Thermodynamic Basis of Temperature Adaptation in Three Outward Proton Pump Rhodopsins Distributed Across Diverse...

Area of Science:

• Biophysics
• Environmental Microbiology
• Protein Biochemistry

Background:

• Microbial rhodopsins are vital for solar energy conversion via light-driven proton pumping.
• Understanding their function across temperatures is crucial for ecosystem insights.
• Thermodynamic principles governing rhodopsin function at varied temperatures are poorly understood.

Purpose of the Study:

• To investigate the thermodynamic basis of proton pump rhodopsin function across different temperature environments.
• To analyze the photocycle kinetics and activation parameters of microbial rhodopsins from distinct thermal habitats.
• To elucidate the molecular adaptation strategies enabling proton transport in diverse thermal conditions.

Main Methods:

• Flash photolysis was used to study the kinetics of three proton pump rhodopsins (HnPR, PR, TR) across a wide temperature range.
• Transition-state theory was applied to determine thermodynamic activation parameters for each photocycle transition.
• Comparative analysis of kinetic and thermodynamic data was performed.

Main Results:

• The P1 → P2 transition's thermodynamic balance (enthalpy vs. entropy) correlates with the rhodopsin's native environment temperature.
• TR showed temperature-dependent photocycle alterations, including shifts in the P2 → P3 transition kinetics.
• Cold-adapted HnPR exhibited reduced activation enthalpy and entropy in the P4 → P0 transition, facilitating low-temperature function.

Conclusions:

• A thermodynamic framework explains how outward proton pump rhodopsins function across diverse thermal habitats.
• Distinct molecular strategies balance protein dynamics and stability for light-driven proton transport.
• These findings highlight microbial rhodopsin adaptation to environmental temperature extremes.