Fig. 2.

Directional flow of electrons through complex I produce ROS. A Complex I consists of the peripheral and membrane arms. The peripheral arm contains the NADH oxidation site at docking to flavin mononucleotide (FMN, orange) from which, electrons are transferred through a series of iron-sulfur cluster subunits (N1a, N3, N1b, N4, N5, N6a, N6b and N2, yellow) to the ubiquinone (UQ, purple) at the UQ-binding site, neighboring the NDUFS2 subunit. B Electron flow (dashed red lines) from NADH to reduce UQ to UQH₂ through forward electron transfer (FET). Low protonmotive force (Δp) and low UQH₂/UQ during FET promotes the generation of relatively low level of ROS from FMN (site I_F). C Favored by high Δp and high UQH₂/UQ, electrons are transferred from reduced UQH₂ during reverse electron transfer (RET) to generate bursts of ROS from the UQ-binding site (site I_Q).