title: Specific damage induced by X-ray radiation and structural changes in the primary photoreaction of bacteriorhodopsin.
abstract: Bacteriorhodopsin, the sole membrane protein of the purple membrane of Halobacterium salinarum, functions as a light-driven proton pump. A 3-D crystal of bacteriorhodopsin, which was prepared by the membrane fusion method, was used to investigate structural changes in the primary photoreaction. It was observed that when a frozen crystal was exposed to a low flux of X-ray radiation (5 x 10(14)photons mm(-2)), nearly half of the protein was converted into an orange species, exhibiting absorption peaks at 450 nm, 478 nm and 510 nm. The remainder retained the normal photochemical activity until Asp85 in the active site was decarboxlyated by a higher flux of X-ray radiation (10(16)photons mm(-2)). The procedure of diffraction measurement was improved so as to minimize the effects of the radiation damage and determine the true structural change associated with the primary photoreaction. Our structural model of the K intermediate indicates that the Schiff base linkage and the adjacent bonds in the polyene chain of retinal are largely twisted so that the Schiff base nitrogen atom still interacts with a water molecule located near Asp85. With respect to the other part of the protein, no appreciable displacement is induced in the primary photoreaction.
authors.affiliation: Graduate School of Science, Nagoya University, Chikusa, 464-8602, Nagoya, Japan.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Matsui
authors.forename: Keisuke
authors.initials: K
authors.lastname: Sakai
authors.forename: Midori
authors.initials: M
authors.lastname: Murakami
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Shiro
authors.forename: Shin ichi
authors.initials: Si
authors.lastname: Adachi
authors.forename: Hideo
authors.initials: H
authors.lastname: Okumura
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Kouyama
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects*; D018360:Crystallography, X-Ray; D005615:Freezing; D008958:Models, Molecular; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism / Q000528:radiation effects; D011487:Protein Conformation; D018717:Purple Membrane / Q000528:radiation effects; D013053:Spectrophotometry / Q000379:methods; D017356:Synchrotrons; D013696:Temperature; D014965:X-Rays
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D001436:Bacteriorhodopsins