Asp563 of the horizontal helix of subunit NuoL is involved in proton translocation by the respiratory complex I.

The NADH:ubiquinone oxidoreductase couples the electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. It contains a 110 A long helix running parallel to the membrane part of the complex. Deletion of the helix resulted in a reduced H+/e− stoichiometry indicating its direct involvement in proton translocation. Here, we show that the mutation of the conserved amino acid D563L, which is part of the horizontal helix of the Escherichia coli complex I, leads to a reduced H+/e− stoichiometry. It is discussed that this residue is involved in transferring protons to the membranous proton translocation site.