| Bacteriorhodopsin is a light-driven ion pump that generates the electrochemical gradient necessary for ATP synthesis in Halobacterium salinarum. Based on the 1.55 Å resolution structure of ground state bacteriorhodopsin (BR), the work here presents the structure of the purple membrane. An almost complete crystallographic model of the native bilayer is described. Very specific lipid-protein interactions support the notion of lipids as mediators of function. Furthermore, a detailed characterization of the intrahelical π-bulge in helix G of BR is presented together with its implications on the ion-pumping photocycle.; Crystal structures are reported for the D85S and D85S/F219L bacteriorhodopsin mutants. Both structures exhibit similar and substantial differences from the wild-type ground state structure, suggesting that these are a direct result of the neutralization of the Schiff base counterion Asp-85. These structures provide a tentative model for the last photocycle intermediate (O) of bacteriorhodopsin, in which Asp-85 is protonated, the proton release group is deprotonated, and the retinal has reisomerized to all-trans. Also discussed are the halide pumping properties of the mutant proteins.; In order to better understand the π-bulge in helix G of BR, the Protein Data Bank (PDB) was data-mined for similar intrahelical motifs. We find that 5.2% of all PDB entries contain intrahelical deformations. These deformations are typically characterized by at least one i←i-5 main chain hydrogen bond, with energetically favorable main chain dihedral angles, followed by a variable number of main chain carbonyl hydrogen bond acceptors that do not participate in intrahelical main chain hydrogen bonds. Stabilization of these non-bonded moieties commonly occurs via hydrogen bonding to water molecules or polar side chains. |
