Involvement of Acidic Amino Acid Residues in Zn2+ Binding to Respiratory Complex I

Proton transfer across membranes and membrane proteins is a central process in biological systems. Zn²⁺ ions are capable of binding to acidic residues, often found within such specific pathways, thereby leading to a blockage. Here we probed Zn²⁺inhibition of the proton‐pumping NADH:ubiquinone oxidoreductase from Escherichia coli by means of electrochemically induced FTIR difference spectroscopy. Numerous conformational changes were identified including those that arise from the reorganization of the membrane arm upon electron transfer in the peripheral arm of the protein. Signals at very high wavenumbers (1781 and 1756 cm^?1) point to the perturbation of acidic residues in a highly hydrophobic environment upon Zn²⁺ binding. In variant D563N^L, which lacks part of the proton pumping activity (residue located on the horizontal amphipathic helix), the spectral signature of Zn²⁺ binding is changed. Our data support a role for this residue in proton translocation.