AN ANTIBODY-CATALYZED ISOMERIZATION REACTION

Monoclonal antibodies (mAbs) were generated against the coplanar trans ition state (TS double dagger) analogue 1 and assayed for their abilit y to catalyze the isomerization of bridged biphenyls 4, 6, and 7. This is a relatively simple unimolecular reaction whose activation barrier arises from unfavorable steric interactions between the two benzylic methylene groups and strain in the bridging ring system. Seven mAbs we re found that catalyzed the isomerization of 4 to 6; the most efficien t (mAb 64D8E10) has k(cat) and K-M values of 4.3 x 10(-5) s(-1) and 42 0 mu M, respectively. This corresponds to a rate enhancement over the unimolecular uncatalyzed reaction (k(cat)/k(uncat)) of 2900. The disso ciation constant for the TS double dagger analogue, K-d, was determine d to be 210 nM. For both the antibody (64D8E10) catalyzed and uncataly zed reactions, the free energy of activation (Delta G double dagger) i s comprised largely of the enthalpy term; the antibody decreases the e nthalpy of activation by 5 kcal/mol. Despite relatively large differen ces in the values of k(cat)/k(uncat) for the five antibodies, the rati os of K-d to K-M(4) are very similar. It is likely that the antibodies catalyze this reaction by reducing both ring strain and nonbonded ste ric interactions in the transition state.