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      • Structural Stability of High-Temperature State of Bacteriorhodopsin: A Model of Multi-state Membrane Proteins

        Mitaku, Shigeki,Yokoyama, Yasunori,Sonoyama, Masashi Korean Society of Photoscience 2002 Journal of Photosciences Vol.9 No.2

        A state of bacteriorhodopsin at high temperature was studied by various spectral measurements. The stability measurements indicated that the onset temperature of the denaturation was 70$^{\circ}C$ in the dark and 60$^{\circ}C$ under illumination. The reactivity of hydroxylamine with the Schiff's base also significantly increased in the temperature range between 60 and 70$^{\circ}C$. A spectral band at about 470 nm appeared in the temperature range higher than 60$^{\circ}C$. The circular dichroism spectra in the visible region started to change from a bilobed exiton type to a positive band at about 60$^{\circ}C$, suggesting that the two-dimensional configuration of bacteriorhodopsin molecules changed from crystalline to amorphous. All the measurements suggested a new state between 60 and 70$^{\circ}C$ in which bacteriorhodopsin is stable only in the dark.

      • Enhancement of Hydroxylamine Reactivity of Bacteriorhodopsin at High Temperature

        Sonoyama, Masashi,Mitaku, Shigeki Korean Society of Photoscience 2002 Journal of Photosciences Vol.9 No.2

        Recent denaturation experiments of bacteriorhodopsin (bR) in the dark and under illumination at high temperatures revealed that irreversible thermal bleaching occurs above ~ 70°C and the preceding reversible structural changes in the dark above 60°C are closely related to irreversible photobleaching observed in the same temperature range (Yokoyama et al. (2002). J Biochem. 131,785). In this study, structural properties of bacteriorhodopsin (bR) at high temperatures were extensively probed by hydroxylamine reactivity with the Schiff base in the dark and hydrogen-deuterium (H-D) exchange in the peptide groups. In the Arrhenius plot from kinetics measurements of the hydroxylamine reaction, a good linear relationship between the reaction time constant and the inverse of the absolute temperature was observed below 60°C, while significant increase started above 60°C, suggesting that remarkable increase in water accessibility of the Schiff base in the temperature region. FT-IR spectroscopic studies on the H-D exchange suggested increase in the deuterium exchanges rate of the peptide hydrogen in the same temperature region.

      • Kinetic Measurements of Irreversible Photobleaching of Bacteriorhodopsin in A High Temperature State

        Yokoyama, Yasunori,Sonoyama, Masashi,Mitaku, Shigeki Korean Society of Photoscience 2002 Journal of Photosciences Vol.9 No.2

        Irreversible photobleaching of bacteriorhodopsin (bR), namely denaturation induced by illumination of visible light, was investigated by absorption kinetic measurements. The denaturation kinetics revealed that light illumination significantly enhanced the structural decay of bR. The kinetic analyses showed that the molecular structure of bR denatures according to a single-exponential decay, whereas irreversible photobleaching has two decay components. The decay constant of the slow component of photobleaching is almost same as that in the dark. An Arrhenius plot of the denaturation kinetic constants for the fast and slow components showed similar activation energies of approximately 19 kcal/mol.

      • Structural Stability of Bacteriorhodopsin Solubilized by Triton X-100

        Sasaki, Takanori,Sonoyama, Masashi,Mukai, Yuri,Nakazawa, Chieko,Mitaku, Shigeki Korean Society of Photoscience 2002 Journal of Photosciences Vol.9 No.2

        The structural stability of bacteriorhodopsin (bR) solubilized by Triton X-100 (TX-100) was studied by measuring the denaturation kinetics in the dark and under illumination, and compared with the structural stability of bR solubilized by octyl-${\beta}$-glucoside (OG). In the dark, bR solubilized by TX- 100 was more stable than bR solubilized by OG. Under illumination, bR solubilized by TX-100 showed light-induced denaturation in the same manner as bR solubilized by OG. These results in the dark well correlated with the experimental results of the visible CD band. Although solubilized bR in the TX-100 concentration range of 2-50 mM showed almost identical positive CD band and did not denature in the dark at 35$^{\circ}$C, the kinetic constant of the photobleaching increased with the increase of TX-100 concentration. These results suggested that photo-intermediates of solubilized bR are destabilized by TX-100 micelles.

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