The esterases are ubiquitous. The acid may be an organic or an inorganic acid. Usually, the esterase is specific for either the acid or the alcohol moiety but not both. The esterases may be subdivided into several groups primarily on the basis of specificity for the acid involved in the ester substrate: carboxylic ester hydrolases, thiolester hydrolases, phosphoric monoester hydrolases, phosphoric diester hydrolases, and sulfuric ester hydrolases. Organophosphorus compounds inhibit acetylcholinesterase by alkylation of a single seryl hydroxyl group just as in a-chymotrypsin The general features of the mechanisms of hydrolysis are identical for acetylcholinesterase and chymotrypsin except in one feature. Both carboxylesterase and lipase hydrolyze methyl butyrate. In contrast to the nonspecific acid and alkaline phosphoric monoester hydrolases, there are a number of specific phosphoric monoesterases in which both the alcohol and the orthophosphate moiety are of importance in determining the specificity.