Why is a pH of 6.0 Required for Crystallization?
Mathew T. Mizwicki, Donald Keidel, Anthony W. Norman. Department of Biochemistry, University of California, Riverside CA 92521. All x-ray crystal structures of the nuclear vitamin D receptor (VDR) ligand binding domain (LBD), complexed with various vitamin D sterols, have required a mildly acidic environment to obtain crystals of good quality (pH= 6.0). The VDR LBD can be fundamentally split into two halves, the conserved tri-helical sandwich and the variable ligand binding pocket. The tri-helical sandwich, composed of helix 1 (H1), H4, H5, H8, H9, H10 and portions of H3 and H7, form the structural foundation of the LBD. Importantly, the 2 structural entities that form the ligand binding pocket, H2, H3, H6, H11, and the b-sheet, extend from this tightly packed region. The region of the ligand binding pocket that shows the highest degree of structural divergence across the nuclear receptor superfamily is the H2/ b-sheet region. It is hypothesized here that the molecular flexibility of s
