This is the all-important reaction in which the protonmotive force produced by proton translocation is coupled to the synthesis of ATP from ADP and phosphate. ATP Synthase is a complex structure consisting of two domains F_o and F₁. F₁ is a spherical

The central stalk is driven by the retrolocation of protons through F_o (counter-clockwise as seen from above), and rotates in 120°) stages. At each stage each of the β-subunits in turn change conformation: L changes to T (after binding ADP and phosphate), T to

structure, which, in the case of mitochondria, sticks out into the matrix and is anchored to the membrane by a stator to prevent rotation. It consists of three α- and three β- subunits, all of which can bind nucleotides, but only the β-subunits can take part in the reactions. F_o is a cylindrical structure capable of rotation when driven by translocated protons, and which is linked to a central stalk that can revolve inside F₁.

The mechanism that drives ATP synthesis seems to depend upon a binding charge conception in which catalytic sites on the β-subunits have different affinities for nucleotides and are designated loose (L), tight (T), and Open(O). .These are coloured pink, blue, and green respectively. The loose (L) sites bind the substrates (ADP and phosphate)

O, and O to L (after releasing ATP). The new L site then binds new ADP and phosphate and begins a new reaction sequence. One complete revolution of F_o therefore results in the formation of 3ATP, one from each of the β-subunits. In this example about 10 protons need to be retrolocated for each complete revolution of Fo, which means that the formation of 1ATP requires about 3.3, though other species may be different. ATP synthase is thought to revolve at more than 100Hz (revolutions/sec.), which is sufficient to produce a turnover of the equivalent of the weight of our body of ATP each day.

Many other subunits are involved, some of which are understood, but others are the centre of on-going research. This animation is a simplified "interpretation" of the complex integrated movements which

reversibly. The T sites then bind the reactants so tightly that ATP is formed. The O sites, which have a low affinity for substrates, then release the ATP already formed in the T state

are characteristic of ATP synthase. It is hoped that, viewed as an animation, it may stimulate an appreciation that, although of very different architecture, in elegance and beauty and in biochemical significance it is akin to DNA.