The F₁ Complex of ATP Synthase

ATP is synthesized on the inner mitochondrial membrane by an enzyme complex composed of a proton-conducting, hydrophobic F₀ unit and a catalytic, hydrophilic F₁ unit. We are going to look at the F₁ complex, whose role is to catalyze the synthesis of ATP. The F₁ complex forms the spherical projections seen on the inner membrane. (See Campbell: Fig. 9.14, p. 171).

As the figures below show, the central portion of the F₁ complex is made up of 7 protein chains or subunits(3 alpha (α), 3 beta (β), and 1gamma (γ). The three pairs of α and β subunits form a flattened sphere around the shaft-like γ subunit (shown in light green below). Note that the γ subunit is composed of a number of long alpha helices and the α and β subunits are made up of a mix of alpha helices and β-sheets. What level of protein structure are these?

As H+ ions pass through the F₀ complex embedded in the membrane (see below right), the F₀ complex rotates. This, in turn, rotates the shaft of the F₁ complex. The rotation of the shaft causes a conformational change in the immobile β subunits leading to differences in the ability of each β subunit to bond with the adenine nucleotides used to make ATP. There is a cyclical progression such that at any given time, one of the β subunits is able to bind ATP tightly, another binds ATP more loosely, and the third binds ATP very loosely. A single cycle of the three β subunits through the three binding phases goes like this: 1) the tight binding subunit is converted to very loose binding, causing it to give up the ATP it holds; 2) the moderately loose site (which holds an ADP and P_i previously picked up from its surroundings) is converted into the tight binding conformation, causing ATP to form; 3) and finally the very loose site is converted to the moderately loose conformation, causing it to take up ADP and P_i from the surrounding fluid. Note that the diagram below left shows the three β subunits in their different conformations. One is bound to ADP (yellow) - this is the loose conformation, one is bound to ATP (red) - this is the tight conformation, and one is empty - the very loose conformation.

In a sense, the proton motive force is not used to make ATP so much as it is used to release ATP from the synthase. The binding of ADP and P_i to one β site promote the release of ATP from another. In other words, ATP synthase shows cooperativity.

Mitochondrial ATP synthase complex. (click to enlarge)

Rotation in the ATP synthase complex demonstrated experimentally.

NOTE: As an additional optional exercise, you may want to explore the structure of the F₁ unit of ATP synthase on your own using Protein Explorer. The PDB code is: 1BMF.

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