Fundamental Electrostatics Explains Why Ferredoxin- and HiPIP-Type Proteins Favor Different Redox Transitions in...

Area of Science:

• Biochemistry and Biophysics
• Bioinorganic Chemistry
• Protein Science

Background:

• [4Fe-4S] clusters are crucial cofactors in proteins, existing in three redox states: [4Fe-4S]³⁺, [4Fe-4S]²⁺, and [4Fe-4S]¹⁺.
• Proteins with [4Fe-4S] clusters exhibit two potential redox transitions: low-potential (LPT) and high-potential (HPT).
• Ferredoxins (Fds) use LPT, while high-potential iron-sulfur proteins (HiPIPs) use HPT, despite sharing the same inorganic cofactor.

Purpose of the Study:

• To investigate the physical basis for the distinct redox behaviors of Fds and HiPIPs.
• To understand how protein environments differentiate the LPT and HPT for [4Fe-4S] clusters.

Main Methods:

• Continuum electrostatics calculations were employed to determine redox potentials.
• Calculations were performed on representative high-resolution protein structures of Fds and HiPIPs.
• The study analyzed the impact of cluster transfer from water to protein and protein background charges.

Main Results:

• Transferring [4Fe-4S] clusters into proteins lowers redox potentials, with a more significant shift for LPT due to desolvation penalties.
• Protein background charges, including peptide bond dipoles, shift both transitions similarly but stabilize the reduced state more in Fds.
• These environmental effects result in a ~1 V separation between LPT and HPT, aligning native transitions with physiological windows.

Conclusions:

• The distinct redox behavior of Fds and HiPIPs arises from specific protein microenvironments.
• Protein electrostatics fine-tune the [4Fe-4S] cluster redox potentials, positioning one transition physiologically and excluding the other.
• This mechanism ensures the functional specificity of ferredoxins and HiPIPs in biological redox processes.