Structural aspects of Vitamin D endocrinology.

1α,25-Dihydroxvitamin D3 (1,25(OH)2 D3 ) is the hormonally active form of vitamin D3 . Its synthesis and its metabolites, their transport and elimination as well as action on transcriptional regulation involves the harmonic cooperation of diverse proteins with vitamin D binding capacities such as vitamin D binding protein (DBP), cytochrome P450 enzymes or the nuclear vitamin receptor (VDR). The genomic mechanism of 1,25(OH)2 D3 action involves its binding to VDR that functionally acts as a heterodimer with retinoid X receptor. The crystal structures of the most important proteins for vitamin D3 , VDR, DBP, CYP2R1 and CYP24A1, have provided identification of mechanisms of actions of these proteins and those mediating VDR-regulated transcription. This review will present the structural information on recognition of the vitamin D3 and metabolites by CYP proteins and DBP as well as the structural basis of VDR activation by 1,25(OH)2 D3 and metabolites. Additionally, we will describe, the implications of the VDR mutants associated with hereditary vitamin D-resistant rickets (HVDRR) that display impaired function.