A nontraditional role for water in the cytochrome c oxidase reaction.

The passage of electrons through cytochrome c oxidase is directly related to the activity of water. Reducing the activity in a system containing reductant, oxygen, and cytochrome oxidase blocks electron transfer between reduced cytochrome a and oxidized cytochrome a3. The extent of the block is directly related to the osmotic pressure of the system, implying that the protein shell of the oxidase acts as a semipermeable membrane that excludes osmotic perturbants but not water. It appears that approximately 10 water molecules must enter and leave the oxidase in order for internal electron transfer to occur.