FIGURE 12-23 Electron and proton transport through complexes I and II. (a) Model of complex I based on its three-dimensional structure. The outline of the shape of the core complex I from the yeast Y. lipolytica, as determined by x-ray crystallography, is shown in light blue, and the borders separating several distinct structural subunits are indicated by thin dashed black lines. From NADH, electrons flow first to a flavin mononucleotide (FMN) and then, via iron-sulfur clusters (Fe-S, red and yellow balls), to CoQ, to which two protons from the matrix bind (red arrow) to form CoQH₂. Conformational changes due to the electron flow, which may be induced by changes in the charge of the CoQ and may include a piston-like horizontal movement of the t-helix, drive proton pumping through the transmembrane subunits from the matrix to the intramembrane space (red arrows). (b) Model of complex II based on its three-dimensional structure. Electrons flow through complex II from succinate to CoQ via FAD/FADH₂ and iron-sulfur clusters (Fe-S), and from complex II to complex III via CoQ/CoQH₂. Electrons released during oxidation of succinate to fumarate in complex II are used to reduce CoQ to CoQH₂ without translocating additional protons.

[Part (a) data from V. Zickermann et al., 2015, Science 347:44–49, PDB ID 3m9s. Part (b) data from F. Sun et al., 2005, Cell 121:1043–1057, PDB ID 1zoy.]