Infrared and Visual Absolute and Difference Spectra of Bacteriorhodopsin Photocycle Intermediates Part 2: Interpretation of IR Difference Spectra

Published

September 1, 2011

Author(s)

Curtis W. Meuse, R W. Hendler, Mark S. Braiman

Abstract

We have re-examined the sequential proton-binding and -releasing amino acid side chains involved in the energy-driven transport of protons across the membrane protein bacteriorhodopsin (bR), by using new procedures for obtaining IR absolute spectra for intermediates of the bacteriorhodopsin photocycle described here and in Part 1 of this series. In addition, we have quantified the extent of changes in conformation or environment of bR accompanying each step in the photocycle. In general, our results confirm the consensus view on the major steps in proton translocation that were based on earlier IR studies, even though those earlier studies relied on partial enrichment techniques, rather than the kinetic isolation of pure intermediate spectra as in the current work. However, in comparing specific spectral signals associated with protonation changes of various candidate groups, to the known kinetics of proton release, our results uniquely support the identification of Arg82 as the primary proton-release group (PRG), rather than a protonated water cluster or H-bonded grouping of carboxylic residues. Notably, the Arg82 deprotonation occurs exclusively in the MF pathway of the parallel cycles model of the photocycle. Our results support the view that the MF pathway preferentially leads to the formation of ΔpH gradient as opposed to the MS pathway, which primarily builds membrane potential (Δψ).

Citation

Biochemistry

Volume

65

Issue

9

Pub Type

Journals

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DOI Link

Keywords

time resolved infrared spectroscopy, step scan, bacteriorhodopsin, photocycle, singular value decomposition, kinetics

Citation

Meuse, C. , Hendler, R. and Braiman, M. (2011), Infrared and Visual Absolute and Difference Spectra of Bacteriorhodopsin Photocycle Intermediates Part 2: Interpretation of IR Difference Spectra, Biochemistry, [online], https://doi.org/10.1366/11-06302 (Accessed April 26, 2024)