The proton-pumping NADH:ubiquinone oxidoreductase (complex I) couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. Electron transfer is accomplished by FMN and a series of iron-sulfur clusters. Its coupling with proton translocation is not yet understood. Here, we report that the redox reaction of the FeS cluster N2 located on subunit NuoB of the Escherichia coli complex I induces a protonation/deprotonation of tyrosine side chains. Electrochemically induced FT-IR difference spectra revealed characteristic tyrosine signals at 1,515 and 1,498 cm ؊1 for the protonated and deprotonated form, respectively. Mutants of three conserved tyrosines on NuoB were generated by complementing a chromosomal in-frame deletion strain with nuoB on a plasmid. Though the single mutations did not alter the electron transport activity of complex I, the EPR signal of cluster N2 was slightly shifted. The tyrosine signals detected by FT-IR spectroscopy were roughly halved in the mutants Y114C and Y139C while only minor changes were detected in the Y154H mutant. The enzymatic activity of the Y114C/Y139F double mutant was 80% reduced, and FT-IR difference spectra of the double mutant revealed a complete loss the modes characteristic for protonation reactions of tyrosines. Therefore, we propose that tyrosines 114 and 139 on NuoB were protonated upon reduction of cluster N2 and were thus involved in the proton-transfer reaction coupled with its redox reaction.The NADH:ubiquinone oxidoreductase, also known as respiratory complex I, links the electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. By this means, complex I establishes a proton motive force required for energy consuming processes (1-3). Homologues of the complex are present in archaea, bacteria, and eukaryotes (4, 5). Complex I from bacteria generally consists of 14 different subunits (4 -6). Seven of these are peripheral proteins including those subunits that bear all known redox groups of complex I, namely one FMN and up to nine ironsulfur (FeS) 1 clusters. The remaining 7 subunits are hydrophobic proteins predicted to fold into 54 ␣-helices across the membrane (1, 2). They are most likely involved in ubiquinone reduction and proton translocation. The mitochondrial complex I of eukaryotes contains at least 29 extra proteins in addition to the homologues of the 14 prokaryotic complex I subunits (1, 2). The genes of the Escherichia coli complex I are named nuoA to nuoN (7). nuoC and D are fused in E. coli giving rise to 13 different subunits assembling the complex (8). The preparation of the E. coli complex I contains one non-covalently bound FMN, two binuclear (N1a and N1b), and five tetranuclear (N2, N3, N4, N6a, and N6b) FeS clusters (Refs. 9, 10, and 11).2 The tetranuclear cluster N5 present in complex I from other species has not yet been detected in this preparation. However, it should be present because of the preservation of the corresponding binding motif (7,12,13). The ...