Fourier transform infrared difference spectroscopy of bacteriorhodopsin and its photoproducts.

Abstract

Fourier transform infrared difference spectroscopy has been used to obtain the vibrational modes. in the chro-*mophore and apoprotein 'that change in intensity or position between light-adapted bacteriorhodopsin and the K and M-intermediates in its photocycle and between dark-adapted and lightadapted bacteriorhodopsin. Our infrared' measurements provide independent verification of resonance Raman results that in lightadapted bacteriorhodopsin the protein-chromophore linkage is a protonated Schiff base and in the M state the Schiff base is un-,protonated. Although we cannot unambiguously identify the Schiff base stretching frequency in the K state, the most'likely interpretation of deuterium shifts of the chromophore hydrogen out-ofplane vibrations is that the Schiff base in K is protonated. The intensity of the hydrogen out-of-plane vibrations in the K state compared with the intensities of.those in light-adapted and'darkadapted bacteriorhodopsin shows that the conformation of the chromophore in K is considerably distorted. In addition, we find evidence that the conformation of the protein changes during the photocycle.Bacteriorhodopsin (bR) is the light-energy transducing protein found in the purple membrane (PM) of the extreme halophile Halobacterium halobium (1-4). The chromophore in bacteriorhodopsin is a single molecule of retinal, covalently bound to the £-amino group of a lysine (Lys-216) via a Schiffbase linkage (Fig 1). Upon absorption of light, the light-adapted form of bR (bR ) undergoes a photocycle, bRLA +--* K --L --M -O 0 bRLA, during which protons are pumped from the inside of the cell to the extracellular medium. The resulting proton gradient is used by the cell to generate chemical energy in the form ofATP and drive other energy-requiring processes. In the dark, bRLA thermally converts to the dark-adapted form of bR (bRDA).The mechanism of this light driven proton pump has been studied by using visible and ultraviolet, resonance Raman (5), and infrared (IR) (6-8) spectroscopies and chemical extraction techniques. These investigations strongly suggest that during the photocycle changes occur in both the isomeric state of the chromophore and the state ofprotonation of the Schiff'base. In particular, chemical extraction experiments have provided evidence that the chromophore in bRLA is in an all-trans configuration, that in the L and M states it is in a 13-cis configuration, and that in bRDA the chromophore exists in two isomeric forms, all-trans and 13-cis, in a ratio of approximately 1: 1 (9-11).Evidence for the conformation of the chromophore in situ comes primarily from comparisons between the resonance Raman vibrational spectra in both 'H20 and 2H20 of native bR, bR in which analogs ofretinal have been incorporated, and retinal 'Schiff bases. Analysis of the results from such work is dif-CH3 CH3

Proton currents and protein motion in membranes

“…Hydrogen exchange experiments [26] could test for rapid exchange of the peptide protons in active membrane-spanning proteins. Fourier-transform infrared spectroscopy on the a-helical membrane-4 spanning protein bacteriorhodopsin shows unusual amide signals that have been attributed to altered conformation or orientation of the cy-helices [10,27,28], but which might be consistent with the altered w and unit twist suggested here. High resolution structural studies of membrane proteins would show any unusual conformation in the cyhelices.…”

Infrared-Detectable Group Senses Changes in Charge Density on the Nickel Center in Hydrogenase from Chromatium vinosum

“…Such bands are absent from the low-temperature spectra of myoglobin (Alben & Caughey, 1968), cytochrome oxidase (Fiamingo et al, 1981), rhodopsin (Bagley et al, 1985(Bagley et al, , 1989, and bacteriorhodopsin (Bagley et al, 1982). Such bands are also absent from the infrared spectra of another Ni,Fe enzyme, CO dehydrogenase (Kumar & Ragsdale, 1992).…”

The synthesis of 13C-labelled retinals

“…Recently FTlRdifference spectroscopy has also been applied to acquire vibrational difference spectra between the intermediates and the starting material (Ref. 4). In addition to information about the chromophore this technique gives information about the part of the protein that is subject to changes when going from starting material to the intermediates.…”