This chapter describes the role of the redox couple H2O/O2 as the cornerstone of biological Gibbs free energy transformation in all higher forms of life on earth.
After a short introduction into basic thermodynamic principles, a short description summarizes the general functional pattern of the two processes that perform these activities: (1) solar energy exploitation via photosynthetic water splitting and (2) Gibbs free energy extraction from food through oxygenic respiration. These reactions take place in multimeric protein complexes, which are anisotropically incorporated into specialized membrane systems, thus giving rise to transient “storage” of Gibbs free energy in form of a transmembrane electrochemical potential difference of protons that provides the driving force for ATP synthesis. Of key relevance in the overall processes are the reaction sequences of the redox couple H2O/O2 either in the forward (O2-formation) or in the backward (O2-consumption) direction taking place within two “molecular machines”: (1) Photosystem II (PS II) which acts as light driven plastoquinone-water:oxido-reductase and (2) cytochrome c oxidase (COX) acting as cytochrome c-oxygen:oxido-reductase.
In this chapter our current knowledge on the structural and functional properties of these two systems is described. Oxidative water splitting into molecular oxygen and four protons requires. the formation of a strongly oxidizing species and the cooperation of four electron abstraction steps from two water molecules. The former goal is achieved through functionalizing of chlorophyll a by a special protein matrix and the second task performed at manganese containing catalytic metal cluster. In COX the Gibbs free energy available from exergonic dioxygen reduction to water is transformed into a protein motive force by a special mechanism of proton pumping.
Open questions on the mechanism of both system (e.g. O–O bond formation in PS II and proton pumping in COX) are outlined and future perspectives discussed.