Protein structure patterns bacteriorhodopsin

Henderson et al. [223] presented a detailed pattern of the structure of bacteriorhodopsin using high-resolution cryoelectron microscopy. Using X-ray and neutron diffraction techniques, Dencher et al. [224—227] could decode the secondary and tertiary structure of bacteriorhodopsin during the photocycle. Nevertheless, we should emphasize that the resolution still shows transitions in the active site (protonation of counterions, deprotonation of Schiff base, and reprotonation of counterions), leading to a metastable state of the protein. 

A three-dimensional structure also has been elucidated for bacteriorhodopsin, an integral membrane protein of the halophilic (salt-loving) bacterium Halobacterium halobium. This protein has been studied intensively because of its remarkable activity as a light-driven proton pump (see chapter 14). It forms well-ordered arrays in two-dimensional sheets that can be studied by electron diffraction. Measurements of the diffraction patterns show clearly that bacteriorhodopsin has seven transmembrane helices (fig. 17.12). 

---