic attack helps bind it to the active site.
The active site of the enzyme has an amino acid that can carry out this binding. The nucleophilic attack is catalyzed by either a serine or cysteine group, as they can develop a negative charge upon themselves.
It can also be carried out by an acidic amino acid, as it can develop a negative charge. This happens at physiological pH, and requires an aqueous medium, which is seldom found near the active site.