Opening and Closing of the Catalytic Site

Opening Image: α/β cross-section of F₁. The open (empty) β subunit is shown in gold.

The catalytic β subunit exists in two conformations, open and closed. In keeping with the convention for allosteric proteins, the unliganded form is called the T state, whereas the liganded form is designated the R state. Two β subunits are always in the closed or R state. One β_R subunit is liganded to MgADP and P_i, the other to MgATP. The third β subunit, designated β_T, does not have any ligands.

Let's examine the folding of the closed β subunit. Each subunit has three domains:

• β-barrel domain on top
• α/β domain in the middle
• α-helical domain at the bottom

A major conformational change accompanies the opening of the catalytic site in a β subunit. The helical domain rotates away from the catalytic site, causing the β sheet in the middle domain to rupture.

Toggle closed versus open states.

Add the α/β cross-sections. Note the spatial relationship of the crankshaft (the γ subunit) to the closed and open β subunits.

During ATP synthesis, the flow of protons through F_O generates a torque which is transmitted to F₁ by the asymmetric γ subunit. This acts as a rotating crankshaft within F₁, driving sequential release of ATPs from the three active sites. ADP and P_i bind to the open catalytic site on the β_T subunit. The T → R transition entraps MgADP and P_i in the closed catalytic site, where they react to form bound MgATP. Next the R → T conversion releases MgATP from the closed catalytic site. The site, which is now empty, is ready to start a new catalytic cycle.

Top view of F₁, including the α, β and γ subunits. Animate the transition.

In intact ATP synthase, the γ subunit rotates within the α₃β₃ cylinder. We can simulate the catalytic cycle by "holding" one of the β subunits in place as we step through the cycle:

• The empty site binds ADP and P_i.
• The T → R transition entraps the ADP and P_i.
• ADP and P_i condense to form ATP.

During the catalytic cycle the rotating crankshaft forces the open β subunit to close as the crankshaft passes by. The energy-dependent T → R transition is tightly coupled to the R → T transition that releases the newly-formed ATP. We might say that the rotating γ shaft "cranks out" ATPs.

Animate the transition.