INVESTIGATION OF MEDIUM EFFECTS IN A FAMILY OF DECARBOXYLASE ANTIBODIES

A family of naphthalenedisulfonate-binding antibodies catalyze the dec arboxylation of 5-nitro-3-carboxybenzisoxazole, providing a series of structurally similar catalysts for investigating the influence of prot ein microenvironment on reactivity. Eight representative catalysts and one noncatalytic hapten binder have been characterized in detail to a ssess their catalytic properties. The Michaelis-Menten parameters, inh ibition constants for hapten and product, and thermodynamic activation parameters are reported for each catalyst. The rate accelerations pro vided by the catalytic antibodies range from 1620 to 23 200. The antib odies display similar affinities for the hapten, substrate, and produc t, suggesting that binding differences are not the major cause of the variation in catalytic activity. Instead, catalytic efficiency appears to correlate roughly with-the hydrophobicity of the antibody active s ite as judged by fluorescence spectroscopy. Furthermore, five of the-c atalytic antibodies fit an isokinetic relationship, displaying a wide variation in thermodynamic activation parameters characterized by enth alpy-entropy compensation. The data illustrate the potential of simila r proteins to solve a specific chemical problem in slightly different ways and warrant detailed structural investigations to determine the p recise combination of hydrogen bonding, electrostatic, and dispersive interactions that constitute medium effects in these related proteins.