Xanthorhodopsin: a bacteriorhodopsin-like proton pump with a carotenoid antenna

Abstract

Xanthorhodopsin is a light-driven proton pump like bacteriorhodopsin, but made more effective for collecting light by its second chromophore, salinixanthin, a carotenoid. Action spectra for transport and fluorescence of the retinal upon excitation of the carotenoid indicate that the carotenoid functions as an antenna to the retinal. The calculated center-to-center distance and angle of the transition moments of the two chromophores are 11 A and 56 degrees , respectively. As expected from their proximity, the carotenoid and the retinal closely interact: tight binding of the carotenoid, as indicated by its sharpened vibration bands and intense induced circular dichroism in the visible, is removed by hydrolysis of the retinal Schiff base, and restored upon reconstitution with retinal. This antenna system, simpler than photosynthetic complexes, is well-suited to study features of excited-state energy migration.

Fig. 1

Absorption spectra (A) and action spectra (B) of xanthorhodopsin. In A: spectrum 1, xanthorhodopsin; spectrum 2, after hydrolysis of the retinal Schiff base with hydroxylamine; spectrum 3, after borohydride reduction of the retinal Schiff base. Hydroxylamine releases the retinal and that decreases the resolution of the vibronic bands of the carotenoid, while borohydride leaves the reduced retinal in its binding site and does not significantly alter the carotenoid spectrum. In B: spectra 1 and 2, action spectra for photoinhibition of respiration by S. ruber and Archaebacterium sp. (Halorubrum sp.) cells, respectively. After [13, 22, 25].

Fig. 2

Chemical structure of salinixanthin, from [20].

Fig. 3

Fluorescence of the chromophores of xanthorhodopsin A: 1, absorption spectrum of xanthorhodopsin; 2, fluorescence emission spectrum when excited at 470 nm, on an arbitrary scale, pH 5.5; 3, excitation spectrum for emission at 700 nm. Emission from salinixanthin is a set of structured bands at 530–600 nm, emission from the retinal is a broad band at 680 nm. Energy transfer from salinixanthin to the retinal is indicated by the carotenoid bands in the excitation spectrum. B: difference in the fluorescence emission spectrum upon reduction of the retinal with borohydride, pH 8.5. The broad retinal emission band is removed, but the carotenoid emission bands have increased in amplitude because the excited state decay channel to the energy acceptor is no longer present. After [25].

Fig. 4

Simplified scheme of excited-state energy transfer from carotenoid to retinal. Absorption of a photon by the carotenoid produces the shortlived S2 state, as S1 is forbidden. Energy migration to the S1 state of the retinal occurs only from S2, because the carotenoid S1 level is lower than the retinal S1. It competes with internal conversion to S1 of carotenoid (dashed vertical arrow). Decay of the all trans retinal S1 state produces 13-cis,15-anti configuration.