Transition state stabilization and binding energy use by catalytic antibody 17E8: Studies with alternative substrates

The esterolytic antibody 17128 catalyzes the hydrolysis of n-formyl norleucine phenyl ester. The crystal structure of the antibody Fab fragment complexed to the transition state analog has been solved to 2.5 ± (Zhou et al, Science(1994)265,1059). The structure contains both an amino acid sidechain binding pocket and a pocket for the phenyl leaving group. These pockets seem to be the main determinants for binding to the transition state attalog and, presumably, the transition state. Previous kinetic results from our laboratory indicate that the noncovalent interactions between the Sl-pocket and the Pl-side chain of the substrate are essential for catalysis (Wade & Scanlan. JACS(1996)118,6510). In-this study, protein-substrate/transition state inter actions (S1-P1) were probed by a series of alternative substrates, substrate analogs, and transition state analogs to investigate the chemical requirements for substrate and transition state recognition. Several specific issues addressed by the alternative substrates include: number of VDW contacts, hydrophobic nature of the contacts, and the conformational requirements of the side chain. The results from these studies reveal the nature of the S1-P1 interactions and suggest how the binding energy from interactions involving this "less-evolved" protein pocket/active site is manifested in catalysis. This work is supported by the National Institutes of Health and the National Science Foundation.