Role of Thr218 in the Light-Driven Anion Pump Halorhodopsin from Natronomonas pharaonis
| Autor: | Keiichi Kawano, Takashi Kikukawa, Hiroaki Shigemura, Kousuke Shibasaki, Naoki Kamo, Tomoyasu Aizawa, Makoto Demura, Masakatsu Kamiya |
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| Rok vydání: | 2013 |
| Předmět: |
Models
Molecular Threonine Photochemistry Biochemistry Membrane Potentials Ion chemistry.chemical_compound Chlorides Pressure Schiff Bases Halobacteriaceae Schiff base Cell Membrane Osmolar Concentration Wild type Proton-Motive Force Water Biological Transport Retinal Partial molar property Photochemical Processes Recombinant Proteins Halorhodopsin Kinetics Amino Acid Substitution chemistry Ion pump Spectrophotometry Halogen Biocatalysis Mutant Proteins Halorhodopsins |
| Zdroj: | Biochemistry. 52:9257-9268 |
| ISSN: | 1520-4995 0006-2960 |
| Popis: | 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. |
| Databáze: | OpenAIRE |
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