Structure-function analysis of vitamin D and VDR model

In the first section, the general three-dimensional structure of the ligand-binding domain (LBD) of nuclear receptors (NR) was briefly described on the basis of their x-ray crystal structures. Emphasis was placed on the three major conformations of NR-LBD and their role in the transactivation function. In the second part, the structure-function relationship of vitamin D was analyzed based on the ligand structure, in particular by using systematic conformational analysis as a tool. On the basis of the conformational analysis of the vitamin D side chain and studies using conformationally restricted synthetic vitamin D analogs, we suggested the active space region concept of vitamin D: The vitamin D side-chain region was grouped into five regions (A, G, EA, EG and F). Activity orders, in terms of the spatial region, found by these studies are as follows: Affinity for vitamin D receptor (VDR), EA>A>F>G>EG; Affinity for vitamin D binding protein (DBP), A>>C,EA,EG; Target gene transactivation, EA>F>A>EG greater than or equal to G: Cell differentiation, EA>F>A>EG greater than or equal to G; Bone calcium mobilization, EA>G greater than or equal to A>F greater than or equal to EG Intestinal calcium absorption, EA=A greater than or equal to G>>EG. In the third section, homology modeling of VDR-LBD and docking of the natural ligand, 1,25-(OH)(2)D-3, into the ligand binding cavity of the model are described. Amino acid residues forming hydrogen bonds with the biologically important 1 alpha- and 25-OH groups were identified: 1 alpha-OH forms a pincer-type hydrogen bond with R273 and S237 and 25-OH with H397. This VDR-LBD/1,25-(OH)(2)D-3 docking model was firmly substantiated by mutation analysis. Using this VDR model, the structure-function relationship of highly potent vitamin D analogs was discussed.