Quantitative model for the cooperative interaction of the bacteriorhodopsin molecules in purple membranes

Abstract

The trimeric, asymmetric and sequential model for the cooperative interaction of the bacteriorhodopsin molecules in purple membranes [Zs. Tokaji, Biophys. J. 65 (1993) 1130^1134] is being extended in the paper. Analyses of data from absorption kinetic measurements with preexcitation and green background illumination, and photoselection measurements on oriented samples confirm the main features of this cooperative interaction and support the validity of the extended model. This model includes the observed heterogeneity of the non-excited state of the bacteriorhodopsin molecules in purple membranes, and the agreement with the data suggests that molecules in any other state than the bacteriorhodopsin ground state can alter the photocycle of their neighbors. The presented results seem to contradict other models for the cooperation of the bacteriorhodopsin molecules.z 1998 Federation of European Biochemical Societies.

Bacteriorhodopsin photocycle at cryogenic temperatures reveals distributed barriers of conformational substates

“…The reason for heterogeneity or branching would be, for example, partial protonation states of critical residues depending on the pH (38), nonequivalence of the monomers in the bacteriorhodopsin trimer (39), or a neighbor/cooperativity effect of photocycling molecules on one another in the membrane (40,41). However, the temperature up-shift experiment (Fig.…”

Structure of the bacteriorhodopsin mutant F219L N intermediate revealed by electron crystallography

“…The effect is seen as an increase in the proportion of a slow‐decaying M species at increasing light intensities at the expense of a fast‐decaying species. It has been explained variously as resulting from heterogeneity in the photoexcited molecules or from cooperativity within a trimer or within the purple membrane (Hendler et al ., 1994; Shrager et al ., 1995; Váró et al ., 1996; Tokaji, 1998). The same effect is seen when a second light flash is given within a few milliseconds after the first (Tokaji and Dancsházy, 1991).…”

“…This explanation of photocooperativity implies that it occurs between different trimers, not within a trimer as assumed in some models (Korenstein et al ., 1979; Tokaji, 1998). That this is indeed the case is shown by the observation that mild Triton X‐100 treatment abolished cooperativity before the trimers were disrupted (Mukhopadhyay et al ., 1994).…”

“…Even at very low light intensities, the slow decay component is never totally absent (Fukuda and Kouyama, 1992; Hendler et al ., 1994; Tokaji, 1998). The easiest explanation is that the ground state is heterogeneous and contains some molecules with a tilted‐helix conformation.…”

Actinic Light-Energy Dependence of Proton Release from Bacteriorhodopsin

“…More accurately, the ratio of amplitudes of the two components (fast and slow) of M decay changes with fluence. These effects were explained by interactions, i.e., ''cooperativity'' between the BR molecules forming triplets in purple membrane (pm) (3,4). The main function of BR is to pump protons from the cytoplasm of the bacterium to the medium and this way to transform light energy into electrochemical energy (5).…”