Figure 20.11 Coupled electron–proton transfer reactions through NADH-Q oxidoreductase. Electrons flow in Complex I from NADH through FMN and a series of iron–sulfur clusters to ubiquinone (Q), forming Q²⁻. The charges on Q²⁻ are electrostatically transmitted to hydrophilic amino acid residues (shown as red and blue balls) that power the movement of HL and bH components. This movement changes the conformation of the transmembrane helices and results in the transport of four protons out of the mitochondrial matrix.