Cytochrome c, a hub linking energy, redox, stress and signaling pathways in mitochondria and other cell compartments

Cytochrome c ( CYT c ) is a soluble redox‐active heme protein that transfers electrons from complex III to complex IV in the cyanide‐sensitive mitochondrial respiratory pathway. CYT c biogenesis is a complex process that requires multiple steps until the mature active protein is obtained. CYT c levels and activity are finely regulated, revealing the importance of this protein not only as electron carrier but also in many other processes. In this article, we describe the role of CYT c in mitochondrial respiration, from its canonical role as electron carrier for ATP production to its involvement in protein import and the stabilization of respiratory complexes and supercomplexes. In plants, CYT c is connected to the synthesis of the antioxidant ascorbate and the detoxification of toxic compounds. Finally, CYT c is also a multi‐functional signaling molecule that influences the balance between life and death, acting in energy provision for cellular functions or triggering programmed cell death. The confluence of several metabolic routes into a single protein that links redox reactions with energy producing pathways seems logical from the point of view of cellular economy, control and organization.