A study led by the Institute of Bioengineering of Catalonia (IBEC) reveals that protons and reactive oxygen species act as mediators in long-distance charge transport within the mitochondrial respiratory chain, a fundamental process in the cellular respiration of organisms. Understanding these mechanisms is crucial, as mitochondria act as the powerhouses of all cells and their dysfunction is associated with numerous diseases.
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 in 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, an ICREA Research Professor and head of the Nanoprobes and Nanoswitches group at IBEC, with Anna Lagunas, a senior researcher in the Nanobioengineering group at the same centre, as first author. Both are also members of CIBER-BBN.
Although this is fundamental research, the impact of this 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 powerhouses of cells and their dysfunction is linked to many diseases,” explains Lagunas. “Furthermore, this finding could inspire the development of new proton devices, equivalent to electronic devices but operating with a positive charge,” adds Gorostiza.
This work culminates a line of research that the teams have been developing collaboratively for years, the first results of which were published in 2018, when the researchers demonstrated for the first time that two proteins could transfer electrons over surprisingly long distances through an aqueous solution, without needing to form a stable complex. Subsequently, in 2022, a second study was published revealing how phosphorylation regulates this process and its relevance to cell signalling. The new paper completes this ‘trilogy’, in the words of Pau Gorostiza: ‘They are three parts of the same story. This chapter in the series leaves some intriguing questions unanswered but gives us a much broader perspective on how this fundamental mechanism might work.’
Although 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. Indeed, the new work from IBEC tackles this challenge using nanoscale and single-protein techniques, enabling the observation of a fundamental process that until now had been very difficult to investigate using macroscopic techniques.
The key role of protons
To discover what acts as the ‘mediator’ in this long-distance transport of electrons through water, the team carried out several experiments. Firstly, they varied the acidity (i.e. the concentration of protons) of the solution within a range compatible with protein stability and observed that transport was more efficient under slightly acidic conditions (with more protons). They then replaced the usual medium with heavy water — a variant of water in which the 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, where the exchange of an electron is closely linked to that of a proton, and which could involve proton transport mechanisms such as the Grotthuss mechanism, in which chains of water molecules pass the proton along as if they were holding hands’.
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 emphasise the significance of this finding. They point out 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, optimising 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 advances our fundamental knowledge but also provides insights into 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