Differences between the photocycles of halorhodopsin and the acid purple form of bacteriorhodopsin analyzed with millisecond time-resolved FTIR spectroscopy

Photocycle of dried acid purple form of bacteriorhodopsin

The photocycle of dried bacteriorhodopsin, pretreated in a 0.3 M HCl solution, was studied. Some properties of this dried sample resemble that of the acid purple suspension: the retinal conformation is mostly all-trans, 15-anti form, the spectrum of the sample is blue-shifted by 5 nm to 560 nm, and it has a truncated photocycle. After photoexcitation, a K-like red-shifted intermediate appears, which decays to the ground state through several intermediates with spectra between the K and the ground state. There are no other bacteriorhodopsin-like intermediates (L, M, N, O) present in the photocycle. The K to K' transition proceeds with an enthalpy decrease, whereas during all the following steps, the entropic energy of the system decreases. The electric response signal of the oriented sample has only negative components, which relaxes to zero. These suggest that the steps after intermediate K represent a relaxation process, during which the absorbed energy is dissipated and the protein returns to its original ground state. The initial charge separation on the retinal is followed by limited charge rearrangements in the protein, and later, all these relax. The decay times of the intermediates are strongly influenced by the humidity of the sample. Double-flash experiments proved that all the intermediates are directly driven back to the ground state. The study of the dried acid purple samples could help in understanding the fast primary processes of the protein function. It may also have importance in technical applications.

Chloride ion binding to bacteriorhodopsin at low pH: an infrared spectroscopic study

Bacteriorhodopsin (bR) and halorhodopsin (hR) are light-induced ion pumps in the cell membrane of Halobacterium salinarium. Under normal conditions bR is an outward proton transporter, whereas hR is an inward Cl- transporter. There is strong evidence that at very low pH and in the presence of Cl-, bR transports Cl- ions into the cell, similarly to hR. The chloride pumping activity of bR is connected to the so-called acid purple state. To account for the observed effects in bR a tentative complex counterion was suggested for the protonated Schiff base of the retinal chromophore. It would consist of three charged residues: Asp-85, Asp-212, and Arg-82. This quadruplet (including the Schiff base) would also serve as a Cl- binding site at low pH. We used Fourier transform infrared difference spectroscopy to study the structural changes during the transitions between the normal, acid blue, and acid purple states. Asp-85 and Asp-212 were shown to participate in the transitions. During the normal-to-acid blue transition, Asp-85 protonates. When the pH is further lowered in the presence of Cl-, Cl- binds and Asp-212 also protonates. The binding of Cl- and the protonation of Asp-212 occur simultaneously, but take place only when Asp-85 is already protonated. It is suggested that HCl is taken up in undissociated form in exchange for a neutral water molecule.

Nano- and microsecond time-resolved FTIR spectroscopy of the halorhodopsin photocycle

Step-scan Fourier transform infrared spectroscopy with 50 ns time resolution was applied to the early stages of the photocycle of halorhodopsin (hR) for the temperature range 3-42 degrees C. Kinetic data analysis with global fitting revealed two distinct kinetic processes associated with relaxations of the early red-shifted photoproduct hK; these processes have time constants tau 1 approximately equal to 280 ns and tau 2 approximately equal to 360 microns at 20 degrees C. Spectral features demonstrate that the tau 1 process corresponds to a transition between two distinct bathointermediates, hKE and hKL. The vibrational difference bands associated with both tau 1 and tau 2 transitions are spread throughout the whole 1800-900 cm-1 range. However, the largest bands correspond to ethylenic C=C stretches, fingerprint C-C stretches and hydrogen out-of-plane (HOOP) wags of the retinal chromophore. The time evolution of these difference bands indicate that both the tau 1 and tau 2 decay processes involve principally a relaxation of the chromophore and its immediate environment. The decay of the intense HOOP vibrations is nearly equally divided between the tau 1 and tau 2 processes, indicating a complex chromophore relaxation from a twisted nonrelaxed conformation in the primary (hKE) bathointermediate, to a less-twisted structure in hKL, and finally to a roughly planar structure in the hypsochromically shifted hL intermediate. This conclusion is also supported by the unexpectedly large positive entropy of activation observed for the tau 1 process. The two relaxations from hKE to hL are largely analogous to corresponding relaxations (KE-->KL-->L) in the bacteriorhodopsin photocycle, except that the second step is slowed down by over 200-fold in hR.

A large photolysis-induced pKa increase of the chromophore counterion in bacteriorhodopsin: implications for ion transport mechanisms of retinal proteins

The proton-pumping mechanism of bacteriorhodopsin is dependent on a photolysis-induced transfer of a proton from the retinylidene Schiff base chromophore to the aspartate-85 counterion. Up until now, this transfer was ascribed to a > 7-unit decrease in the pKa of the protonated Schiff base caused by photoisomerization of the retinal. However, a comparably large increase in the pKa of the Asp-85 acceptor also plays a role, as we show here with infrared measurements. Furthermore, the shifted vibrational frequency of the Asp-85 COOH group indicates a transient drop in the effective dielectric constant around Asp-85 to approximately 2 in the M photointermediate. This dielectric decrease would cause a > 40 kJ-mol-1 increase in free energy of the anionic form of Asp-85, fully explaining the observed pK alpha increase. An analogous photolysis-induced destabilization of the Schiff base counterion could initiate anion transport in the related protein, halorhodopsin, in which aspartate-85 is replaced by Cl- and the Schiff base proton is consequently never transferred.