Charge stabilization mechanism in the visual and purple membrane pigments.

Abstract

The effects of charged groups of rhodopsin and bacteriorhodopsin on the potential energy surface of their chromophore are examined, taking into account the protein dielectric effect. It is found that the barriers for twisting double bonds of an isolated chromophore can be drastically reduced when the chromophore interacts with the protein charges. New types of local minima are found in the ground-state potential surface of the protein-chromophore complex. These minima correspond to "charge-stabilized intermediates" which are formed when a shift of the chromophore positive charge to the ring is stabilized by the ionization of a properly placed acidic group of the protein and by partial alternation of the bond len s of the chromophore. It is suggested that the absorption of light by rhodopsin and bacteriorhodopsin may be used not only for isomerization about double bonds, but also for trapping such charge-stabilized intermediates. Thus, for example, it is concluded that prelumirhodopsin might be still in the cis configuration. Both the mechanism of the proton pump system of the purple membrane and the dark reaction of the visual and purple membrane pigments are considered. The connection between the finding of the present work and the mechanism of storage of light energy in photobiology is indicated. A protonated Schiff base of retinal (PSBR) is the chromophore of several important photoreceptors, including the visual and purple membrane pigments. Absorption of light by such systems leads to a series of events that involve several intermediates (see refs. 1 and 2 for reviews). Characterization of these intermediates is essential for detailed understanding of the reaction mechanism of PSBR-containing pigments. There is only indirect experimental information about the structure of these intermediates, and the proposed models for their identity are ill defined. (3 Fig. 1. It is composed of the PSBR chromophore, two acids (A1 and A2); and a three-dimensional grid of induced dipoles that simulate the dielectric effect of the protein atoms. The potential surface of this system includes the following contributions: (a) The intramolecular potential of the chromophore, evaluated by the QCFF/PI method (9, 10) (the parameters used are given in ref.3). (b) The electrostatic interactions between the chromophore and the protein, evaluated by the recently developed microscopic dielectric approach (5). In this approach the chromophore and the assumed protein charges are surrounded by a cubic grid of induced dipoles (see Fig. 1) with the average density (1/8 A-3) and polarizability (a = 0.6 A-3) of the protein atoms. The interaction between these charges and induced Abbreviation: PSBR, protonated Schiff base of retinal. 2558

The Involvement of Water at the Retinal Binding Site in Rhodopsin and Early Light‐induced Intramolecular Proton Transfer

“…Removal of water could then result in a displacement of positive charge away from Schiff base nitrogen and toward this side group, effectively neutralizing its negative charge while decreasing the magnitude of the local electrostatic field near the Schiff base nitrogen. In our opinion, such a mechanism could readily be incorporated into several recent models of retinylidene-opsin interaction (Harosi et al, 1978;Warshel, 1978;Honig et al, 1979a).…”

“…The visible absorption maximum of rhodopsin (498 nm) is substantially red-shifted compared t o that of the unbound prosthetic group, 1 1-cis, 12s-cis-retinal (376 nm in ethanol). Considerable experimental evidence (Honig and Ebrey, 1974; Sulkes et a/., 1978;Eyring and Mathies, 1979; Aton et a/., 1980) supports the idea that much of this bathochromic shift is due t o direct protonation of the Schiff base which links the retinylidene group to opsin, although partial protonation involving hydrogen bonding has also been suggested (Harosi et a/., 1978). Additional secondary interactions between the retinylidene group and the protein are required t o explain the full magnitude of the bathochromic shift.…”

“…Intramolecular proton transfer in rhodopsin has been incorporated into several recent models of early events in visual excitation (Aton et d., 1980;Harosi et a/., 1978;Fransen et a/., 1980;van der Meer et al, 1976;Peters et al, 1977;Warshel, 1978; Honig et a/., 1979b), stimulated in part by the detection of major deuterium isotope effects on the kinetics of bathorhodopsin formation (Peters et a/., 1977). We have obtained additional evidence that proton transfer may occur, a t least by the metarhodopsin I stage, based on the photochemistry of dehydrated rod outer segment membranes.…”

Electrostatics of Proteins: Principles, Models and Applications

“…Reliable analysis of the energetics of ion pairs in proteins presents a major challenge and has been the subject of significant conceptual confusions (see discussions in Refs [3] and [12]). The most common confusion is the intuitive assumption that ion pairs are more stable in nonpolar environments rather than less stable in nonpolar environments (see analysis in Refs [154] and [155]). Excellent examples of this confusion are provided even in recent studies (e.g., [156]) and in most of the desolvation models (e.g., [157][158][159]) which are based on the assumption that enzymes destabilize their ground states by placing them in nonpolar environments.…”

Molecular mechanism for the initial process of visual excitation. IV. Energy surfaces of visual pigments and photoisomerization mechanism

“…If similar interaction energy surfaces as ours could be obtained by the other mechanism, we would not adhere to the steric hindrance mechanism. From this point of view, it is interesting to see what energy surfaces would be obtained in the charge stabilization model of Warshel (1978).…”