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Reversible steps in the bacteriorhodopsin photocycle.

R H Lozier1, A Xie, J Hofrichter

  • 1Laboratory of Chemical Physics, National Institutes of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892.

Proceedings of the National Academy of Sciences of the United States of America
|April 15, 1992
PubMed
Summary

The light-driven proton-pumping cycle of bacteriorhodopsin (bR) was modeled using kinetic analysis. Including back-reactions improved data fitting, but suggested the model is incomplete, possibly due to unobserved states.

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Area of Science:

  • Biophysics
  • Photochemistry
  • Protein Dynamics

Background:

  • Bacteriorhodopsin (bR) is a light-driven proton pump crucial for cellular energy generation.
  • Understanding its photocycle kinetics is key to elucidating energy transduction mechanisms.

Purpose of the Study:

  • To analyze the kinetic model of the bacteriorhodopsin photocycle using absorbance changes.
  • To investigate the role of back-reactions and temperature/pH dependencies in the photocycle.

Main Methods:

  • Global fitting of absorbance changes across various times, wavelengths, temperatures, and pH.
  • Optimization of rate constants, activation energies, and extinction coefficients.
  • Comparison of reversible and irreversible kinetic models.

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Main Results:

  • A kinetic model incorporating back-reactions provided a good fit to experimental data.
  • Optimized parameters included rate constants at 20°C, activation energies, and intermediate extinction coefficients.
  • Back-reactions were essential for accurate data fitting, outperforming irreversible models.

Conclusions:

  • The current kinetic model, while improved by back-reactions, remains an incomplete description of the bacteriorhodopsin photocycle.
  • Discrepancies suggest the existence of additional, spectrally similar intermediate states.
  • Further investigation is needed to fully resolve the photocycle dynamics and identify all transient species.