<P> The F region also shows significant similarity to hexameric DNA helicases, and the F region shows some similarity to H + - powered flagellar motor complexes . The α β hexamer of the F region shows significant structural similarity to hexameric DNA helicases; both form a ring with 3-fold rotational symmetry with a central pore . Both have roles dependent on the relative rotation of a macromolecule within the pore; the DNA helicases use the helical shape of DNA to drive their motion along the DNA molecule and to detect supercoiling, whereas the α β hexamer uses the conformational changes through the rotation of the γ subunit to drive an enzymatic reaction . </P> <P> The H + motor of the F particle shows great functional similarity to the H + motors that drive flagella . Both feature a ring of many small alpha - helical proteins that rotate relative to nearby stationary proteins, using a H + potential gradient as an energy source . This link is tenuous, however, as the overall structure of flagellar motors is far more complex than that of the F particle and the ring with about 30 rotating proteins is far larger than the 10, 11, or 14 helical proteins in the F complex . </P> <P> The modular evolution theory for the origin of ATP synthase suggests that two subunits with independent function, a DNA helicase with ATPase activity and a H + motor, were able to bind, and the rotation of the motor drove the ATPase activity of the helicase in reverse . This complex then evolved greater efficiency and eventually developed into today's intricate ATP synthases . Alternatively, the DNA helicase / H + motor complex may have had H + pump activity with the ATPase activity of the helicase driving the H + motor in reverse . This may have evolved to carry out the reverse reaction and act as an ATP synthase . </P> <P> E. coli ATP synthase is the simplest known form of ATP synthase, with 8 different subunit types . </P>

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