title: Formation of M-Like Intermediates in Proteorhodopsin in Alkali Solutions (pH ≥ ∼8.5) Where the Proton Release Occurs First in Contrast to the Sequence at Lower pH.
abstract: Proteorhodopsin (PR) is an outward light-driven proton pump observed in marine eubacteria. Despite many structural and functional similarities to bacteriorhodopsin (BR) in archaea, which also acts as an outward proton pump, the mechanism of the photoinduced proton release and uptake is different between two H(+)-pumps. In this study, we investigated the pH dependence of the photocycle and proton transfer in PR reconstituted with the phospholipid membrane under alkaline conditions. Under these conditions, as the medium pH increased, a blue-shifted photoproduct (defined as Ma), which is different from M, with a pKa of ca. 9.2 was produced. The sequence of the photoinduced proton uptake and release during the photocycle was inverted with the increase in pH. A pKa value of ca. 9.5 was estimated for this inversion and was in good agreement with the pKa value of the formation of Ma (∼ 9.2). In addition, we measured the photoelectric current generated by PRs attached to a thin polymer film at varying pH. Interestingly, increases in the medium pH evoked bidirectional photocurrents, which may imply a possible reversal of the direction of the proton movement at alkaline pH. On the basis of these findings, a putative photocycle and proton transfer scheme in PR under alkaline pH conditions was proposed.
authors.affiliation: College of Pharmaceutical Sciences, Matsuyama University , Matsuyama, Ehime 790-8578, Japan.|Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Tamogami
authors.affiliation: Department of Physics, Faculty of Science and Engineering, Chuo University , Tokyo 112-8551, Japan.
authors.forename: Keitaro
authors.initials: K
authors.lastname: Sato
authors.affiliation: Department of Physics, Faculty of Science and Engineering, Chuo University , Tokyo 112-8551, Japan.
authors.forename: Sukuna
authors.initials: S
authors.lastname: Kurokawa
authors.affiliation: Department of Physics, Faculty of Science and Engineering, Chuo University , Tokyo 112-8551, Japan.
authors.forename: Takumi
authors.initials: T
authors.lastname: Yamada
authors.affiliation: College of Pharmaceutical Sciences, Matsuyama University , Matsuyama, Ehime 790-8578, Japan.
authors.forename: Toshifumi
authors.initials: T
authors.lastname: Nara
authors.affiliation: Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Demura
authors.affiliation: Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.|Faculty of Pharmaceutical Sciences, Toho University , Funabashi 274-8510, Japan.
authors.forename: Seiji
authors.initials: S
authors.lastname: Miyauchi
authors.affiliation: Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kikukawa
authors.affiliation: Department of Physics, Faculty of Science and Engineering, Chuo University , Tokyo 112-8551, Japan.
authors.forename: Eiro
authors.initials: E
authors.lastname: Muneyuki
authors.affiliation: College of Pharmaceutical Sciences, Matsuyama University , Matsuyama, Ehime 790-8578, Japan.|Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Kamo
mesh_terms: D000465:Algorithms; D019943:Amino Acid Substitution; D059001:Aquatic Organisms / Q000378:metabolism / Q000528:radiation effects; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D055162:Biocatalysis / Q000528:radiation effects; D001692:Biological Transport / Q000528:radiation effects; D005051:Eubacterium / Q000378:metabolism / Q000528:radiation effects; D020563:Gammaproteobacteria / Q000378:metabolism / Q000528:radiation effects; D019617:Halobacterium salinarum / Q000378:metabolism / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D055767:Immobilized Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008051:Lipid Bilayers / Q000737:chemistry; D008567:Membranes, Artificial; D008958:Models, Molecular; D009154:Mutation; D010713:Phosphatidylcholines / Q000737:chemistry; D055668:Photochemical Processes; D017494:Proton Pumps / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D025604:Rhodopsins, Microbial / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D055767:Immobilized Proteins; D008051:Lipid Bilayers; D008567:Membranes, Artificial; D010713:Phosphatidylcholines; D017494:Proton Pumps; D011994:Recombinant Proteins; D025604:Rhodopsins, Microbial; C413548:proteorhodopsin