ATP-Synthase

ATP production was probably one of the earliest cellular processes to evolve, and the synthesis of ATP from two precursor molecules is the most prevalent chemical reaction in the world. The enzyme that catalyzes the synthesis of ATP is the ATP-synthase or F₀F₁-ATPase, one of the most ubiquitous proteins on Earth. The F₁F₀-ATPases comprise a huge family of enzymes with members found not only in the thylakoid membrane of chloroplasts but also in the bacterial cytoplasmic membrane and in the inner membrane of mitochondria. The source of energy for the functioning of ATP-synthase is provided by photosynthetic metabolism in the form of a proton gradient across the thylakoid membrane, that is, a higher concentration of positively charged protons in the thylakoid lumen than in the stroma.

The F₀F₁-ATPase molecule is divided into two portions termed F₁ and F₀. The F₀ portion is embedded in the thylakoid membrane, while the F₁ portion projects into the lumen. Each portion is in turn made up of several different proteins or subunits. In F₀, the subunits are named a, b, and c. There is one a subunit, two b subunits, and 9–12 c subunits. The large a subunit provides the channel through which H⁺ ions flow back into the stroma. Rotation of the c subunits, which form a ring in the membrane, is chemically coupled to this flow of H⁺ ions. The b subunits are believed to help stabilize the F₀F₁ complex by acting as a tether between the two portions. The subunits of F₁ are called α, β, γ, δ, and ε. F₁ has three copies each of α and β subunits which are arranged in an alternating configuration to form the catalytic “head” of F₁. The γ and ε subunits form an axis that links the catalytic head of F₁ to the ring of c subunits in F₀. When proton translocation in F₀ causes the ring of c subunits to spin, the γ–ε axis also spins because it is bound to the ring. The opposite end of the γ subunit rotates within the complex of α and β subunits. This rotation causes important conformational changes in the β subunits resulting in the synthesis of ATP from ADP and Pi (inorganic phosphate) and to its release.