Figure 14–30 ATP synthase. the three-dimensional structure of the F1Fo atp synthase, determined by x-ray crystallography. also known as an F-type atpase, it consists of an Fo part (from “oligomycin-sensitive factor”) in the membrane and the large, catalytic F1 head in the matrix. Under mild dissociation conditions, this complex separates into its F1 and Fo components, which can be isolated and studied individually. (a) Diagram of the enzyme complex showing how its globular head portion (green) is kept stationary as proton-flow across the membrane drives a rotor (blue) that turns inside it. (B) In bovine heart mitochondria, the Fo rotor ring in the membrane (light blue) has eight c subunits. It is attached to the γ subunit of the central stalk (dark blue) by the ε subunit (purple). the catalytic F1 head consists of a ring of three α and three β subunits (light and dark green), and it directly converts mechanical energy into chemical-bond energy in atp, as described in the text. the elongated peripheral stalk of the stator (orange) is connected to the F1 head by the small δ subunit (red) at one end, and to the a subunit in the membrane (pink oval) at the other. together with the c subunits of the ring rotating past it, the a subunit creates a path for protons through the membrane. (C) the symbol for atp synthase used throughout this book.
the closely related atp synthases of mitochondria, chloroplasts, and bacteria synthesize atp by harnessing the proton-motive force across a membrane. this powers the rotation of the rotor against the stator in a counterclockwise direction, as seen from the F1 head. the same enzyme complex can also pump protons against their electrochemical gradient by hydrolyzing atp, which then drives the clockwise rotation of the rotor. the direction of operation depends on the net free-energy change (∆G) for the coupled processes of h+ translocation across the membrane and the synthesis of atp from aDp and pi (Movie 14.5 and Movie 14.6).
Measurement of the torque that the atp synthase can produce by atp hydrolysis reveals that the atp synthase is 60 times more powerful than a diesel engine of equal dimensions. (B, courtesy of K. Davies. pDB codes: 2WpD, 2CLy, 2WSS, 2BO5.) through the second half-channel into the matrix. Thus proton flow causes the rotor ring to spin against the stator like a proton-driven turbine.
The mitochondrial ATP synthase is of ancient origin: essentially the same enzyme occurs in plant chloroplasts and in the plasma membrane of bacteria or archaea. The main difference between them is the number of c subunits in the rotor ring. In mammalian mitochondria, the ring has 8 subunits. In yeast mitochondria, the number is 10; in bacteria and archaea, it ranges from 11 to 13; in plant chloroplasts, there are 14; and the rings of some cyanobacteria contain 15 c subunits.