Carotenoid Antenna Binding and Function in Retinal Proteins

Abstract : Xanthorhodopsin, a proton pump from the eubacterium Salinibacter ruber, is a unique dual chromophore system that contains, in addition to retinal, the carotenoid salinixanthin as a light-harvesting antenna. The key factors affecting binding and function of the antenna were established and examined. The first is the binding of the carotenoid ring near the retinal ring. Substitution of the small glycine with bulky tryptophan in this site eliminates binding. The second factor is the 4-keto group in the carotenoid ring. A close analogue of salinixanthin, salinixanthol, in which the 4-keto group (C=O) is reduced to hydroxyl (C-OH), does not bind. Third, the binding site itself. Another protein capable of carotenoid binding was found. Gloeobacter rhodopsin, was shown to bind salinixanthin and echinenone but not beta-carotene or salinixanthol. Using femtosecond absorption spectroscopy, a large transient electrochromic shift of the salinixanthin band was detected 150 fs after excitation of the retinal chromophore. It indicates that excitation of retinal is accompanied by charge separation and a strong electrostatic field. It partially remains in the first photoproduct K. Thermal conversion of the latter involves at least two substates which were characterized with FTIR spectroscopy. The kinetics of photocycle and proton transport by xanthorhodopsin was examined.