Researchers from the Institute of Bioengineering of Catalonia (IBEC) and CIBER-BBN, in collaboration with teams from the University of Barcelona (UB) and the Institute of Chemical Research – cicCartuja, University of Seville-CSIC, have discovered that long-distance charge transport between two key proteins of the mitochondrial respiratory chain—cytochrome c and respiratory complex III—is mediated by protons and the superoxide ion, a reactive oxygen species. The study, recently published in the journal Small, was led by Pau Gorostiza, ICREA Research Professor and head of the Nanosondes and Nanoswitches group at IBEC, with Anna Lagunas, senior researcher in the Nanobioengineering group at the same institute, as first author. Both are also members of CIBER-BBN.
Although this is fundamental research, the impact of the work is twofold: on the one hand, it helps to better understand the regulation of cellular respiration, and on the other, it could inspire new applications in emerging fields. “Understanding these basic mechanisms is essential, because mitochondria are the cell’s power plants and their dysfunction is linked to many diseases,” explains Lagunas. “In addition, this finding could inspire the development of new protonic devices, analogous to electronic ones but operating with positive charge,” adds Gorostiza.
This work culminates a line of research that the teams have been developing collaboratively for years. In 2018, the first results were published, when the researchers demonstrated for the first time that two proteins could transfer electrons over surprisingly long distances through an aqueous solution, without the need to form a stable complex. Subsequently, in 2022, a second study revealed how phosphorylation regulates this process and its relevance in cellular signaling. The new article completes this “trilogy,” in the words of Pau Gorostiza: “They are three pieces of the same story. This chapter of the series leaves some intriguing questions open, but it gives us a much broader perspective on how this fundamental mechanism may work.”
While electron transport within a single protein or complex has been studied in detail using structural and functional techniques, transfer between proteins still holds some mysteries. This is due to the presence of the aqueous solvent and the dynamic and transient nature of both the molecular interaction between proteins and the electron transfer event itself. Precisely, the new work from IBEC addresses this challenge using nanometric and single-protein techniques, allowing the observation of a fundamental process that until now had been very difficult to investigate with macroscopic techniques.
The fundamental role of protons
To uncover who the “mediator” of this long-distance electronic transport through water is, the team carried out several experiments. First, they varied the acidity (that is, the proton concentration) of the solution within ranges compatible with protein stability and observed that transport was more efficient under slightly acidic conditions (with more protons). Next, they replaced the usual medium with heavy water—a variant of water in which hydrogen atoms are replaced by deuterium, a heavier form of this element—and found that deuterium hindered the process. Both results point to the essential role of protons in charge transport. Finally, they repeated the measurements in solutions with different concentrations of dissolved oxygen and found that the absence of oxygen shortened the transport distance between the proteins.
As Lagunas explains: “These results indicate that protons and oxygen play a central role in this mechanism. Everything points to a proton-coupled electron transfer (PCET) process, in which the exchange of an electron is intimately linked to that of a proton, and which could involve proton transport mechanisms such as the Grotthuss mechanism, where chains of water molecules pass the proton along as if handing it from one to another.”
The researchers also suggest that the superoxide anion, a reactive oxygen species that is naturally produced in complex III and is relatively stable, could act as a mediator in this process.
From a biological perspective, Professors Irene Díaz-Moreno and Miguel A. De la Rosa highlight the relevance of this finding. They note that “the efficiency of mitochondrial respiration directly determines the cell’s ability to produce ATP, the ‘energy currency’ that sustains all vital processes. In a crowded cellular environment, optimizing electron transfer is essential to make the most of the available energy, avoiding losses and reducing the uncontrolled production of reactive oxygen species. Therefore, understanding how protons and superoxide mediate long-distance transfer not only expands basic knowledge, but also provides clues to how cellular energy efficiency is regulated and what happens when this process is disrupted in metabolic and degenerative diseases.”
Reference article:
Anna Lagunas, Alexandre M. J. Gomila, Alba Nin-Hill, Alejandra Guerra-Castellano, Gonzalo Pérez-Mejías, Josep Samitier, Carme Rovira, Miguel A. De la Rosa, Irene Díaz-Moreno, Pau Gorostiza. Long-Distance Charge Transport between Cytochrome c and Complex III is Mediated by Protons and Reactive Oxygen Species. Small (2025). DOI: 10.1002/smll.202501286