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Proteorhodopsin is a light-driven proton pump with variable vectoriality.

Thomas Friedrich1, Sven Geibel, Rolf Kalmbach

  • 1Max-Planck-Institute of Biophysics, Department of Biophysical Chemistry, Kennedyallee 70, D-60596 Frankfurt am Main, Germany.

Journal of Molecular Biology
|September 11, 2002
PubMed
Summary
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Proteorhodopsin, a marine bacterial protein, functions as a light-driven proton pump. Its proton pumping direction reverses at acidic pH due to Asp97 protonation, unlike its alkaline pH function.

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Marine Microbiology

Background:

  • Proteorhodopsin is a light-activated ion pump found in marine bacteria.
  • It is a homologue of archaeal bacteriorhodopsin (BR) and plays a role in microbial energy metabolism.
  • Understanding its function is crucial for comprehending marine microbial energy balance.

Purpose of the Study:

  • To clone, express, and functionally characterize proteorhodopsin.
  • To investigate the photocycle characteristics and pH-dependence of proteorhodopsin.
  • To determine the mechanism and directionality of proton pumping.

Main Methods:

  • Chemical gene synthesis and heterologous expression in Escherichia coli.
  • Protein purification and reconstitution into proteoliposomes.

Related Experiment Videos

  • Time-resolved absorption spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and electrophysiology (voltage clamp).

    • Main Results:

    • Proteorhodopsin exhibits a photocycle similar to BR, with pH-dependent formation of intermediates.
    • The pK(a) of Asp97 was determined to be 7.68.
    • Light-induced proton pumping was observed, with an inverted direction at acidic pH compared to alkaline pH, linked to Asp97 protonation.

    Conclusions:

    • Proteorhodopsin functions as a light-driven proton pump, analogous to BR at alkaline pH.
    • The proton pumping direction is reversible and dependent on the protonation state of Asp97.
    • This reversible proton pumping mechanism has implications for energy transduction in marine environments.