Leaving group activation, peptide hydrolysis

Serine peptidases, serine proteases the most studied class of peptidases. They have a reactive serine residue, for example, the hydrolysis of a peptide substrate involves an acyl enzyme intermediate in which the hydroxyl group of Ser (chymotrypsin numbering system) is acylated by the acyl moiety of the substrate, thus releasing the amine fragment of the substrate as the first product. The formation of the acyl enzyme is the slow step in peptide bond hydrolysis, but acylenzyme often accumulates in the hydrolysis of ester substrates. The acyl enzyme thus formed will be the same for a series of substrates which differ in their leaving group. The active site of serine peptidases is complementary in stmc-ture to the transition state of the reaction, a structure which is very close to the tetrahedral adduct of Ser and the carbonyl... 

Water is required eventually to hydrolyse the acyl group attached to the serine residue. However, this is a much easier step than the hydrolysis of a peptide link since esters are more reactive than amides. Furthermore, the hydrolysis of the peptide link means that one half of the peptide can drift away from the active site and leave room for a water molecule to enter. 

If this should be the case, either apparent acyl transfer or amine transfer would be possible by a direct condensation of the preferentially retained product and an acceptor that can readily displace the product that leaves more easily. That such condensation reactions are catalyzed by pepsin in the case of oligopeptides was demonstrated many years ago (54), and is consistent with the neglible free energy decrease in the hydrolysis of interior peptide bonds (55). For transpeptidation reactions in which an apparent E-Tyr amino-enzyme has been postulated, the free energy change in the condensation of an acceptor such as Ac-Phe with tyrosine would be more unfavorable in free solution, but the possibility must be considered that the ammonium pKa of the tyrosine retained at the active site may be lower than that of tyrosine in free solution, perhaps by virtue of the interaction of the carboxylate group of the amino acid with a complementary cationic group of the active site. 

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