Mitochondrial complex I as a master regulator of redox signaling: From structural architecture to directionality of...

Mitochondrial complex I (MCI) is the largest enzyme of the electron transport chain, catalyzing oxidation of NADH, reduction of ubiquinone, and translocation of protons across the inner mitochondrial membrane (IMM). In addition to driving ATP synthesis through oxidative phosphorylation (OxPhos), MCI is a dynamic redox regulator that couples bidirectional catalysis with redox signaling. MCI conducts electron transfer in both the forward and reverse directions. While forward electron transport (FET) is essential for OxPhos and ATP synthesis, reverse electron transport (RET), driven by high membrane potential and ubiquinol pool, transfers electrons from ubiquinol to NAD⁺ and produces excessive ROS. MCI-derived ROS and NAD⁺/NADH changes act as physiologically regulated signals mediating hypoxia sensing, immune activation, stem-cell metabolism, but they can also contribute to pathology when dysregulated as in ischemia-reperfusion, cancer, neurodegeneration, and aging. Recent cryo-EM structures, time-resolved studies, and multiscale molecular dynamics (MD) simulations have provided near-atomic views of MCI architecture and operational mechanics. Here we review these developments from a redox-centered perspective. By positioning MCI as a dynamic redox regulator within a spatially organized mitochondrial network, we aim to provide a unifying framework for understanding how directional electron transfer, proton translocation, and redox signaling are intertwined in mitochondrial biology.