Structural Insights from Recent CB1 X-Ray Crystal Structures

Abstract

Over the past 2 years, X-ray crystal structures of the antagonist-and agonist-bound CB1 receptor have been reported. Such structures are expected to accelerate progress in the understanding of CB1 and should provide an exceptional starting point for structurebased drug discovery. This chapter examines the consistency of these X-ray structures with the CB1 experimental literature, including mutation, NMR and covalent labeling studies. These comparisons reveal discrepancies between this literature and the TMH1-2-3 region of each CB1 crystal structure. The chapter also examines crystal packing issues with each X-ray structure and shows that the discrepancies with the experimental literature can be attributed to crystal packing problems that force the N-terminus deep in the binding pocket of the two inactive state structures and force TMH2 to bend at G2.53/ S2.54 and invade the binding pocket in the activated state structure. Revision is advisable before these structures are used for structure-based drug discovery.

GPR6 Structural Insights: Homology Model Construction and Docking Studies

“…S1P1 was used as a template due to its 33% sequence homology (similarity) with GPR6 and because both share essential features such as (1) the absence of helix kinking proline residues in TMH2 (2.58 or 2.59) and TMH5 (5.50); (2) both possess an acidic residue E1.49 before the highly conserved N1.50 in TMH1 (most Class A GPCRs have a G1.49 here); and (3) the presence of an internal disulfide bridge in the EC2 loop. Neither the Lysophosphatidic acid receptor 1 (LPA1) [PDB identifier: 4Z35] [36] or CB1 [PDB ID: 5TGZ and 5U09] [58,59], the other closely related crystallized GPCRs, were used because the LPA1 shares less essential structural features than S1P1 and the CB1 structures suffer from crystal packing problems [60]. Before mutating, the sequence of GPR6 was aligned first with the sequences of other class A GPCRs such as Rhodopsin (Rho), CB1, CB2, δ-opioid receptor (DOR), β2-AR, LPA1 and S1P1 receptors using the highly conserved residues/motifs as alignment guides (N1.50, D2.50, R3.50, W4.50, Y5.58, CWXP in TMH6, and NPXXY in TMH7) (blue residues in Figure 1).…”