ATP Synthase Comprises Two Multiprotein Complexes Termed F0 and F1

The FoFi complex, or ATP synthase, has two principal components, F0 and F1, both of which are multimeric proteins (Figure 8-24). The F0 component contains three types of integral membrane proteins, designated a, b, and c. In bacteria and in yeast mitochondria the most common subunit composition is a1b2c10, but F0 complexes in animal mitochondria have 12 c subunits and those in chloroplasts have 14. In all cases the c subunits form a donut-shaped ring in the plane of the membrane. The a and two b subunits are rigidly linked to one another but not to the ring of c subunits.

The F1 portion is a water-soluble complex of five distinct polypeptides with the composition a3p378e. The lower part of the F1 7 subunit is a coiled coil that fits into the center of the c-subunit ring of F0 and appears rigidly attached to it. The F1 e subunit is rigidly attached to 7 and also forms rigid contacts with several of the c subunits of F0. The F1 a and p subunits associate in alternating order to form a hexamer, a^a^ap, or (ap)3, which rests atop the single long 7 subunit. The F1 8 subunit is permanently linked to one of the F1 a subunits and also to the b subunit of F0. Thus the F0 a and b subunits and the 8 subunit and (ap)3 hexamer of the F1 complex form a rigid structure anchored in the membrane. The rodlike b subunits form a "stator" that prevents the (ap)3 hexamer from moving while it rests on the 7 subunit (see Figure 8-24).

When ATP synthase is embedded in a membrane, the F1 component forms a knob that protrudes from the cytosolic in tu in

► FIGURE 8-24 Model of the structure and function of ATP synthase (the F0Fi complex) in the bacterial plasma membrane. The F0 portion is built of three integral membrane proteins: one copy of a, two copies of b, and on average 10 copies of c arranged in a ring in the plane of the membrane. Two proton half-channels lie at the interface between the a subunit and the c ring. Half-channel I allows protons to move one at a time from the exoplasmic medium and bind to aspartate-61 in the center of a c subunit near the middle of the membrane. Half-channel II (after rotation of the c ring) permits protons to dissociate from the aspartate and move into the cytosolic medium. The F1 portion contains three copies each of subunits a and p that form a hexamer resting atop the single rod-shaped y subunit, which is inserted into the c ring of F0. The e subunit is rigidly attached to the y subunit and also to several of the c subunits. The 8 subunit permanently links one of the a subunits in the F1 complex to the b subunit of F0. Thus the F0 a and b subunits and the F1 8 subunit and (ap)3 hexamer form a rigid structure anchored in the membrane (orange). During proton flow, the c ring and the attached F1 e and y subunits rotate as a unit (green), causing conformation changes in the F1 p subunits leading to ATP synthesis. [Adapted from M. J. Schnitzer, 2001, Nature 410:878, and P D. Boyer, 1999, Nature 402:247]

Inner mitochondrial membrane

Treatment with ultrasonic vibrations

"Inside-out" submitochondrial vesicles capable of both electron transport and ATP synthesis

Inner mitochondrial membrane

Mechanical agitation

Membrane vesicles capable of electron transport, but not of ATP synthesis

Treatment with ultrasonic vibrations

Mechanical agitation

Membrane vesicles capable of electron transport, but not of ATP synthesis

F-| particles incapable of electron transport or ATP synthesis but with ATPase activity

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