ATP synthase (F0 portion)
Escherichia coli (bacteria)
Adenosine triphosphate (ATP) is the main provider of chemical energy for the processes of life. Splitting the phosphate-phosphate bond in ATP releases more energy than the act of splitting the bond requires, called an exoenergetic reaction. ATP is converted into adenosine diphosphate (ADP) and a phosphate group P. To regenerate ATP from ADP and P requires a chemical process. This ATP creation is carried out by a protein called ATP synthase. The energy required for this chemical synthesis step comes from a difference in proton concentration across a membrane boundary. ATP synthase (or F1F0 ATP synthase) is a rather complex molecules with a membrane-embedded part F0 and an extracellular part F1. The extracellular F1 portion, where the ATP synthesis takes places, consists of nine protein units of five different kinds. The membrane-bound F0 portion has a simpler structure, made from only two different kinds of proteins, one copy of one kind, the a subunit and 12 copies of another kind, the c subunit.
The structure of these subunits is extremely simple, consisting just of two long helices, aligned parallel to each other in the direction normal to the membrane plane and connected by a short loop. Twelve copies of these are then arranged in a circle. The four transmembrane helices of subunit A are attached at one side of this circle. The non-membrane part of subunit a is omitted in this structure, making it superficially look like two additional c-type subunits. The interface between the ring of c subunits and the a subunits forms the proton pump which produces the energy for ATP synthesis. This flow produces changes in the arrangement of c type helices which results in a large-scale motion of the wheel. This motion is transmitted to the F1 portion of the synthase through the contact between c subunits and parts of F1 and ultimately drives the catalysis of ATP from ADP and P. Imagine a creek with its water flowing down a slope. Insert a wheel into the creek and the water flow will turn the wheel. When connecting the turning wheel to a grinder, this energy can be used to grind wheat. This is how it was done in the middle ages in Europe and this mechanical analogy can give you an idea how nature uses a similar concept to make ATP.
Protein Data Bank (PDB)
author: Arno Paehler