Abstract
Cytochrome c oxidase catalyzes the reduction of oxygen to water. This process is accompanied by the vectorial transport of protons across the mitochondrial or bacterial membrane ("proton pumping"). The mechanism of proton pumping is still a matter of debate. Many proposed mechanisms require structural changes during the reaction cycle of cytochrome c oxidase. Therefore, the structure of the cytochrome c oxidase was determined in the completely oxidized and in the completely reduced states at a temperature of 100 K. No ligand exchanges or other major structural changes upon reduction of the cytochrome c oxidase from Paracoccus denitrificans were observed. The three histidine Cu(B) ligands are well defined in the oxidized and in the reduced states. These results are hardly compatible with the "histidine cycle" mechanisms formulated previously.
Highlights
Cytochrome c oxidase (EC 1.9.3.1) is the terminal enzyme in the respiratory chains of mitochondria and many aerobic bacteria
Single Crystal Absorbance Spectra—Spectral measurements with single crystals of the cytochrome c oxidase show that the enzyme is completely oxidized under the crystallization conditions (Fig. 1)
There is no difference between the optical spectra of the oxidized and of the reduced cytochrome c oxidase in the crystals and spectra of the same species in solution
Summary
Cytochrome c oxidase (EC 1.9.3.1) is the terminal enzyme in the respiratory chains of mitochondria and many aerobic bacteria (see for reviews Refs. 1 and 2). Binuclear center, and indirect mechanisms where the coupling is achieved by major conformational changes in the protein. Many research groups favor a directly coupled proton pump mechanism in combination with a gating element because the energy for the proton pumping is generated by oxygen reduction, which takes place in the binuclear center, the characteristic feature of the heme-copper-containing terminal oxidase superfamily [4]. Because there was no electron density for the CuB ligand His-325 in the oxidized, azide-treated enzyme, Iwata et al [10] adapted the “histidine cycle” on the grounds of possible multiple conformations of this ligand and two different proton access routes, maintaining strict electroneutrality for the redox changes around the binuclear center [12]
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