The ligand binding domain of the human vitamin D receptor (VDR) was modeled based on the crystal structure of the retinoic acid receptor. The ligand binding pocket of our VDR model is spacious at the helix 11 site and confined at the ␤-turn site. The ligand 1␣,25-dihydroxyvitamin D3 was assumed to be anchored in the ligand binding pocket with its side chain heading to helix 11 (site 2) and the A-ring toward the ␤-turn (site 1). Three residues forming hydrogen bonds with the functionally important 1␣-and 25-hydroxyl groups of 1␣,25-dihydroxyvitamin D3 were identified and confirmed by mutational analysis: the 1␣-hydroxyl group is forming pincer-type hydrogen bonds with S237 and R274 and the 25-hydroxyl group is interacting with H397. Docking potential for various ligands to the VDR model was examined, and the results are in good agreement with our previous three-dimensional structure-function theory.T he steroid hormone 1␣,25-dihydroxyvitamin D 3 [1,25-(OH) 2 D 3 ] is unique not only in its structure but also in its function. Its structure is long in length and flexible unlike any other steroid hormone. In addition to its classical role of regulating calcium metabolism, it is involved in such basic functions as regulation of proliferation and differentiation of cells and the immune response (1). 1,25-(OH) 2 D 3 exerts these effects through a ligand-activated transcription factor, vitamin D receptor (VDR) (2). VDR is a member of the nuclear receptor (NR) superfamily (3), which includes the receptors for the steroid and thyroid hormones and retinoic acids and numerous orphan receptors for which currently no natural ligands are known. All NRs exhibit a common modular structure consisting of six distinct domains with an evolutionary highly conserved DNA binding domain and a moderately conserved ligand binding domain (LBD), which functions as a multifunctional domain. Besides the ligand recognition, it is involved in dimerization and ligand-dependent transactivation. So far, crystallographic structures of six NR-LBDs [retinoid X receptor (4), retinoic acid receptor (RAR) (5, 6), thyroid hormone receptor (7,8), estrogen receptor (ER) (9-11), progesterone receptor (PR) (12), and peroxisome proliferator-activated receptor (PPAR) (13, 14)] have been solved, including the structure of holo and apo forms, complexes with the natural ligand, synthetic agonists and antagonists, and ternary complexes with the ligand and a coactivator. These structures reveal not only the common fold of NR-LBDs but also the structural role of the ligands in inducing conformational changes in LBD, which makes the recruitment of coactivator possible to initiate the action of the general transcriptional machinery. Crystal structures also afforded structural basis of the mechanism of the action of antagonists.From structure-function relationship analysis of more than 500 vitamin D analogs, it was shown that variable side-chain structures are accommodated in VDR, whereas only limited structural modifications are tolerated for the A-ring for binding to...

Novel 19-nor analogues of 1alpha,25-dihydroxyvitamin D(3) were prepared and substituted at C-2 with an ethylidene group. The synthetic pathway was via Wittig-Horner coupling of the corresponding A-ring phosphine oxides with the protected 25-hydroxy Grundmann's ketones. Selective catalytic hydrogenation of 2-ethylidene analogues provided the 2alpha- and 2beta-ethyl compounds. The 2-ethylidene-19-nor compounds with a methyl group from the ethylidene moiety in a trans relationship to the C(6)-C(7) bond (E-isomers) were more potent than the corresponding Z-isomers and the natural hormone in binding to the vitamin D receptor. Both geometrical isomers (E and Z) of (20S)-2-ethylidene-19-norvitamin D(3) and both 2alpha-ethyl-19-norvitamins (in the 20R- and 20S-series) have much higher HL-60 differentiation activity than does 1alpha,25-(OH)(2)D(3). Both E-isomers (20R and 20S) of 2-ethylidene vitamins are characterized by very high calcemic activity in rats. The three-dimensional structure model of the rat vitamin D receptor and the computational docking of four synthesized (20R)-19-norvitamin D(3) analogues into its binding pocket are also reported.

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