ATP synthase

An ATP synthase (EC 3.6.3.14) is a general term for an enzyme that can synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate by utilizing some form of energy. The overall reaction sequence is:

ADP + P_i → ATP

These enzymes are of crucial importance in almost all organisms, because ATP is the common "energy currency" of cells.

In mitochondria, the F_OF₁ ATP synthase has a long history of scientific study. The F₁ portion of the ATP synthase is above the membrane, the F_O portion is within the membrane. It's easy to visualize the F_OF₁ particle as resembling the fruiting body of a common mushroom, with the head being the F₁ particle, the stalk being the gamma subunit of F₁, and the base and "roots" being the F_O particle embedded in the membrane. The F₁ particle was first isolated by Ephraim Racker in 1961.

The F₁ particle is large and can be seen in the transmission electron microscope by negative staining (1962, Fernandez-Moran et al., Journal of Molecular Biology, Vol 22, p 63). These are particles of 9 nm diameter that pepper the inner mitochondrial membrane. They were originally called elementary particles and were thought to contain the entire respiratory apparatus of the mitochondrion, but through a long series of experiments, Ephraim Racker and his colleagues were able to show that this particle is correlated with ATPase activity in uncoupled mitochondria and with the ATPase activity in submitochondrial particles created by exposing mitochondria to ultrasound. This ATPase activity was further associated with the creation of ATP by yet another long series of experiments in many laboratories.

Binding change mechanism

In the 1960s through the 1970s, Paul Boyer developed his binding change, or flip-flop, mechanism, which postulated that ATP synthesis is coupled with a conformational change in the ATP synthase generated by rotation of the gamma subunit. John E. Walker crystallized the ATP synthase and was able to determine that Boyer's conformational model was essentially correct. In the crystal structure, the F₁ particle can be seen to be composed of a cylinder of 6 subunits, alternating alpha and beta subunits, that form a ring around an asymmetrical gamma subunit. Facilitated diffusion of protons causes the F_O particle to rotate, rotating the gamma subunit of F₁, while the major F₁ subunits are fixed in place. This rotation forces a conformational change in the F₁ particle, eventually leading to the synthesis of ATP. For elucidating this Boyer and Walker shared in the 1997 Nobel Prize in Chemistry.

Physiological role

The F₁F_O ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient, this is used by fermenting bacteria which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient, which they use for flagella and transport of nutrients into the cell.

In respiring bacteria under physiological conditions, ATP synthase generally runs in the opposite direction, creating ATP while using the protonmotive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. Same process takes place in mitochondria, were ATP synthase is located in the inner mitochondrial membrane (so that F₁-part sticks into mitochondrial matrix, were ATP synthesis takes place).

Plant ATP synthase

In plants ATP synthase is also present in chloroplasts (CF_OF₁-ATP synthase). The enzyme is integrated into thylakoid membrane; the CF₁-part sticks into stroma, were dark reactions of photosynthesis (Calvin cycle) and ATP synthesis take place. The overall structure and the catalytic mechanism of the chloroplast ATP synthase are almost the same as those of the mitochondrial enzyme. However, in chloroplasts the protonmotive force is generated not by respiratory electron transport chain, but by primary photosynthetic proteins - photosystems I and II and cytochrome b₆f.

E. coli ATP synthase

E. coli ATP synthase is the simplest known form of ATP synthase, with 8 different subunit types.

Yeast ATP synthase

Yeast ATP synthase is the most complex known and is made of 20 different types of subunits.

See also

• mitochondrion
• chloroplast
• electron transfer chain
• proton pump
• transmembrane ATPase

External links

• "ATP synthase - a splendid molecular machine"