A modular mechanism determines the proton-pumping stoichiometry of respiratory complex I

Respiratory complex I, a crucial metabolic enzyme, uses the reducing potential of NADH to pump protons across energy-transducing membranes, but its mechanism of catalysis remains unknown. Here, guided by detailed structural information, we combine targeted mutagenesis and biophysical analyses to define the proton-pumping pathways in the three antiporter-like (proton-pumping) subunits of complex I from Paracoccus denitrificans and Escherichia coli . In Pd -CI, a four-proton pump, one of the three proton-uptake pathways is inactive, reconciling the four-proton/three-subunit mismatch. In Ec -CI two pathways are inactive – and Ec -CI pumps only two protons. We define the structural determinants for redox-coupled proton uptake, establish a mechanistic framework for proton pumping, and reveal how the proton-pumping stoichiometry has been adapted to deliver efficient energy conservation in diverse bioenergetic environments.