The crystal structures of the human androgen receptor (hAR) and human progesterone receptor ligandbinding domains in complex with the same ligand metribolone (R1881) have been determined. Both threedimensional structures show the typical nuclear receptor fold. The change of two residues in the ligandbinding pocket between the human progesterone receptor and hAR is most likely the source for the specificity of R1881 to the hAR. The structural implications of the 14 known mutations in the ligand-binding pocket of the hAR ligand-binding domains associated with either prostate cancer or the partial or complete androgen receptor insensitivity syndrome were analyzed. The effects of most of these mutants could be explained on the basis of the crystal structure. Androgen (AR)1 and progesterone receptors (PR) are members of the superfamily of nuclear receptors that includes the steroid receptors, among others, as well as the vitamin D, thyroid, retinoic acid receptors, and the so-called orphan receptors. In addition, AR and PR are members of a group of four closely related steroid receptors including the mineralocorticoid receptor and the glucocorticoid receptor recognizing the same hormone response element. In general, steroid receptors are comprised of five to six domains and act as ligand-activated transcription factors that control the expression of specific genes. To date, no experimentally determined three-dimensional structure is available for a complete receptor. During the past few years, x-ray structures have been published for two of the domains, the DNA-binding domain as well as for a number of ligand-binding domains (LBD) including LBD⅐ligand complexes of the estrogen receptor ␣ and ␤, the PR, the vitamin D receptor, the retinoic acid receptors (X,RXR; acid, RAR), the thyroid hormone receptor, and the peroxisome proliferatoractivated receptors (1-13). Despite the low sequence homology of as low as 20% between the LBDs of different nuclear receptor families, all these proteins share a similar fold. They are comprised of up to 12 helices and a small ␤-sheet arranged in a so-called ␣-helical sandwich, a kind of fold that up to now has only been observed for the LBDs of nuclear receptors. Depending on the nature of the bound ligand, agonist, or antagonist, the carboxyl-terminal helix H12 is found in either one of two orientations. In the agonist-bound conformation, helix H12 serves as a "lid" to close the ligand-binding pocket (LBP), whereas in the antagonist-bound conformation helix H12 is positioned in a different orientation thus opening the entrance to the LBP.Androgens and their receptors play an important role in male physiology and pathology.

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The primary and three-dimensional structures of a [NiFe] hydrogenase isolated from D. desulfitricans ATCC 27774 were determined, by nucleotide analysis and single-crystal X-ray crystallography. The three-dimensional structural model was refined to R=0.167 and Rfree=0.223 using data to 1.8 A resolution. Two unique structural features are observed: the [4Fe-4S] cluster nearest the [NiFe] centre has been modified [4Fe-3S-3O] by loss of one sulfur atom and inclusion of three oxygen atoms; a three-fold disorder was observed for Cys536 which binds to the nickel atom in the [NiFe] centre. Also, the bridging sulfur atom that caps the active site was found to have partial occupancy, thus corresponding to a partly activated enzyme. These structural features may have biological relevance. In particular, the two less-populated rotamers of Cys536 may be involved in the activation process of the enzyme, as well as in the catalytic cycle. Molecular modelling studies were carried out on the interaction between this [NiFe] hydrogenase and its physiological partner, the tetrahaem cytochrome c3 from the same organism. The lowest energy docking solutions were found to correspond to an interaction between the haem IV region in tetrahaem cytochrome c3 with the distal [4Fe-4S] cluster in [NiFe] hydrogenase. This interaction should correspond to efficient electron transfer and be physiologically relevant, given the proximity of the two redox centres and the fact that electron transfer decay coupling calculations show high coupling values and a short electron transfer pathway. On the other hand, other docking solutions have been found that, despite showing low electron transfer efficiency, may give clues on possible proton transfer mechanisms between the two molecules.

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This is the first known three-dimensional structure in which multiple copies of a tetrahaem cytochrome c3-like fold are present in the same polypeptide chain. Sequence homology was found between this cytochrome and the C-terminal region (residues 229-514) of the high molecular weight cytochrome c from Desulfovibrio vulgaris Hildenborough (DvH Hmc). A new haem arrangement in domains III and IV of DvH Hmc is proposed. Kinetic experiments showed that 9Hcc can be reduced by the [NiFe] hydrogenase from D. desulfuricans ATCC 27774, but that this reduction is faster in the presence of tetrahaem cytochrome c3. As Hmc has never been found in D. desulfuricans ATCC 27774, we propose that 9Hcc replaces it in this organism and is therefore probably involved in electron transfer across the membrane.

Crystals of the tetraheme cytochrome c3 from sulfate-reducing bacteria Desulfovibrio gigas (Dg) (MW 13 kDa, 11 1 residues, four heme groups) were obtained and X-ray diffraction data collected to 1.8 A resolution. The structure was solved by the method of molecular replacement and the resulting model refined to a conventional R-factor of 14.9%. The three-dimensional structure shows many similarities to other known crystal structures of tetraheme c3 cytochromes, but it also shows some remarkable differences. In particular, the location of the aromatic residues around the heme groups, which may play a fundamental role in the electron transfer processes of the molecule, are well conserved in the cases of hemes I, 111, and IV. However, heme I1 has an aromatic environment that is completely different to that found in other related cytochromes c 3 . Another unusual feature is the presence of a Ca2+ ion coordinated by oxygen atoms supplied by the protein within a loop near the N-terminus. It is speculated that this loop may be stabilized by the presence of this CaZ+ ion, may contribute to heme-redox perturbation, and might even be involved in the specificity of recognition with its redox partner.

The crystal structure of a mutant androgen receptor (AR) ligand-binding domain (LBD) in complex with the agonist 9alpha-fluorocortisol has been determined at 1.95 A resolution. This mutant AR contains two mutations (L701H and T877A) and was previously reported as a high-affinity cortisol/cortisone responsive AR (AR(ccr)) isolated from the androgen-independent human prostate cancer cell lines MDA PCa 2a and 2b (Zhao et al. Nature Med. 2000, 6, 703-6). The three-dimensional structure of the AR(ccr) LBD complexed with 9alpha-fluorocortisol shows the typical conformation of an agonist-bound nuclear receptor in which helix 12 is precisely positioned as a "lid" for the ligand-binding pocket. Binding of 9alpha-fluorocortisol to the AR(ccr) involves favorable hydrogen bond patterns on the C17 and C21 substituents of the ligand due to the mutations at 701 and 877 in the AR(ccr). Our studies provide the first structural explanation for the glucocorticoid activation of AR(ccr), which is important for the development of new therapeutic treatments for androgen-independent prostate cancer.

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