ATP Synthase - Evolution of ATP Synthase

Evolution of ATP Synthase

The evolution of ATP synthase is thought to be an example of modular evolution during which two functionally independent subunits became associated and gained new functionality. This association appears to have occurred early in evolutionary history, because essentially the same structure and activity of ATP synthase enzymes are present in all kingdoms of life. The F-ATP synthase displays high functional and mechanistic similarity to the V-ATPase. However, whereas the F-ATP synthase generates ATP by utilising a proton gradient, the V-ATPase generates a proton gradient at the expense of ATP, generating pH values of as low as 1.

The F₁ domain also shows significant similarity to hexameric DNA helicases, and the F_O domain shows some similarity to H+-powered flagellar motor complexes. The α₃β₃ hexamer of the F₁ domain 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 α₃β₃ hexamer uses the conformational changes through the rotation of the γ subunit to drive an enzymatic reaction.

The H+ motor of the F_O particle shows great functional similarity to the H+ motors seen in flagellar motors. 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_O particle and the ring with ca. 30 rotating proteins is far larger than the 10, 11, or 14 helical proteins in the F_O complex.

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 drive the ATPase activity of the helicase in reverse. This complex then evolved greater efficiency and eventually developed into today's complex 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.