光駆動プロトンポンプの四次元X線構造解析 [in Japanese] Four-Dimensional X-ray Crystallography of Light-Driven Proton Pumps [in Japanese]
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Bacteriorhodopsin (bR), a membrane protein found in <I>Halobacterium salinarum, </I> contains retinal as chromophore and functions as a light-driven proton pump. We have carried out four-dimensional X-ray crystallographic studies of bR and its homologous proteins, to elucidate the proton pumping mechanism. A novel crystallization method, called the membrane fusion method, has been developed to prepare a 3D crystal that is stable over a wide pH range. This enabled us to investigate pH-induced as well as light-induced structural changes. Structural analyses of reaction intermediates revealed that water relocation, which affects the pK<SUB>a</SUB> values of ionizable residues in the proton channel, takes place in the early stage of the proton pumping cycle. On the basis of this observation, we hypothesized that bR is a water/proton anti-porter. Since some of internal cavities are highly conserved during the evolution, it is suggested that their morphological changes initiated by the retinal photo-isomerization play an important role for water relocation and proton tranlocation across the membrane.
X-RAYS 48(5), 359-366, 2006-10-31
The Crystallographic Society of Japan