Three-dimensional structure-function relationship of vitamin D and vitamin D receptor model

Abstract

On the basis of conformational analysis of the vitamin D side chain and studies using conformationally restricted synthetic vitamin D analogs, we have suggested the active space region concept of vitamin D: The vitamin D side-chain region was grouped into four regions (A, G, EA and EG) and the A and EA regions were suggested to be important for vitamin D actions. We extended our theory to known highly potent vitamin D analogs and found a new region F. The analogs which occupy the F region have such modifications as 22-oxa, 22-ene, 16-ene and 18-nor. Altogether, the following relationship between the space region and activity was found: Affinity for vitamin D receptor (VDR), EA > A> F > G > EG; Affinity for vitamin D binding protein (DBP), A ≫ G,EA,EG; Target gene transactivation, EA > F > A > EG ≧ G; Cell differentiation, EA > F > A > EG ≧ G; Bone calcium mobilization, EA > GA > F ≧ EG; Intestinal calcium absorption, EA = A ≧ G ≫ EG. We modeled the 3D structure of VDR-LBD (ligand binding domain) using hRARγ as a template, to develop our structure-function theory into a theory involving VDR. 1α,25(OH) 2D 3 was docked into the ligand binding pocket of the VDR with the side chain heading the wide cavity at the H-11 site, the A-ring toward the narrow β-turn site, and the β-face of the CD ring facing H3. Amino acid residues forming hydrogen bonds with the 1α- and 25-OH groups were specified: S237 and R274 forming a pincer type hydrogen-bond for the 1α-OH and H397 for the 25-OH. Mutants of several amino acid residues that are hydrogen-bond candidates were prepared and their biologic properties were evaluated. All of our mutation results together with known mutation data support our VDR model docked with the natural ligand.