Two forms of intermediates of frog rhodopsin in rod outer segments

Nanosecond photolytic interruption of bacteriorhodopsin photocycle: K-590 --> BR-570 reaction

The molecular processes comprising the room temperature bacteriorhodopsin (BR) photocycle are examined through the properties of the photo-induced reverse reaction, K-590 + hnu --> BR-570 (K --> BR). Two sequential pumping pulses, each of 10-ns duration, are used, respectively, to initiate the photocycle via the forward BR-570 + hnu --> K-590 (BR --> K) reaction (532 nm) and to photolytically interrupt the thermal BR photocycle after a 20-ns delay via K --> BR (620-700 nm). The ground-state BR-570 population, monitored by 633-nm absorption 200 mus after the photocycle begins, provides a quantitative measure of the efficiency with which K --> BR interrupts the photocycle to reform BR-570. The quantum yield (Phi) for K --> BR is found to be 1.6 +/- 0.1 times larger than that for BR --> K which, when compared to a Phi of 0.64 for BR --> K, suggests that Phi for K --> BR is approximately 1.0. The significance of such a high efficiency K --> BR reaction with respect to mechanistic descriptions of the BR photocycle is discussed.

Torsion Potential Works in Rhodopsin¶

We investigate the role of protein environment of rhodopsin and the intramolecular interaction of the chromophore in the cis-trans photoisomerization of rhodopsin by means of a newly developed theoretical method. We theoretically produce modified rhodopsins in which a force field of arbitrarily chosen part of the chromophore or the binding pocket of rhodopsin is altered. We compare the equilibrium conformation of the chromophore and the energy stored in the chromophore of modified rhodopsins with those of native rhodopsins. This method is called site-specific force field switch (SFS). We show that this method is most successfully applied to the torsion potential of rhodopsin. Namely, by reducing the twisting force constant of the C11=C12 of 11-cis retinal chromophore of rhodopsin to zero, we found that the equilibrium value of the twisting angle of the C11=C12 bond is twisted in the negative direction down to about -80 degrees. The relaxation energy obtained by this change amounts to an order of 10 kcal/mol. In the case that the twisting force constant of the other double bond is reduced to zero, no such large twisting of the bond happens. From these results we conclude that a certain torsion potential is applied specifically to the C11=C12 bond of the chromophore in the ground state of rhodopsin. This torsion potential facilitates the bond-specific cis-trans photoisomerization of rhodopsin. This kind of the mechanism is consistent with our torsion model proposed by us more than a quarter of century ago. The origin of the torsion potential is analyzed in detail on the basis of the chromophore structure and protein conformation, by applying the SFS method extensively.

Spectral and kinetic evidence for the existence of two forms of bathorhodopsin

Transient-absorption difference spectra from 320 nm to 700 nm were obtained at times ranging from 30 ns to 1200 ns after 532-nm photolysis of rhodopsin at room temperature. Kinetics on this time scale at various wavelengths are also presented. The isosbestic points between spectra acquired at successive times after photolysis shift from 510 nm to 530 nm. This shift is inconsistent with a simple process of one bathorhodopsin (BathoR) intermediate being transformed into one lumirhodopsin (LumiR) intermediate on this time scale. The kinetics at 425 nm, 515 nm, and 575 nm could not be fit well to a single-exponential expression. The data are consistent with the existence of two forms of BathoR (BathoR1 and BathoR2) that exhibit different spectra and decay kinetics. The BathoR1 absorption maximum lies near 565 nm, and the BathoR1/LumiR1 isosbestic point is near 430 nm. The BathoR2 absorption maximum lies near 535 nm, and the BathoR2/LumiR2 isosbestic point is near 480 nm. The kinetics at the isosbestic wavelengths were fit to single-exponential expressions corresponding to BathoR1 and BathoR2 lifetimes of 170 +/- 20 ns and 36 +/- 15 ns, respectively.