title: Role of Thr218 in the light-driven anion pump halorhodopsin from Natronomonas pharaonis.
abstract: Halorhodopsin (HR) is an inward-directed light-driven halogen ion pump, and NpHR is a HR from Natronomonas pharaonis. Unphotolyzed NpHR binds halogen ion in the vicinity of the Schiff base, which links retinal to Lys256. This halogen ion is transported during the photocycle. We made various mutants of Thr218, which is located one half-turn up from the Schiff base to the cytoplasm (CP) channel, and analyzed the photocycle using a sequential irreversible model. Four photochemically defined intermediates (P(i), i = 1-4) were adequate to describe the photocycle. The third component, P₃, was a quasi-equilibrium complex between the N and O intermediates, where a N ↔ O + Cl⁻ equilibrium was attained. The K(d,N↔O) values of this equilibrium for various mutants were determined, and the value of Thr (wild type) was the highest. The partial molar volume differences between N and O, ΔV(N→O), were estimated from the pressure dependence of K(d,N↔O). A comparison between K(d,N↔O) and ΔV(N→O) led to the conclusion that water entry by the F-helix opening at O may occur, which may increase K(d,N↔O). For some mutants, however, large ΔV(N→O) values were found, whereas the K(d,N↔O) values were small. This suggests that the special coordination of a water molecule with the OH group of Thr is necessary for the increase in K(d,N↔O). Mutants with a small K(d,N↔O) showed low pumping activities in the presence of inside negative membrane potential, while the mutant activities were not different in the absence of membrane potential. The effect of the mutation on the pumping activities is discussed.
authors.affiliation: Faculty of Advanced Life Science, Hokkaido University , Sapporo 060-0810, Japan.
authors.forename: Kousuke
authors.initials: K
authors.lastname: Shibasaki
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Shigemura
authors.forename: Takashi
authors.initials: T
authors.lastname: Kikukawa
authors.forename: Masakatsu
authors.initials: M
authors.lastname: Kamiya
authors.forename: Tomoyasu
authors.initials: T
authors.lastname: Aizawa
authors.forename: Keiichi
authors.initials: K
authors.lastname: Kawano
authors.forename: Naoki
authors.initials: N
authors.lastname: Kamo
authors.forename: Makoto
authors.initials: M
authors.lastname: Demura
mesh_terms: D019943:Amino Acid Substitution; D055162:Biocatalysis; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism*; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism; D006216:Halobacteriaceae / Q000378:metabolism*; D025602:Halorhodopsins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D008958:Models, Molecular; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D009994:Osmolar Concentration; D055668:Photochemical Processes; D011312:Pressure; D018892:Proton-Motive Force; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012545:Schiff Bases / Q000737:chemistry; D013053:Spectrophotometry; D013912:Threonine / Q000737:chemistry*; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: D002712:Chlorides; D025602:Halorhodopsins; D050505:Mutant Proteins; D011994:Recombinant Proteins; D012545:Schiff Bases; D014867:Water; D013912:Threonine