Insight into odorant perception: The crystal structure and binding characteristics of antibody fragments directed against the musk odorant traseolide

Monoclonal antibodies were elicited against the small hydrophobic hapten tr aseolide, a commercially available musk fragrance. Antibody variable region sequences were found to belong to different sequence groups, and the bindi ng characteristics of the corresponding antibody fragments were investigate d. The antibodies M02/01/01 and M02/05/01 are highly homologous and differ in the binding pocket only at position H93. M02/ 05/01 (H93 Val) binds the hapten traseolide about 75-fold better than M02/01/01 (H93 Ala). A traseoli de analog, missing only one methyl group, does not have the characteristic musk odorant fragrance. The antibody M02/05/01 binds this hapten analog abo ut tenfold less tightly than the original traseolide hapten, and mimics the odorant receptor in this respect, while the antibody M02/01/01 does not di stinguish between the analog and traseolide. To elucidate the structural ba sis for the fine specificity of binding, we determined the crystal structur e of the Fab fragment of M02/05/01 complexed with the hapten at 2.6 Angstro m resolution. The crystal structure showed that only van der Waals interact ions are involved in binding. The somatic Ala H93 Val mutation in M02/05/01 fills up an empty cavity in the binding pocket. This leads to an increase in binding energy and to the ability to discriminate between the hapten tra seolide and its derivatives. The structural understanding of odorant specif icity in an antibody gives insight in the physical principles on how specif icity for such hydrophobic molecules may be achieved. (C) 1999 Academic Pre ss.