Antibody Catalysis Approaching the Activity of Enzymes

Article abstract of DOI:10.1021/ja00215a045

Antibodies were raised to phosphonate esters that model a carboxyl esterolytic transition state.Twenty monoclonal antibodies were screened by a direct assay for hydrolysis of the carboxylic esters deduced from the structure of the transition-state analogue.Five of these were found to be esterases.The transition-state analogue is a specific inhibitor of the activity.One antibody accelerates the hydrolysis of a related substrate with k_cat of 20 s^-1 and K_m of 1.5 mM at pH 8.0.This represents an acceleration of several million times above the spontaneous rate of hydrolysis.The kinetic constants for two substrates and the inhibition constant suggest the better binding of transition states than ground states to the antibody.The pH dependence of the activity can be explained by the titration of an amino acid side chain with a pK_a of about 8.9.Activity is abolished by protein nitration that is specific for tyrosyl groups, and the nitration of two or more groups is prevented by the presence of the phosphonate ligand.Thus, the residues that constitute the combining site of the antibody may act analogously to residues in the active site of enzymes.These results demonstrate that an esterolytic antibody can be relatively efficient as compared with similar enzymes.