Diverse structural solutions to catalysis in a family of antibodies

Background: Small organic molecules coupled to a carrier protein elicit an antibody response on immunisation. The diversity of this response has been found to be very narrow in several cases. Some antibodies also catalyse che mical reactions. Such catalytic antibodies are usually identified among tho se that bind tightly to an analogue of the transition state (TSA) of the re levant reaction; therefore, catalytic antibodies are also thought to have r estricted diversity. To further characterise this diversity, we investigate d the structure and biochemistry of the catalytic antibody 7C8, one of the most efficient of those which enhance the hydrolysis of chloramphenicol est ers, and compared it to the other catalytic antibodies elicited in the same immunisation. Results: The structure of a complex of the 7C8 antibody Fab fragment with t he hapten TSA used to elicit it was determined at 2.2 Angstrom resolution. Structural comparison with another catalytic antibody (6D9) raised against the same hapten revealed that the two antibodies use different binding mode s. Furthermore, whereas 6D9 catalyses hydrolysis solely by transition-state stabilisation, data on 7C8 show that the two antibodies use mechanisms whe re the catalytic residue, substrate specificity and rate-limiting step diff er. Conclusions: Our results demonstrate that substantial diversity may be pres ent among antibodies catalysing the same reaction. Therefore, some of these antibodies represent different starting points for mutagenesis aimed at bo osting their activity. This increases the chance of obtaining more proficie nt catalysts and provides opportunities for tailoring catalysts with differ ent specificities.