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 Q^2-. The charges on Q^2- are electrostatically transmitted to hydrophilic amino acid residues (shown as red (glutamate) and blue (lysine or histidine) balls) that power the movement of HL and βH components. This movement changes the conformation of the transmembrane helices and results in the transport of four protons out of the mitochondrial matrix.

[Information from R. Baradaran et al., Nature 494:443–448.]