Photoisomerization, energy storage, and charge separation: A model for light energy transduction in visual pigments and bacteriorhodopsin

Abstract

Visual pigments are a class of proteins found in the membranes of photoreceptor cells (for reviews, see refs. 1 and 2). Their chromophoric unit is 11-cis-retinal covalently bound in the form of a Schiff base to the c-amino group of a lysine. The absorption of a photon by a visual pigment initiates a sequence of biochemical events that eventually lead to the generation of a neural signal by a photoreceptor cell. The identity of the primary photochemical event has been a subject of considerable interest and controversy. It was originally suggested that the primary event was an isomerization of the chromophore from its 1 1-cis to an all-trans conformation (3, 4). The strongest evidence favoring this mechanism was the observation (based on spectral data at low temperature) that an artificial pigment containing a 9-cis chromophore had the same photoproduct as rhodopsin itself. It was quite reasonably concluded that the most plausible common photoproduct formed from the two cis isomers is a trans isomer. A number of picosecond absorption studies of the primary event have raised widespread doubts as to the validity of the original model. It was found (5) that the primary event is complete in less than a few picoseconds at room temperature, and it was argued that this is too short a time for isomerization to occur [although other picosecond studies have reached the opposite conclusion (6)]. More recent evidence has come from the observation that at low temperature the rate of the process is significantly inhibited by deuterium replacement of the exchangeable protons on the pigment (7). Since only one proton on the chromophore is exchangeable, it is unlikely that this would have a measurable effect on the rate of isomerization. The picosecond measurements have generated numerous models (7-11) whose major feature is a photochemical proton transfer followed by a thermal cis-trans isomerization that occurs at a later stage. However, the original evidence upon which the suggestion of a cis-trans isomerization was originally based has never been discredited and, thus, it appears necessary to find a molecular model that is consistent with the entire body of available evidence. The purpose of this paper is to present such a model.There are, in fact, a fairly large number of observations that can be used in the construction and evaluation of alternative models. For example, we have shown, by using simple thermodynamic arguments, that a significant fraction of the photon's energy is "stored" in the primary photoproduct (12). Clearly a mechanism for energy storage must be an important component of any model that is proposed. Another energetic constraint may be derived from psychophysical and electrophysiological measurements of the level of thermal noise in photoreceptor cells. We show below that these observations may be interpreted directly in molecular terms and that they require an unusually high thermal activation energy for the primary event that appears to be inconsistent with many of the models that have been proposed.The ...

Crystal structure of the ligand-free G-protein-coupled receptor opsin

“…Light induced retinal cis→trans isomerization in rhodopsin causes rearrangements in the retinal binding site, leading to the disrupture of the Schiff base/ Glu-113 salt bridge by proton transfer to Glu-113 [161]. Glu-113 was suggested as Schiff base counterion and predicted to be the cause of energy increase and spectral red shift in the primary photon-induced event [58]. The role of Glu-113 as counterion was confirmed by the spectral shift observed in the E113D (λ max = 505 nm) and E113Q (λ max = 380 nm) mutants [50].…”

“…Protein structural alterations and release of the constraints are induced by cis/trans isomerization of the chromophore following light absorption [51,52,53,54,77,129,130]. Due to retinal isomerization, the stabilizing salt bridge present in the inactive rhodopsin ground state between the protonated 11-cis-retinal Schiff base and the Glu-113 (on TM3) counterion is broken [25,31,58]. As a consequence, conformational rearrangements of the opsin moiety occur, giving raise to a succession of several different photoproducts in the activation process, i.e.…”

Polar residues drive association of polyleucine transmembrane helices

“…Such induced helix rearrangement may be a switching mechanism underlying the activation͞inactivation of TM receptors or the opening͞ closing of ion channels͞transporters. For example, proteins in the G protein-coupled receptor family, especially the rhodopsin subfamily, are thought to be constrained in their inactive states by interhelical interactions such as hydrogen bonds (27)(28)(29)(30). Upon light absorption or ligand binding, the polar residues may no longer participate in the original hydrogen bonds, releasing the constraints and allowing receptor conformational changes and, thus, signal transduction activation.…”

“…A number of conserved polar residues in the TM helices are found to be vital to ligand binding and͞or signal transduction; interactions among these residues are proposed to conformationally constrain the receptors in their inactive states in the absence of light or ligands (28,30). Incorporating hydrogen bonding interactions between conserved polar residues as constraints has improved molecular modeling of the TM helix bundle of rhodopsin-like receptors (31).…”

Computational Studies of the Primary Phototransduction Event in Visual Rhodopsin

“…The underlying molecular rearrangements responsible for storage of electrostatic energy are thus significantly different from previously postulated charge separation processes based upon displacement of the polyene chain linkage away from the carboxylate counterion toward a nonpolar environment. [12][13][14]30 The molecular rearrangements due to the photoisomerization in visual rhodopsin have been recently analyzed at the QM/MM (CASPT2//CASSCF/6-31G*: Amber) level 6 through partial geometry relaxation with respect to the configuration of the chromophore, Lys-296, and nearby water molecules. The bathorhodopsin product was found to be only partially isomerized with φ(C11-C12) ) -145°, and the endothermicity was predicted to be only 26 kcal/mol, mainly due to strain energy within the chromophore.…”