BPS2026 – Do metalloproteins regulate net charge to control the energetics of electron transfer?

When and to what extent a metalloprotein broadcasts the charge state of its active site in intermolecular interactions depends in ways not quantitatively understood on the number, composition, and distribution of protonatable residues around a redox-active site. Positions of the protonation equilibria at a given pH set the net charge of the protein (Q net ) and also shift in response to a change in electrostatic potential from oxidation/reduction at the active site, a phenomenon dubbed charge regulation capacitance (C reg ). Furthermore, Q net and C reg are claimed to influence the redox potential (E°) and reorganization energy (λ), two components of the electron transfer activation barrier, but the relationships are obscured by the use of highly dissimilar proteins in prior studies. Here, a new method, called Empirical Charge Capacitance (ECC), is developed to rapidly assess Q net and C reg at any pH, and validated against experimental data for C reg in Pseudomonas aeruginosa azurin variants. The method is then applied to an orthologous series of structurally conserved high potential iron sulfur proteins (HiPIPs) that have previously reported E°s from 93–458 mV vs. SHE and predicted Q net from -12 to +5e. Strong correlations (r > |0.720|, p = 0.026) at pH 7 are found among E°, Q net , and C reg . Notably, HiPIPs from alkaliphilic/halophilic organisms exhibit negative Q net with high C reg and low E°, while HiPIPs from neutrophilic organisms show positive Q net with low C reg and high E°. Just as alkaliphilic HiPIPs evolved increased negative charge and shielding capacity for their environment, cancer cells that overexpress Arg-to-His substitutions may exploit similar charge modifications to dysregulate redox signaling to promote survival at elevated pH conditions. The relationship to λ is under examination.