Photoreactions of the Photointermediates of Bacteriorhodopsin

Abstract

The photochemical reactions of the intermediates of the photochemical cycle of bacteriorhodopsin (bR) are reviewed. These reactions constitute photochemical control of the cycle and provide an independent approach for the investigation of the mechanism of light energy transduction in the purple membrane. The absorption of a light quantum by the K, L, or M intermediates converts them back to bR. These transformations interrupt the photocycle so that no proton transfer occurs after absorption of the second quantum. The action of blue light on the M intermediate causes structural changes of the chromophore, as a result of which the Schiff base is reprotonated from Asp-85, not from Asp-96 as in the usual thermal transition of M. The photoreactions of the L, M, N, and 0 intermediates lead to the formation of new photoproducts. Studies of the photoconversion ofthe intermediates can serve as an additional source of information on the nature of photoprocesses in bR: they reveal several conformers of K and bR at 90 K, different M states, two N intermediates, and provide direct evidence for the existence of a thermal back reaction from N to M. The study of the photoreactions of the J, K, L, M, N, and 0 intermediates is a promising method for elucidating the structures and roles of these states. Reversible photoconversions of bR and its photointermediates provide a basis for potential applications of bR in optical registration of information.This paper is dedicated to the memory of my colleagues, Dr.

Energy transformations early in the bacteriorhodopsin photocycle revealed by DNP-enhanced solid-state NMR

“…The major product can be assigned to the K state on multiple grounds: first, irradiation with red light results in the expected reversion to bR 568 ; second, as discussed further below, thermal relaxation (in the dark) at 150-170 K gives rise to the observed L photocycle intermediate, confirming that the newly observed species is an intermediate in the functional photocycle. The minor product disappears over a few hours at 90 K. Thus, it appears to be an unstable side-product of the formation of K. According to visible spectroscopy, the generation of K is accompanied by the formation of iso-bR and pseudo-bR, both of which relax to bR 568 in the dark at 77 K (20). Based on the yields reported in the literature, we tentatively assign the small, most downfield signal in Fig.…”

Spectral Characteristics of the Photocycle of Channelrhodopsin-2 and Its Implication for Channel Function

“…25 The best known example is the blue-light effect in BR, where the absorption of blue light by the M state leads to a return to the ground state without vectorial proton transport. 26 In the case of ChR2, the spectral difference of the intermediate P 520 is maximal in the region between 520 and 540 nm.…”

Photoexcitation of the O intermediate of bacteriorhodopsin and its mutant E204Q

“…To understand the molecular mechanism of light-energy transduction many methods are used [3]. One of them, photoexcitation of the intermediates, provides valuable information on their particular rôle in the photocycle [4]. Electric responses of the photoreactions of the intermediates turned out to be indispensable in understanding the mechanism of proton translocation.…”