Protein-Based Determination of Model Redox Potentials for the High-Potential and Low-Potential Redox Transition of...

Area of Science:

• Biochemistry
• Structural Biology
• Bioinorganic Chemistry

Background:

• Iron-sulfur ([4Fe-4S]) clusters are vital redox-active cofactors in numerous proteins.
• These clusters commonly exist in three redox states: [4Fe-4S]3+, [4Fe-4S]2+, and [4Fe-4S]1+.

Purpose of the Study:

• To determine model redox potentials for [4Fe-4S] cluster transitions using high-resolution crystal structures.
• To investigate the influence of protein environment and solvent accessibility on cluster microstates.

Main Methods:

• Application of the Virtual Model Compound Approach.
• Rerefinement of high-resolution protein crystal structures.
• Calculation of model redox potentials for redox state transitions.

Main Results:

• The model redox potential for the oxidized ([4Fe-4S]3+) to semireduced ([4Fe-4S]2+) transition is +653 mV.
• The model redox potential for the semireduced ([4Fe-4S]2+) to reduced ([4Fe-4S]1+) transition is -157 mV.
• Solvent accessibility influences microstate populations in ferredoxins and nitrogenase iron protein, while buried clusters in high-potential iron-sulfur proteins show uniform microstate populations.

Conclusions:

• Provides accurate redox potentials for [4Fe-4S] cluster transitions, crucial for understanding electron transfer.
• Highlights the role of protein microenvironment in modulating cluster redox properties and stability.
• Introduces a novel chiral volume-based labeling scheme for unambiguous charge assignment in complex clusters.