Modulating Radical Propagation in Proteins by Proton-Coupled Electron Transfer and Hydrogen Bonding

Abstract

Long-range protein electron transfer (ET) often depends on tryptophan and tyrosine residues acting as radical relay sites. For example, cytochrome c peroxidase (CcP) generates a W191^•+ radical to increase ET from cytochrome c (Cc) to the active center. W191 substitution to Tyr reduces ET rates, but introduction of an adjacent general base (as Glu or His) at position 232 (Y191:E/H232 CcP) recovers activity. E232 fluorination lowers the pK_a of the conjugate base and confirms that a hydrogen bond is critical to elevate the Y191^• formal potential for effective ET. Photoinitiated ET between Zn-porphyrin (ZnP) CcP (ZnCcP) and Cc also depends on activating Y191 with a basic residue but through a different mechanism than for the peroxide-driven system. In ZnCcP, pH dependencies and solvent isotope effects indicate that proton-coupled electron transfer to the basic residue and ZnP^•+, respectively, facilitates Y191^• formation. Replacing Cc with the irreversible oxidant [Co(NH3)₅Cl]²⁺ isolates distinct protein radicals for characterization by electron paramagnetic resonance (EPR) spectroscopy. Radical distributions and computation indicate that W191^•+ lies close in potential to ZnP^•+ and that the two radicals exchange on a slow time scale despite their close separation. Remarkably, Y191:E/H232 ZnCcP variants propagate radicals differently to peripheral sites depending on the nature of the 232 residue. QM/MM calculations support radical exchange between ZnP^•+/Trp^•+ and the importance of a hydrogen bond to Y191^• for maintaining a high potential to oxidize peripheral donors. These resolved reactivity patterns of CcP/ZnCcP have general relevance for engineering proton management to separate and migrate charge in proteins and potentially other molecular systems.