Complete Charge Regulation by a Redox Enzyme Upon Single Electron Transfer

• Published in: Angewandte Chemie International Edition Vol. 59; no. 27; pp. 10989 - 10995
• Main Authors: Zhang, Ao Yun, Koone, Jordan C., Dashnaw, Chad M., Zahler, Collin T., Shaw, Bryan F.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 26.06.2020
• Edition: International ed. in English
• Subjects: Azurin; Charge transfer; Copper; Cytochrome; Cytochrome c; Cytochromes; Cytochromes c - metabolism; Electron transfer; Electron Transport; electrophoresis; Histidine; Kinetics; Ladders; metalloenzyme; Metalloproteins - metabolism; Myoglobin - metabolism; Myoglobins; Oxidation; Oxidation-Reduction; Protein kinase A; Proteins; Protonation; Single electrons; SOD1; Superoxide dismutase; Superoxide Dismutase - metabolism; copper; electron transfer; electrophoresis; metalloenzyme; SOD1
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202001452

The degree by which metalloproteins partially regulate net charge (Z) upon electron transfer (ET) was recently measured for the first time using “protein charge ladders” of azurin, cytochrome c, and myoglobin [Angew. Chem. Int. Ed. 2018, 57(19), 5364–5368; Angew. Chem. 2018, 130, 5462–5466]. Here, we show that Cu, Zn superoxide dismutase (SOD1) is unique among proteins in its ability to resist changes in net charge upon single ET (e.g., ΔZET(SOD1)=0.05±0.08 per electron, compared to ΔZET(Cyt‐c)=1.19±0.02). This total regulation of net charge by SOD1 is attributed to the protonation of the bridging histidine upon copper reduction, yielding redox centers that are isoelectric at both copper oxidation states. Charge regulation by SOD1 would prevent long range coulombic perturbations to residue pKa’s upon ET at copper, allowing SOD1’s “electrostatic loop” to attract superoxide with equal affinity (at both redox states of copper) during diffusion‐limited reduction and oxidation of superoxide. Complete charge regulation: Capillary electrophoresis and protein charge ladders reveal that SOD1 maintains a constant net charge upon reduction of CuII to CuI. This tight charge regulation enables the protein to guide superoxide to the copper at diffusion limited rates, regardless of metal oxidation state.