Kunitoshi Shimokata

The low-spin heme of cytochrome c oxidase as the driving element of the proton-pumping process.

Mitochondrial cytochrome c oxidase plays an essential role in aerobic cellular respiration, reducing dioxygen to water in a process coupled with the pumping of protons across the mitochondrial inner membrane. An aspartate residue, Asp-51, located near the enzyme surface, undergoes a redox-coupled x-ray structural change, which is suggestive of a role for this residue in redox-driven proton pumping. However, functional or mechanistic evidence for the involvement of this residue in proton pumping has not yet been obtained. We report that the Asp-51 → Asn mutation of the bovine enzyme abolishes its proton-pumping function without impairment of the dioxygen reduction activity. Improved x-ray structures (at 1.8/1.9-Å resolution in the fully oxidized/reduced states) show that the net positive charge created upon oxidation of the low-spin heme of the enzyme drives the active proton transport from the interior of the mitochondria to Asp-51 across the enzyme via a water channel and a hydrogen-bond network, located in tandem, and that the enzyme reduction induces proton ejection from the aspartate to the mitochondrial exterior. A peptide bond in the hydrogen-bond network critically inhibits reverse proton transfer through the network. A redox-coupled change in the capacity of the water channel, induced by the hydroxyfarnesylethyl group of the low-spin heme, suggests that the channel functions as an effective proton-collecting region. Infrared results indicate that the conformation of Asp-51 is controlled only by the oxidation state of the low-spin heme. These results indicate that the low-spin heme drives the proton-pumping process.

The proton pumping pathway of bovine heart cytochrome c oxidase

X-ray structures of bovine heart cytochrome c oxidase have suggested that the enzyme, which reduces O2 in a process coupled with a proton pumping process, contains a proton pumping pathway (H-pathway) composed of a hydrogen bond network and a water channel located in tandem across the enzyme. The hydrogen bond network includes the peptide bond between Tyr-440 and Ser-441, which could facilitate unidirectional proton transfer. Replacement of a possible proton-ejecting aspartate (Asp-51) at one end of the H-pathway with asparagine, using a stable bovine gene expression system, abolishes the proton pumping activity without influencing the O2 reduction function. Blockage of either the water channel by a double mutation (Val386Leu and Met390Trp) or proton transfer through the peptide by a Ser441Pro mutation was found to abolish the proton pumping activity without impairment of the O2 reduction activity. These results significantly strengthen the proposal that H-pathway is involved in proton pumping.

Cell-free synthesis of cytochrome c oxidase, a multicomponent membrane protein

Cell-free protein synthesis is a useful technique that can site-specifically incorporate isotope-labeled amino acids into proteins. This incorporation is essential for infrared analyses of the electronic state of a specific amino acid residue used to elucidate protein function. Although 17 membrane proteins have been synthesized in their active state by cell-free systems, to date no hetero-subunit protein has been synthesized with this technique, suggesting that there are serious technical limitations. Here we report the cell-free synthesis of Paracoccus denitrificans cytochrome c oxidase, a membrane protein complex composed of three distinct subunits that contain two heme A molecules and two redox-active copper centers. The synthesized protein exhibited normal Soret/vis absorption spectra and ferrocytochrome c oxidation activity.