Thermodynamic Driving Forces of Redox-Dependent CPR Insertion into Biomimetic Endoplasmic Reticulum Membranes

Area of Science:

• Biochemistry
• Biophysics
• Membrane Biology

Background:

• Cytochrome P450 reductase (CPR) is a key NADPH-dependent oxidoreductase in the endoplasmic reticulum (ER).
• CPR is the primary redox partner for most cytochrome P450 enzymes, crucial for various metabolic processes.
• Understanding CPR's interaction with the ER membrane is vital for comprehending P450 function.

Purpose of the Study:

• To quantify the thermodynamic driving forces governing CPR's partitioning into a biomimetic ER membrane.
• To investigate how the redox state of CPR influences its membrane association and insertion thermodynamics.
• To develop a model explaining CPR-membrane interactions based on redox state-dependent properties.

Main Methods:

• Utilized temperature-dependent fluorescence correlation spectroscopy.
• Employed fluorescence single-protein tracking techniques.
• Measured standard state free energies, enthalpies, and entropies of CPR insertion.

Main Results:

• CPR partitioning thermodynamics are significantly dependent on its oxidized (CPR ^ox) and reduced (CPR ^red) states.
• CPR ^ox insertion is exothermic with a small positive entropy change, while CPR ^red insertion is endothermic with a large positive entropy change.
• Both states exhibit strong membrane association (negative free energy), but CPR ^red shows a higher partition coefficient (K _P).

Conclusions:

• The redox state critically modulates CPR's thermodynamic behavior and interaction with ER membranes.
• Distinct thermodynamic profiles for CPR ^ox and CPR ^red suggest different insertion mechanisms and membrane orientations.
• A phenomenological model integrating membrane interactions, orientation, and conformation as a function of redox state was developed.