Cytochrome C Oxidase - Biochemistry

Biochemistry

Summary reaction:

4 Fe2+-cytochrome c + 8 H+_in + O₂ → 4 Fe3+-cytochrome c + 2 H₂O + 4 H+_out

Two electrons are passed from two cytochrome c's, through the Cu_A and cytochrome a sites to the cytochrome a₃- Cu_B binuclear center, reducing the metals to the Fe+2 form and Cu+1. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O₂. The oxygen is rapidly reduced, with two electrons coming from the Fe+2cytochrome a₃, which is converted to the ferryl oxo form (Fe+4=O). The oxygen atom close to Cu_B picks up one electron from Cu+1, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical: The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron arising from another cytochrome c is passed through the first two electron carriers to the cytochrome a₃- Cu_B binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to Cu_B+2 to a water molecule. The fourth electron from another cytochrome c flows through Cu_A and cytochrome a to the cytochrome a₃- Cu_B binuclear center, reducing the Fe+4=O to Fe+3, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a₃- Cu_B center as it was at the start of this cycle. The net process is that four reduced cytochrome c's are used, along with 4 protons, to reduce O₂ to two water molecules.