Directed evolution governed by controlling the molecular recognition between an abzyme and its haptenic transition-state analog.

The catalytic antibody, 6D9, was subjected to directed evolution in the phage-display system using two structurally related transition-state analogs (TSAs) for panning. One analog, TSA 3, was originally used for immunization, and the other, TSA 4, a derivative of TSA 3, was designed to optimize the differential affinity for the transition state relative to the ground state so as to provide variants with improved reaction rates. We previously reported that by panning with TSA 4, we could obtain variants with highly improved catalytic rate enhancement (k(cat)/k(uncat)), and Tyr (L27e) seemed to play a key role in stabilizing the transition-state structure [Nat. Biotechnol. 19 (2001) 563]. Here, we examined in detail a large number of the variants selected by these haptens, in order to elucidate the mechanism of the directed evolution driven by them. ELISA with 3- and 4-bovine serum albumin (BSA) showed that variants selected by these TSAs exhibited distinct binding patterns. All the variants whose rate enhancement was greater than five-fold of that of 6D9 had Tyr (L27e) and were obtained from the library panned with TSA 4, but not from the library panned with TSA 3. Kinetic studies showed that TSA 4 could efficiently select variants with increased differential binding affinity for the transition state relative to the ground state, and these variants exhibited improved rate enhancements. This study verified the difference of in vitro evolution driven by the two structurally related TSAs and stresses the importance of designing an appropriate hapten for panning.