Comparative study of primary photochemical events of two retinal proteins, bacteriorhodopsin and halorhodopsin, by use of subpicosecond time-resolved spectroscopy

Local and distant protein structural changes on photoisomerization of the retinal in bacteriorhodopsin

The photoisomerization of the retinal in bacteriorhodopsin is selective and efficient and yields perturbation of the protein structure within femtoseconds. The stored light energy in the primary intermediate is then used for the net translocation of a proton across the membrane in the microsecond to millisecond regime. This study is aimed at identifying how the protein changes on photoisomerization by using the O-H groups of threonines as internal probes. Polarized Fourier-transform IR spectroscopy of [3-(18)O]threonine-labeled and unlabeled bacteriorhodopsin indicates that 3 of the threonines (of a total of 18) change their hydrogen bonding. One is exchangeable in D(2)O, but two are not. A comprehensive mutation study indicates that the residues involved are Thr-89, Thr-17, and Thr-121 (or Thr-90). The perturbation of only three threonine side chains suggests that the structural alteration at this stage of the photocycle is local and specific. Furthermore, the structural change of Thr-17, which is located >11 A from the retinal chromophore, implicates a specific perturbation channel in the protein that accompanies the retinal motion.

Structural change of threonine 89 upon photoisomerization in bacteriorhodopsin as revealed by polarized FTIR spectroscopy

The all-trans to 13-cis photoisomerization of the retinal chromophore of bacteriorhodopsin occurs selectively, efficiently, and on an ultrafast time scale. The reaction is facilitated by the surrounding protein matrix which undergoes further structural changes during the proton-transporting reaction cycle. Low-temperature polarized Fourier transform infrared difference spectra between bacteriorhodopsin and the K intermediate provide the possibility to investigate such structural changes, by probing O-H and N-H stretching vibrations [Kandori, Kinoshita, Shichida, and Maeda (1998) J. Phys. Chem. B 102, 7899-7905]. The measurements of [3-18O]threonine-labeled bacteriorhodopsin revealed that one of the D2O-sensitive bands (2506 cm(-1) in bacteriorhodopsin and 2466 cm(-1) in the K intermediate, in D2O exhibited 18(O)-induced isotope shift. The O-H stretching vibrations of the threonine side chain correspond to 3378 cm(-1) in bacteriorhodopsin and to 3317 cm(-1) in the K intermediate, indicating that hydrogen bonding becomes stronger after the photoisomerization. The O-H stretch frequency of neat secondary alcohol is 3340-3355 cm(-1). The O-H stretch bands are preserved in the T46V, T90V, T142N, T178N, and T205V mutant proteins, but diminished in T89A and T89C, and slightly shifted in T89S. Thus, the observed O-H stretching vibration originates from Thr89. This is consistent with the atomic structure of this region, and the change of the S-H stretching vibration of the T89C mutant in the K intermediate [Kandori, Kinoshita, Shichida, Maeda, Needleman, and Lanyi (1998) J. Am. Chem. Soc. 120, 5828-5829]. We conclude that all-trans to 13-cis isomerization causes shortening of the hydrogen bond between the OH group of Thr89 and a carboxyl oxygen atom of Asp85.

The projection structure of the low temperature K intermediate of the bacteriorhodopsin photocycle determined by electron diffraction

Bacteriorhodopsin (bR) is an integral membrane protein which absorbs visible light and pumps protons across the cell membrane of Halobacterium salinarium. bR is one of the few membrane-bound pumps whose structure is known at atomic resolution. Changes in the protein structure of bR are a crucial element in the mechanism of proton pumping and can be followed by a variety of spectroscopic, and diffraction methods. A number of intermediates in the photocycle have been identified spectroscopically and a number of laboratories have been successful in reporting the structural changes taking place in the later stages of the photocycle over the millisecond time-scale using diffraction techniques. These studies have revealed significant changes in the protein structure, possibly involving changes in flexibility and/or movement of helices. Earlier intermediates which arise and decay on the picosecond to microsecond time-scale have proven more difficult to trap. Here, we report for the first time the successful trapping and diffraction analysis of bR in a low temperature state resembling the very early intermediate, K. We have calculated a projection difference map to 3.5 A resolution. The map reveals no significant structural changes in the molecule, despite having a very low background noise level. This does not rule out the possibility of movements in a direction perpendicular to the plane of the membrane. However, the data are consistent with other evidence that significant structural changes do not occur in the protein itself.

Chemical dynamics in proteins: the photoisomerization of retinal in bacteriorhodopsin

Chemical dynamics in proteins are discussed, with bacteriorhodopsin serving as a model system. Ultrafast time-resolved methods used to probe the chemical dynamics of retinal photoisomerization in bacteriorhodopsin are discussed, along with future prospects for ultrafast time-resolved crystallography. The photoisomerization of retinal in bacteriorhodopsin is far more selective and efficient than in solution, the origins of which are discussed in the context of a three-state model for the photoisomerization reaction coordinate. The chemical dynamics are complex, with the excited-state relaxation exhibiting a multiexponential decay with well-defined rate constants. Possible origins for the two major components are also discussed.