The energy to drive the metabolic processes of all life forms on earth is ultimately derived from photosynthetic processes. The principle means used by a living organism to store this energy is through the production of adenosine 5’-triphosphate (ATP) or the reduced form of nicotinamide adenine dinucleotide (NADH) and its phosphorylated variant nicotinamide adenine dinucleotide phosphate (NADPH) that is used in photosynthetic processes. Due to the wide variety of metabolic processes that use ATP as an energy source, this molecule is often referred to as the energy currency of a cell. Energy stored in the high-energy phosphoryl bond of ATP is released when this bond is cleaved by an ATPase reaction to form ADP and inorganic phosphate (Pi). The energy stored in reduced NADPH derives from its negative redox potential relative to other cellular components.
During photosynthesis, ATP is synthesized from ADP and Pi by the chloroplast FₒF₁ ATP synthase. This enzyme is composed of the thylakoid membrane-embedded Fₒ protein complex, and the attached F₁ protein complex that protrudes into the aqueous stroma of the chloroplast (Figure 1). The Fₒ is composed of subunits a, b, b', and c (also known as subunits IV, I, II, and III, respectively) while F₁ contains subunits α, β, γ, δ, and ε.