The Ability of Actinic Light to Modify the Bacteriorhodopsin Photocycle

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 M, decays to the 0 intermediate. In contrast, the second form of M, called M-slow or M,, 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.Keywords. Bacteriorhodopsin ; light intensity ; photocycle regulation ; mathematical models ; kinetics ; M-intermediates.Bacteriorhodopsin (BR) is a transmembrane protein that acts as a light-driven pump in the purple membrane of halobacteria Stoeckenius, 1971, 1973). Upon exposure to light, BR cycles through several intermediates which are currently labeled K, L, M, N and 0, with possible subspecies and branchings. Slifkin and Caplan (1975) demonstrated that (at least) two species of M intermediate were present with different lifetimes. Since then, the presence of two distinct species of M has been repeatedly confirmed (see Hendler et al., 1994, for references). A very important finding was made by Ohno et al. (1981), namely that the ratio of fast and slow forms of M (i.e. M, and Ms), was modulated by the intensity of the actinic flash used to initiate the photocycle, producing changes in the shapes of the kinetic curves. At low flash intensities, the MF species was predominant, but with increasing light intensities, the fraction of MR i.e. Mp/(MF + Ms), decreased to <0.5. This finding has also been repeatedly confirmed (see Hendler et al., 1994, for references). More recently, it has been shown that M, decays directly to the 0 intermediate, whereas M, decays directly to BR (Eisfeld et al., 1993;Hendler et al., 1994). At present, there are many different views on the nature of the photocycle, and a general consensus is lacking. An important criterion for narrowing the field of possibly correct photocycle models is the ability of those models to explain (i.e. fit) experimental kinetic data taken over a considerable range of actinic light intensities. It is also important for these models to account for the various decay pathways for the M, and M, intermediates. In this context, we will examine a variety of c...