Cryo-EM structures of intermediates suggest an alternative catalytic reaction cycle for cytochrome c oxidase

Cytochrome c oxidases are among the most important and fundamental enzymes of life. Integrated into membranes they use four electrons from cytochrome c molecules to reduce molecular oxygen (dioxygen) to water. Their catalytic cycle has been considered to start with the oxidized form. Subsequent electron transfers lead to the E -state, the R -state (which binds oxygen), the P -state (with an already split dioxygen bond), the F -state and the O -state again. Here, we determined structures of up to 1.9 Å resolution of these intermediates by single particle cryo-EM. Our results suggest that in the O -state the active site contains a peroxide dianion and in the P -state possibly an intact dioxygen molecule, the F -state may contain a superoxide anion. Thus, the enzyme’s catalytic cycle may have to be turned by 180 degrees. Cytochrome c oxidase is a fundamental enzyme of life and its mechanism is not fully understood yet. Here, the authors present four cryo-EM structures of different intermediate states, which suggest an alternative cytochrome c oxidase reaction cycle.