Na+ translocation by the NADH:ubiquinone oxidoreductase (complex I) from Klebsiella pneumoniae

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Abstract
Complex I is the site for electrons entering the respiratory chain and therefore of prime importance for the conservation of cell energy. It is generally accepted that the complex I‐catalysed oxidation of NADH by ubiquinone is coupled specifically to proton translocation across the membrane. In variance to this view, we show here that complex I of Klebsiella pneumoniae operates as a primary Na+ pump. Membranes from Klebsiella pneumoniae catalysed Na+‐stimulated electron transfer from NADH or deaminoNADH to ubiquinone‐1 (0.1–0.2 μmol min−1 mg−1). Upon NADH or deaminoNADH oxidation, Na+ ions were transported into the lumen of inverted membrane vesicles. Rate and extent of Na+ transport were significantly enhanced by the uncoupler carbonylcyanide‐m‐chlorophenylhydrazone (CCCP) to values of ≈0.2 μmol min−1 mg−1 protein. This characterizes the responsible enzyme as a primary Na+ pump. The uptake of sodium ions was severely inhibited by the complex I‐specific inhibitor rotenone with deaminoNADH or NADH as substrate. N‐terminal amino acid sequence analyses of the partially purified Na+‐stimulated NADH:ubiquinone oxidoreductase from K. pneumoniae revealed that two polypeptides were highly similar to the NuoF and NuoG subunits from the H+‐translocating NADH:ubiquinone oxidoreductases from enterobacteria.

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