II: Reaction 

   The mechanism of reaction of catalysis of acetylcholine is similar to other serine proteases.  Four major steps make up the reaction.  First a nucleophilic attack by the serine hydroxyl of the carbonyl carbon takes place.  This forms a tetrahedral intermediate.  The nucleophilic serine hydroxyl is stabilized by the available lone pair on the nitrogen of a close histidine( these residuses can be seen in the active site link).  Second, the ester oxygen leaves the tetrahedral intermediate.  This forms another ester with the serine.  In this step the same histidine is deprotonated by the choline molecule.   In the third step, the histidine deprotonates a water molecule to add to its nucleophilicity.  The water then attacks the carbonyl carbon and the tetrahedral intermediate is again formed.  Finally, the intermediate is destroyed and the acetate is formed in the last step.  The serine oxygen receives a proton from the nitrogen of the histidine and the enzyme returns to its original state(4).

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