Identification of the long-lived L'species of bacteriorhodopsin as the N intermediate by Raman/absorption quasi-simultaneous measurements

Threonine-89 participates in the active site of bacteriorhodopsin: evidence for a role in color regulation and Schiff base proton transfer

Bacteriorhodopsin (bR) functions as a light-driven proton pump in the purple membrane of Halobacterium salinarium. A major feature of bR is the existence of an active site which includes a retinylidene Schiff base and amino acid residues Asp-85, Asp-212, and Arg-82. This active site participates in proton transfers and regulates the visible absorption of bacteriorhodopsin and its photointermediates. In this work we find evidence that Thr-89 also participates in this active site. The substitution Thr-89 --> Asn (T89N) results in changes in the properties of the all-trans retinylidene chromophore of light-adapted bR including a redshift of the visible lambda(max) and a downshift in C=N and C=C stretch frequencies. Changes are also found in the M and N intermediates of the T89N photocycle including shifts in lambda(max), a downshift of the Asp-85 carboxylic acid C=O stretch frequency by 10 cm(-1), and a 3-5-fold decrease in the rate of formation of the M intermediate. In contrast, the properties of the 13-cis retinylidene chromophore of dark-adapted T89N as well as the K and L intermediates of the T89N photocycle are similar to the wild-type bacteriorhodopsin. These results are consistent with an interaction of the hydroxyl group of Thr-89 with the protonated Schiff base of light-adapted bR and possibly the N intermediate but not the 13-cis chromophore of dark-adapted bR or the K and L intermediates. Thr-89 also appears to influence the rate of Schiff base proton transfer to Asp-85 during formation of the M intermediate, possibly through an interaction with Asp-85. In contrast, the hydroxyl group of Thr-89 is not obligatory for proton transfer from Asp-96 to the Schiff base during formation of the N intermediate.

Fourier transform Raman spectroscopy of the bacteriorhodopsin mutant Tyr-185-->Phe: formation of a stable O-like species during light adaptation and detection of its transient N-like photoproduct

Near-infrared FT-Raman spectroscopy can be used to measure the vibrations of the bacteriorhodopsin (bR) chromophore without the disadvantage of conventional visible resonance Raman spectroscopy, where the visible excitation drives the bR photoreactions. We utilized this technique to investigate the light-dark adaptation of bacteriorhodopsin and the mutant Tyr-185-->Phe (Y185F) at room temperature in solution. Compared to wild-type bR, both the FT-Raman and resonance Raman spectra of the light-adapted Y185F displayed new features characteristic of the vibrations of the O intermediate. Light adaptation of Y185F was found to involve a 13-cis, C=N syn-->all-trans isomerization of the retinal chromophore which produces a species similar to bR570 and a second O-like species. Dark adaptation, which was much slower in Y185F compared to wild-type bR, involved a parallel decay of the bR570 and O-like species and resulted in a decreased all-trans:13-cis ratio compared to wild type. Further evidence for the existence of an O-like species in Y185F comes from pump-probe Raman difference spectroscopy, where a red pump beam is found to produce a species very similar to the N intermediate in the photocycle. This species is shown by stroboscopic Raman measurements to exist transiently even at high pH. We postulate that when the Y185F chromophore has an all-trans structure the effective pKa of Asp-85 and Asp-212 is elevated in Y185F due to the disruption of the Asp-212/Tyr-185 hydrogen bond, thereby accounting for the increased protonation of these residues in the O-like species.