Animals, and most fungi and bacteria, obtain their energy by “burning” foodstuff. This happens stepwise in a very controlled manner. The foodstuff (carbohydrates, fats) is first degraded and biologically fixed hydrogen is generated in the form of NADH. NADH is then oxidized in the so-called respiratory chain, which is located in the inner membrane of mitochondria and bacteria. The respiratory chain consists of complex I (NADH-ubiquinone oxidoreductase), complex II (succinate dehydrogenase), complex III (ubiquinol cytochrome c oxidoreductase or cytochrome bc1 complex) and complex IV (cytochrome c oxidase). Complexes I, III and IV are “proton pumps”, this means that they translocate protons across the membrane. This process leads to the generation of an electric potential and a pH-gradient across the membrane. Protons flow back via the ATP-synthase, where they drive the synthesis of ATP, the universal energy currency of life.

Cytochrome c oxidase is the terminal enzyme of the respiratory chain. It uses four electrons from cytochrome c and four protons to convert one oxygen molecule to two modules of water and to pump four protons. It is therefore the “biosynthesis” of water which drives the pumping of protons. This process is astonishingly complex and not understood at present. Even the question, which of the four electron transfer steps into the enzyme are coupled to proton pumping, is a matter of controversy. Experimental approaches to clarify the issue will be presented. They comprise methods from the fields of molecular biology, X-ray crystallography, various kinds of spectroscopy and electrophysiology.