Charge Regulation in a Rieske Proton Pump Pinpoints Zero, One, and Two Proton-Coupled Electron Transfer.

The degree to which redox-driven proton pumps regulate net charge during electron transfer (ΔZ) remains undetermined due to difficulties in measuring the net charge of solvated proteins. Values of ΔZ can reflect reorganization energies or redox potentials associated with ET and can be used to distinguish ET from proton(s)-coupled electron transfer (PCET). Here, we synthesized protein "charge ladders" of a Rieske [2Fe-2S] subunit from (truncRp) and made 120 electrostatic measurements of ΔZ across pH. Across pH 5-10, truncRp is suspected of transitioning from ET to PCET, and then to two proton-coupled ET (2PCET). Upon reduction, we found that truncRp became more negative at pH 6.0 by one unit (ΔZ = -1.01 ± 0.14), consistent with single ET; was isoelectric at pH 8.8 (ΔZ = -0.01 ± 0.45), consistent with PCET; and became more positive at pH 10.6 (ΔZ = +1.37 ± 0.60), consistent with 2PCET. These ΔZ values are attributed to protonation of H154 and H134. Across pH, redox potentials of Rp (measured previously) correlated with protonation energies of H154 and H134 and ΔZ for truncRp, supporting a discrete proton pumping mechanism for Rieske proteins at the Fe-coordinating histidines.