Ligand recognition by the vitamin D receptor Original Research Article

Three-dimensional structure of the ligand binding domain (LBD) of the vitamin D receptor (VDR) docked with the natural ligand 1α,25-dihydroxyvitamin D3 [1,25-(OH)2D3] has been mostly solved by the X-ray crystallographic analysis of the deletion mutant (VDR-LBDΔ165-215). The important focus, from now on, is how the VDR recognizes and interacts with potent synthetic ligands. We now report the docking models of the VDR with three functionally and structurally interesting ligands, 22-oxa-1,25-(OH)2D3 (OCT), 20-epi-1,25-(OH)2D3 and 20-epi-22-oxa-24,26,27-trihomo-1,25-(OH)2D3. In parallel with the computational docking studies, we prepared twelve one-point mutants of amino acid residues lining the ligand binding pocket of the VDR and examined their transactivation potency induced by 1,25-(OH)2D3 and these synthetic ligands. The results indicate that L233, R274, W286, H397 and Y401 are essential for holding the all ligands tested, S278 and Q400 are not important at all, and the importance of S237, V234, S275, C288 and H305 is variable depending on the side-chain structure of the ligands. Based on these studies, we suggested key structural factors to bestow the selective action on OCT and the augmented activities on 20-epi-ligands. Furthermore, the docking models coincided well with our proposed active space-region theory of vitamin D based on the conformational analyses of ligands.