title: Structural insights into steroid hormone binding: the crystal structure of a recombinant anti-testosterone Fab fragment in free and testosterone-bound forms.
abstract: The monoclonal anti-testosterone antibody (3-C(4)F(5)) has a relatively high affinity (3 x 10(8) m(-1)) with an overall good specificity profile. However, the earlier characterized binding properties have shown that both the affinity and specificity of this antibody must be improved if it is intended for use in clinical immunoassays. In this paper, the crystal structures of the recombinant anti-testosterone (3-C(4)F(5)) Fab fragment have been determined in the testosterone-bound and free form at resolutions of 2.60 and 2.72 A, respectively. The high affinity binding of the (3-C(4)F(5)) Fab is mainly determined by shape complementarity between the protein and testosterone. Only one direct hydrogen bond is formed between the hydroxyl group of the testosterone D-ring and the main-chain oxygen of Gly100(J)H. The testosterone is deeply bound in a hydrophobic pocket, and the close shape complementarity is mainly formed by the third complementarity-determining regions (CDR) of the heavy and light chain. Comparison of the bound structure with the free structure indicates conformational changes in the protein upon testosterone binding. The conformational changes of the side chains of two residues Glu95H and Tyr99H in the CDR-H3 are particularly essential for the binding. Interesting similarities in the binding of different steroids were also observed upon comparison of the available structures of anti-steroid antibodies.
authors.affiliation: Department of Chemistry, University of Joensuu, P. O. Box 111, 80101 Joensuu and VTT Biotechnology, P. O. Box 1500, 02044 VTT, Finland. jarkko.valjakka@joensuu.fi
authors.forename: Jarkko
authors.initials: J
authors.lastname: Valjakka
authors.forename: Kristiina
authors.initials: K
authors.lastname: Takkinenz
authors.forename: Tuija
authors.initials: T
authors.lastname: Teerinen
authors.forename: Hans
authors.initials: H
authors.lastname: Söderlund
authors.forename: Juha
authors.initials: J
authors.lastname: Rouvinen
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D001665:Binding Sites; D022801:Complementarity Determining Regions; D018360:Crystallography, X-Ray; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013739:Testosterone / Q000276:immunology* / Q000378:metabolism
chemical_list: D000911:Antibodies, Monoclonal; D022801:Complementarity Determining Regions; D007140:Immunoglobulin Fab Fragments; D011994:Recombinant Proteins; D013739:Testosterone