20-Oxopregnacalciferols: Vitamin D compounds that bind the progesterone receptor

Model of three-dimensional structure of vitamin D receptor and its binding mechanism with 1alpha,25-dihydroxyvitamin D(3)

Comparative modeling of the vitamin D receptor three-dimensional structure and computational docking of 1alpha,25-dihydroxyvitamin D(3) into the putative binding pocket of the two deletion mutant receptors: (207-423) and (120-422, Delta [164-207]) are reported and evaluated in the context of extensive mutagenic analysis and crystal structure of holo hVDR deletion protein published recently. The obtained molecular model agrees well with the experimentally determined structure. Six different conformers of 1alpha,25-dihydroxyvitamin D(3) were used to study flexible docking to the receptor. On the basis of values of conformational energy of various complexes and their consistency with functional activity, it appears that 1alpha,25-dihydroxyvitamin D(3) binds the receptor in its 6-s-trans form. The two lowest energy complexes obtained from docking the hormone into the deletion protein (207-423) differ in conformation of ring A and orientation of the ligand molecule in the VDR pocket. 1alpha,25-Dihydroxyvitamin D(3) possessing the A-ring conformation with axially oriented 1alpha-hydroxy group binds receptor with its 25-hydroxy substituent oriented toward the center of the receptor cavity, whereas ligand possessing equatorial conformation of 1alpha-hydroxy enters the pocket with A ring directed inward. The latter conformation and orientation of the ligand is consistent with the crystal structure of hVDR deletion mutant (118-425, Delta [165-215]). The lattice model of rVDR (120-422, Delta [164-207]) shows excellent agreement with the crystal structure of the hVDR mutant. The complex obtained from docking the hormone into the receptor has lower energy than complexes for which homology modeling was used. Thus, a simple model of vitamin D receptor with the first two helices deleted can be potentially useful for designing a general structure of ligand, whereas the advanced lattice model is suitable for examining binding sites in the pocket.

Chemistry and conformation of vitamin D molecules

1 alpha,25-Dihydroxyvitamin D3 (1,25) is a structurally unique steroid hormone because it not only possesses the complete 25-hydroxycholesterol side chain, but most notably, it possesses a seco-B triene structure (it lacks a B-ring and is usually depicted in a non-steroidal, extended conformation). In contrast, the classical steroid hormones possess a truncated side chain (progesterone, cortisol, and aldosterone) or no side chain (estradiol and testosterone) and they all possess the fully intact ABCD steroid rings. These structural differences render the seco-B-steroid 1,25 considerably more conformationally flexible. Since 1,25 is now known to target a myriad of tissues where specific interactions occur to produce an array of biological responses, it is of interest to determine whether different topologies of 1,25 (resulting from different conformational orientations of 1,25) are necessary to interact effectively at the different target sites. The array of biological responses include both non-genomic and genomic effects and there is considerable promise for the efficacy of 1,25 analogs as chemotherapeutic agents in a variety of human disease states. For the non-genomic calcium transport response of transcaltachia, the finding that two 6-s-cis locked analogs, 1 alpha,25-dihydroxyprevitamin D3 (pre-1,25) and 1 alpha,25-dihydroxylumisterol3 (1,25-Lumi), are equipotent to 1,25, points strongly to the involvement of the 6-s-cis conformer of 1,25 as the biologically active conformer. Since there is a continuum of easily interconvertible 6,7-single bond conformers of the seco-B ring available to 1,25, conformational minima (either local or global) may have little to do with the manner in which 1,25 is bound to receptor. For the genomic calcium transport response, and for other genomic (or non-genomic) effects, there is no clear evidence whether the steroidal (s-cis) or non-steroidal (s-trans) conformer of 1,25 is involved. In order to address this matter further, efforts are underway to evaluate other conformationally locked analogs of 1,25 which might mimic either the planar 6-s-trans-1,25 or some intermediate conformer between it and the planar-6-s-cis form.