Analysis of an anti-progesterone antibody: variable crystal morphology of the Fab' and steroid-Fab' complexes.

Anti-progesterone monoclonal antibodies are being used for structural studies of antibody-antigen interaction, for their ability to block pregnancy shortly after fertilization, and for hormone immunoassay. A mouse anti-progesterone monoclonal Fab' fragment has been crystallized in its native form and co-crystallized with seven different, but structurally related, steroids. The crystals show interesting preferences in their crystal morphology, depending on the bound steroid ligand. The X-ray crystallographic analysis of this Fab', complexed with a series of related steroid ligands, should reveal details of the chemistry of antibody-antigen union and provide insights into how steroids interact with proteins.