|Animation of ATP Synthase|
ATP Synthase (aka FOF1-ATP synthase or F-type ATPase) is one of the central enzymes in energy metabolism for most organisms. It is a membrane protein found in the inner membrane of mitochondria, the thylakoid membrane of chloroplasts, and the plasma membrane of bacteria. Structurally ATP synthase is composed of two portions: hydrophobic membrane-embedded FO and hydrophilic F1 that protrudes by approximately 10 nm from the membrane plane. Both portions are multisubunit complexes. In E. coli FO is composed of subunits a, b and c in stoichiometry 1:2:10; F1 consists of subunits alpha, beta, gamma, delta and epsilon in stoichiometry of 3:3:1:1:1.
ATP synthase interconverts two main "energy currencies" of a living cell: transmembrane proton electrochemical potential difference and ATP. (In some bacteria, e.g. Propionigenium modestum or Iliobacter tartaricus sodium ion is substituting proton). In ATP synthesis mode the enzyme makes ATP from ADP + Pi using the energy of proton electrochenical potential difference. In the reverse mode the energy of ATP hydrolysis is used to pump protons across the membrane and generate the potential difference. In respiring and photosynthetic organisms ATP synthesis is the primary function, but in organisms living on fermentation ATP synthase works as ATP-driven proton pump. In E. coli both functions are important, depending on the cell environment.
During ATP synthesis protons pass through FO and cause rotation of a ring-shape oligomer of 10 c subunits relative to the peripheral ab2-complex. This rotation is mechanically passed to subunits gamma and epsilon in F1. The rotation of gamma-epsilon complex relative to other F1 subunits causes the co-ordinated binding and release of nucleotides and Pi and results in ATP synthesis.
"Proton translocation and ATP synthesis (or hydrolysis) are coupled by a unique mechanism, subunit rotation. Electrochemical energy contained in the proton gradient is converted into mechanical energy in form of subunit rotation, and back into chemical energy as ATP". 
In E. coli deletion of ATP synthase operon is not lethal: cells can grow by fermentation on appropriate media. This makes E. coli a convenient host for expression of ATP synthase with various mutations, because non-functional enzyme still can be obtained. Growth on succinate, glycerol or other non-fermentative energy sources can be used as a test for fully functional ATP synthase in E. coli.
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- Weber, J () ATP synthase: subunit-subunit interactions in the stator stalk. Biochim. Biophys. Acta 1757 1162-70 PubMed
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