Complete Charge Regulation by a Redox Enzyme Upon Single Electron Transfer.

Area of Science:

• Biochemistry
• Biophysics
• Protein Chemistry

Background:

• Metalloproteins regulate net charge (Z) during electron transfer (ET).
• Previous studies used "protein charge ladders" to measure charge regulation in azurin, cytochrome c, and myoglobin.
• The charge regulation capacity of Cu, Zn superoxide dismutase (SOD1) was previously uncharacterized.

Purpose of the Study:

• To investigate the charge regulation properties of SOD1 upon electron transfer.
• To compare SOD1's charge regulation to other metalloproteins.
• To elucidate the mechanism behind SOD1's unique charge regulation.

Main Methods:

• Utilized "protein charge ladders" to assess charge changes upon electron transfer.
• Quantified the net charge change per electron transferred in SOD1.
• Analyzed the protonation state of the bridging histidine in SOD1 during copper reduction.

Main Results:

• SOD1 exhibits exceptional resistance to net charge changes upon single electron transfer (ΔZ_ET(SOD1) =0.05±0.08 per electron).
• This contrasts significantly with other proteins like cytochrome c (ΔZ_ET(Cyt-c) =1.19±0.02).
• Protonation of the bridging histidine upon copper reduction maintains isoelectricity at both copper oxidation states.

Conclusions:

• SOD1's unique charge regulation prevents long-range electrostatic perturbations to residue pK_a's.
• This mechanism ensures consistent superoxide attraction by SOD1's "electrostatic loop" across redox states.
• Enables efficient superoxide dismutation essential for cellular protection against oxidative stress.