Hydrostatic High-Pressure-Induced Denaturation of LH2 Membrane Proteins

Area of Science:

• Biophysics
• Structural Biology
• Membrane Protein Biochemistry

Background:

• High pressure (HP) denaturation of globular proteins often involves void release and hydrophobic interior exposure.
• Limited data exists on HP effects on membrane proteins, particularly their structural response.
• Light-harvesting 2 (LH2) complexes are integral membrane proteins crucial for photosynthesis.

Purpose of the Study:

• To investigate the impact of hydrostatic pressures up to 12 kbar on LH2 complexes from purple photosynthetic bacteria.
• To compare the pressure resilience of LH2 complexes from a pressure-sensitive mutant *Rhodobacter sphaeroides* and a pressure-robust *Thermochromatium tepidum*.
• To elucidate the role of protein structure versus hydration in pressure-induced denaturation of membrane proteins.

Main Methods:

• Utilized innate chromophores (bacteriochlorophyll-a and tryptophan) within LH2 complexes to monitor structural changes.
• Applied hydrostatic pressures up to 12 kbar to bacterial LH2 complexes.
• Analyzed changes in bacteriochlorophyll-a absorption spectra and tryptophan fluorescence quenching.
• Employed atomistic model structures for detailed structural analysis.

Main Results:

• In pressure-sensitive *R. sphaeroides* LH2 complexes, a correlation was observed between bacteriochlorophyll-a blue shift (tertiary structure breakage) and tryptophan fluorescence quenching (protein solvation).
• No such correlated effects were observed in the pressure-robust *T. tepidum* LH2 complexes.
• Atomistic model structures confirmed the critical role of the protein's intrinsic structure in pressure-induced denaturation.

Conclusions:

• Pressure-induced denaturation of membrane proteins is critically dependent on the protein's intrinsic structure, not solely hydration.
• The study provides insights into the differential pressure resilience of membrane protein complexes.
• Understanding structural mechanisms of pressure denaturation is vital for membrane protein stability studies.