Energy storage in the primary photochemical events of rhodopsin and isorhodopsin

Abstract

The energetics associated with the photoequilibrium (Formula: see text) are measured at 77 K by using pulsed-laser photocalorimetry and a range of excitation wavelengths and relative starting concentrations. Enthalpies for the photochemical transformations R hv----B and I hv----B are measured to be delta HRB = 32.2 +/- 0.9 kcal mol-1 and delta HIB = 27.1 +/- 3.2 kcal mol-1, respectively. Although the value of delta HRB is slightly lower than that reported previously by Cooper of 34.7 +/- 2.2 kcal mol-1 [Cooper, A. (1979) Nature (London) 282, 531-533], the two values are in agreement within experimental error. The energy difference delta HRB - delta HIB = 5.1 +/- 3.3 kcal mol-1 is identical within experimental error with the difference in enthalpies of isorhodopsin and rhodopsin [5.2 +/- 2.3; Cooper, A. (1979) FEBS Lett. 100, 382-384]. We suggest that this result is consistent with the theory that bathorhodopsin is a single, common photochemical intermediate connecting rhodopsin and isorhodopsin.

Resonance Raman Analysis of the Mechanism of Energy Storage and Chromophore Distortion in the Primary Visual Photoproduct

“…Our calculations further indicate that the terminal group does not mechanistically change the nature of the distortions that cause the C 11 =C 12 decoupling. The calculated ATR and ATR-PSB structures have a torsional energy of14.1 and 15.6 kcal/mol, respectively, compared to a total of ∼30 kcal/mol in Batho (12,14). This calculated energy is only a qualitative estimate because all of the protein interactions have not been included, but this result does indicate that torsional distortions are an important energy-storage component.…”

“…The transduction of this torsional energy presumably drives the movements of E2 through the steric interactions at the 9-and 13-methyl groups, as well as the movement of H3 through the electrostatic interaction of Glu113 (on H3) with the PSB. The induced E2 and H3 movements coordinated by the functionally important disulfide bond (63) may further couple to the hydrogen-bond network in the binding pocking that conducts the counterion switch process (19).Torsional strain along the ethylenic chain has been previously proposed as a mechanism for energy storage in the Batho chromophore (12)(13)(14). Since the first crystal structure of rhodopsin was published in 2000 (32), a number of molecular dynamics calculations have been performed to model the Batho structure (64-66).…”

Structural Analysis and Dynamics of Retinal Chromophore in Dark and Meta I States of Rhodopsin from 2H NMR of Aligned Membranes

“…The stress due to torsional twisting (frustration) of retinal is appreciable 34,59,63 in relation to the energy stored by photoisomerization of retinal (≈ 140 kJ mole −1 ) 81 . By comparison, an energy only ≈ 10 kJ mole −1 is sufficient to shift the meta I to meta II equilibrium from reactant to product 73 .…”

QM/MM Study of the NMR Spectroscopy of the Retinyl Chromophore in Visual Rhodopsin

“…[1][2][3] The formation of the product bathorhodopsin is endothermic and stores approximately 50% of the photon energy. 20,[23][24][25] The energy storage is required to promote thermal reactions in the protein bleaching sequence and in the subsequent transducin cycle. The underlying molecular rearrangements responsible for energy storage have been recently analyzed in terms of rigorous DFT QM/MM (molecular orbitals: molecular mechanics) (QM/MM-(MO:MM)) hybrid methods as applied to the refinement of high-resolution X-ray structures of bovine rhodopsin.…”