The nuo genes encode NADH dehydrogenase I (NDH-1), a 13-subunit enzyme complex in Escherichia coli that transfers electrons from NADH to the quinone pool while generating a proton motive force, with a stoichiometry of two protons translocated per electron transferred (H⁺/e⁻ = 2). Inactivation of NDH-1, as in Δnuo mutants, increases the intracellular NADH/NAD⁺ ratio, shifts energy metabolism toward substrate-level phosphorylation, and improves the production of reduced metabolites such as 2,3-butanediol and isobutanol [356].