The Ability of Actinic Light to Modify the Bacteriorhodopsin Photocycle. Heterogeneity and/or Photocooperativity?

Abstract

The focus of this paper is on the established observation that the bacteriorhodopsin (BR) photocycle responds to the level of actinic light by altering the proportions of two forms of the M intermediate. The first form of M, called M-fast or MF, decays to the O intermediate. In contrast, the second form of M, called M-slow or MS, decays directly to the ground state, and its decay rate is slower than that of MF. Any proposed scheme for the BR photocycle must account for this light-dependent phenomenon. Several papers have attempted to explain the observation on the basis of photocooperativity, or on the basis of heterogeneous populations. In this paper, we test previously proposed cooperative models with experimental data, and find those models to be inadequate. We show that two new models, one purely cooperative, the other purely heterogeneous, can both fit the data, hence such modelling will not resolve the mechanism. Taking into account the demonstration of heterogeneity, the trimer structure of BR, and certain experimental evidence in favor of cooperativity, it appears likely that both heterogeneity and cooperativity are involved in the adaptation of the BR photocycle to different levels of actinic light.

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).…”

Protein structural change at the cytoplasmic surface as the cause of cooperativity in the bacteriorhodopsin photocycle

“…In purple membranes, but not in the monomers, the photocycle, which describes the sequence of intermediate states (termed J, K, L, M, N, and 0) after photoexcitation of the retinal chromophore with a light pulse, exhibits distinct kinetic differences when the intensity of the actinic laser beam is varied. The differences consist mainly of what appears to be an increase in the amplitude, although not the time constant, of the slower component in the biphasic decay of the M intermediate at pH near 9 (Tokaji and Dancshazy, 1992; Dancshazy and Tokaji, 1993;Tokaji, 1993Tokaji, , 1995Hendler et al, 1994;Mukhopadhyay et al, 1994;Shrager et al, 1995). Thus, the excitation intensity can evidently influence not only the number of bacteriorhodopsins that are photoexcited but also the rate of at least one of the steps in the proton transport.…”

Lipid patches in membrane protein oligomers: Crystal structure of the bacteriorhodopsin-lipid complex

“…The 2D crystalline state is important for the in vivo physiology of BR (5). Haloarchaeal lipids constitute onefourth of the PM and affect significantly the kinetics of BR (6). Main components are archaeol derivatives uniquely found in Haloarchaea: phosphatidylglycerol (PG), phosphatidylglycerol sulfate (PGS), phosphatidylglycerol phosphate methylester (PGP-Me), and a sulfated triglycoside lipid (S-TGA-1, 3-HSO 3 -Galp␤1-6Manp␣1-2Glcp␣-1-archaeol).…”