Catalytic Antibodies with Acyl-Transfer Capabilities: Mechanistic and Kinetic Investigations

Antibodies have been shown to catalyze acyl-transfer reactions. The processes by which they perform such tasks have often been postulated but largely remain unknown. An extended study is presented on three different monoclonal antibodies that catalyze the hydrolysis of an alkyl ester and an aryl amide bond. Antibodies 2H6 and 21H3 catalyze the hydrolysis of an unactivated benzyl ester and show exquisite specificity for substrates with either the R or S configuration, respectively, while 43C9 catalyzes the cleavage of a p-nitroanilide amide bond. New substrates were synthesized and buffer-assisted reactions were employed to determine antibody-substrate fidelity. Oxygen-18 incorporation experiments were performed providing evidence that these antibody-mediated reactions proceed through attack at the acyl carbonyl, and excluding the possibility of an S_N2 displacement mechanism for the ester hydrolysis reaction. A pH-rate profile study in protium and deuterium oxide was performed on antibody 43C9. This revealed an apparent pK_a of ~9 involved in catalysis, but both the presence and absence of a solvent isotope effect in the pH-dependent and -independent regions suggested a multistep reaction pathway may be operative.