The monoclonal anti-testosterone antibody (3-C 4 F 5 ) has a relatively high affinity (3 ؋ 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 antitestosterone (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 Å, 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 ( The number of different, closely related steroid hormones in human serum is high, and their relative concentrations, which usually are very low, can vary greatly even between normal healthy individuals. Testosterone, the main male sex hormone, is one of the structurally similar steroid hormones. Measurement of serum testosterone levels is important in the evaluation of conditions associated with hyperandrogenism in women (1, 2) and to diagnose hypogonadism, impotence, and problems in spermatogenesis and pubertical development in men (3, 4). Development of monoclonal antibodies that would fulfill the high affinity and selectivity requirements needed for the accurate measurement of testosterone levels in serum samples has not been successful with the use of hybridoma technology. Rabbit polyclonal antibodies are used in most current immunoassays of steroid hormones. However, the supply of polyclonal reagents with consistent quality is a severe problem for the immunodiagnostic industry and requires continuous immunization of many animals. The inherent batch-to-batch variation of the polyclonal sera makes it necessary to optimize the assay parameters for each new serum batch.The three-dimensional structures of steroid binding Fab fragments are important for a general understanding of the molecular basis of specific interactions between antibodies and hydrophobic molecules such as steroids. The molecular basis of steroid antibody interactions has been studied in detail by determining the structures of a progesteronebinding antibody (DB3) in its free and bound form and with different high affinity cross-reactive progesterone derivatives (5-7). The binding site of the DB3 antibody is a hydrophobic pocket, and the binding affinity and specificity are determined by van der Waals interactions and a few hydrogen bonds formed with the progesterone hapten (5). The high affinity binding mode of a number of progesterone derivatives is due to two alternate docking orientations for the steroid skeleton (6). In the cases of the anti-digoxin 26-10 and 40-50 antibodies, the h...

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