Complex V (also called ATP synthase or the F0F1 complex) is a multi-protein structure with three-fold symmetry, resembling a mushroom (Figure 15.14). It consists of a top knob called F1 and a stalk, which joins the knob to the base called F0 in the inner mitochondrial membrane. The F1 knob projects into the mitochondrial matrix and contains three dimers arranged like segments of an orange around the stalk. The stalk contains and proteins; is attached to protein b of the F0 base, which also contains proteins a and c plus others. The abc complex is called a stator.

The F1 complex is the part of complex V that synthesizes ATP as protons pass from the intermembrane space through the stalk and out the top of the F1 complex into the mitochondrial matrix. Movement of the protons through the F1 complex causes it to rotate (Figure 15.20). As seen in Figure 15.19, F1 contains three similar, but not identical, binding sites for ATP or ADP + Pi. These are called L, T, and O, for Loose, Tight, and Open. The tight binding site always contains ATP. ADP and Pi can bind together in the L site. A turn of the rotor (as a result of proton movement through the complex) converts all three binding sites into new conformations. The T site becomes the O site, and ATP is released. The L site becomes a T site, which starts to condense ADP + Pi to ATP. The O site becomes an L site, which prepares to bind the next ADP + Pi. At each turn of the wheel, an ATP is released.

The antibiotic oligomycin binds to a specific protein of the F0 complex, blocks the flow of protons through the F0 channel, and inhibits oxidative phosphorylation directly.