| Complex I (NADH:ubiquinone oxidoreductase) is the largest, L-shaped enzyme complex in the mitochondrial inner membrane, comprising about 45 subunits, with 14 core subunits conserved across species. Its hydrophilic peripheral arm contains the FMN cofactor and a relay of 8–9 iron-sulfur (FeS) clusters (such as N3, N1b, N4, N5, N6a/b, N2) for electron transfer, while the membrane arm features four proton-pumping modules (P_D, P_P, P_NuoL, P_NuoM) powered by charged residues and helical rearrangements. Electrons from matrix NADH enter at the N-module (NDUFV1/2), traverse ~90 Å along the FeS clusters to the Q-module’s N2 [4Fe-4S] cluster near NDUFS2/7, and reduce ubiquinone in a ~30 Å tunnel via semiquinone intermediates. The redox reaction (~-150 mV driving force) is tightly coupled to vectorial translocation of four protons per two electrons through conformational waves, ubiquinone binding elicits a two-state stabilization, rigidifying the complex and propagating allosteric shifts from the Q-site to distal pumps via antiporter-like helices (TMH 38–42 in ND2/4/5), thereby generating membrane potential (Δψ) for ATP synthesis and minimizing superoxide production by keeping the N2-Q distance short (~7 Å). As the entry point for oxidative phosphorylation, Complex I maintains cellular energy balance, links the TCA cycle and NADH homeostasis, and influences ROS signaling and mitochondrial DNA maintenance. Dysfunction due to mutations (e.g., NDUFS4, ND1) or assembly defects leads to elevated ROS, impaired bioenergetics, and diseases such as Leigh syndrome, MELAS, Parkinson’s disease (via α-synuclein inhibition), and Leber’s hereditary optic neuropathy, particularly impacting neurons with high energy demands. |