Structure and Function Study of Light-Driven Ion Pumps

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This research project aims at revealing the correlation between structure and function of light-driven ion pumps that possess retinal molecule as the chromophore. The target molecules are bacteriorhodopsin (bR) as a light-driven proton pump, halorhodopsin (hR) as a light-driven chloride pump, and rhodopsin (Rh) as a light-sensor in our vision. Fourier-transform infrared (FTIR) spectroscopy of these proteins as well as their mutants and isotope-labeled molecules provided the following results.(1) Location and orientation of a water molecule present between the Schiff base and Asp85 of bR were studied by means of polarized FTIR spectroscopy, It was revealed that the water, which plays an important role in proton transfer, is bound to protein through hydrogen bonding.(2) Polarized FTIR spectroscopy of bR revealed the structural changes of protein side upon photoisomerization. Many structural features on hydrogen bonding network in bR are changed.(3) Water structural changes in the N intermediate of bR were measured for the wild-type and mutant proteins, and a functional water at the cytoplasmic region was found. In addition, specific protein structural changes were newly observed by polarized FTIR spectroscopy.(5) Time-resolved step-scan FTIR spectroscopy of bR mutants provided the structural information on the 0 intermediate, an intermediate appearing at the last stage of the photocycle of bR.(4) Two L intermediates of hR were revealed by low-temperature FTIR spectroscopy, which could be correlated with change in accessibility in chloride pumping.(5) Water molecules in Rh were discovered by low-temperature FTIR spectroscopy.