Structure and ligand-binding mechanism of the human OX1 and OX2 orexin receptors.

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Abstract

The orexin (also known as hypocretin) G protein-coupled receptors (GPCRs) regulate sleep and other behavioral functions in mammals, and are therapeutic targets for sleep and wake disorders. The human receptors hOX1R and hOX2R, which are 64% identical in sequence, have overlapping but distinct physiological functions and potential therapeutic profiles. We determined structures of hOX1R bound to the OX1R-selective antagonist SB-674042 and the dual antagonist suvorexant at 2.8-Å and 2.75-Å resolution, respectively, and used molecular modeling to illuminate mechanisms of antagonist subtype selectivity between hOX1R and hOX2R. The hOX1R structures also reveal a conserved amphipathic α-helix, in the extracellular N-terminal region, that interacts with orexin-A and is essential for high-potency neuropeptide activation at both receptors. The orexin-receptor crystal structures are valuable tools for the design and development of selective orexin-receptor antagonists and agonists.

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Linking crystallographic model and data quality.

P. Andrew Karplus, Kay Diederichs (2012)

In macromolecular x-ray crystallography, refinement R values measure the agreement between observed and calculated data. Analogously, R(merge) values reporting on the agreement between multiple measurements of a given reflection are used to assess data quality. Here, we show that despite their widespread use, R(merge) values are poorly suited for determining the high-resolution limit and that current standard protocols discard much useful data. We introduce a statistic that estimates the correlation of an observed data set with the underlying (not measurable) true signal; this quantity, CC*, provides a single statistically valid guide for deciding which data are useful. CC* also can be used to assess model and data quality on the same scale, and this reveals when data quality is limiting model improvement.

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Crystal structure of rhodopsin: A G protein-coupled receptor.

K Palczewski, T Kumasaka, T. Hori … (2000)

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) respond to a variety of different external stimuli and activate G proteins. GPCRs share many structural features, including a bundle of seven transmembrane alpha helices connected by six loops of varying lengths. We determined the structure of rhodopsin from diffraction data extending to 2.8 angstroms resolution. The highly organized structure in the extracellular region, including a conserved disulfide bridge, forms a basis for the arrangement of the seven-helix transmembrane motif. The ground-state chromophore, 11-cis-retinal, holds the transmembrane region of the protein in the inactive conformation. Interactions of the chromophore with a cluster of key residues determine the wavelength of the maximum absorption. Changes in these interactions among rhodopsins facilitate color discrimination. Identification of a set of residues that mediate interactions between the transmembrane helices and the cytoplasmic surface, where G-protein activation occurs, also suggests a possible structural change upon photoactivation.