Cytochrome C oxidase (before oxidized, with carbon monoxide)
Photosynthesis uses light energy to generate oxygen from water. Another enzyme catalyzes the reverse reaction, producing water from oxygen and hydrogen. This enzyme is cytochrome c oxidase. It is one of the key enzymes in the respiration of cells. It is a large enzyme, with about 30,000 atoms, embedded in the membranes of mitochondria.
The enzyme is a dimer, of two identical subunits. The core of each subunits, as is the case in many membrane proteins, is made from long alpha-helices, oriented normal to the membrane plane. Each subunit contains two heme groups. These heme groups are used to bind the oxygen, consumed in the conversion to water. The chemical process drives the transport of hydrogen ions (protons) from one side of the mitochondrial membrane to the opposite side. It acts like a pump, pumping protons from one place to another. It can pump protons in both directions. To be able to do so, the chemical groups inside the protein working as the pump must be accessible only to one side of the membrane at a time. Small changes in the structure of the protein accomplish this and a careful study of these changes is important. The structure shown here shows the protein with carbon monoxide bound to it (see PDB:2OCC as after the ligand oxidized). As known from other oxygen binding proteins, like hemoglobin and myoglobin, carbon monoxide is a powerful poison because it binds to heme groups more efficiently than oxygen.
Protein Data Bank (PDB)
author: Arno Paehler