Acyl exchange is one of the most important reaction types in biochemistry. This is exemplified by the mechanism of enzymatic action of the serine proteases such as chymotrypsin. In the overall mechanism of this enzyme for the hydrolysis of a peptide bond, two acyl exchanges occur. The active site promotes the reactivity of the hydroxyl group of a serine residue to then service as the nucleophile toward the peptide bond, an amide linkage in the parlance of carboxylic acid derivatives. Other active sites in chymotrypsin stabilize the tetrahedral intermediate. Resolution of the intermediate upon the departure of the leaving group forms an acyl enzyme complex which is hydrolyzed (ester hydrolysis, the second acyl exchange) to regenerate the enzyme.