432 CHAPTER 14 . Electron Transport and ATP Synthesis Complex IV contributes to the proton gradient that drives ATP synthesis. Two protons are translocated for each pair of electrons that pass through this complex. Recall that complex I transfers four protons per electron pair, and complex III also translocates four protons per electron pair. So, for every molecule of NADH that is oxidized, the membrane-associated electron transport system pumps ten protons, which cross the membrane. 14.9 Complex V: ATP Synthase Complex V is ATP synthase. It catalyzes the synthesis of ATP from ADP + P, in a reaction driven by the proton gradient generated during membrane-associated electron transport. ATP synthase is a specific F-type ATPase, called FoF, ATPase, and its name is due to the opposite reaction. Despite their name, F-type ATPases are responsible for synthesizing ATP, not hydrolyzing it. They are membrane-bound, and have a characteristic handle-and-stem (or "knob-and-rod," as on doors) structure (Figure 14.14). The Fi component (the handle) contains the catalytic subunits. When released from membrane preparations, component F catalyzes the hydrolysis of ATP. For this reason it has been called F, ATPase, by tradition. This part of the enzyme enters the mitochondrial matrix in eukaryotes, and the cytoplasm in bacteria. (ATP synthase is also found in chloroplast membranes, as you will see in the next chapter.) The Fo component (the stem) is embedded in the membrane. It has a proton channel that crosses the membrane. The passage of protons through the channel, from the outside of the membrane to the inside, is coupled to the formation of ATP by the F1 component. The composition of the subunit in the F component is azBsybe and that of the Fo component is a b>c10-14- The Fy subunits interact to form a cylindrical base within the membrane. The core of the F1 structure (handle) is made up of three copies of each of the a and B subunits arranged as a cylindrical hexamer. The nucleotide binding sites are in the clefts between adjacent a and B subunits. Thus, the binding sites are 120 apart on the surface of the aB3 cylinder. The catalytic site of ATP synthesis is primarily associated with amino acid residues in the B subunit. The aBz membrane oligomer by a multisubunit stem formed by the y and e subunits. The F unit is connected to e subunits that traverse the C-e-and forms a "rotor" that rotates within the membrane. Rotation of the y subunit within the aB1 hexamer alters the conformation of the subunits, opening and closing the active sites. The a, b, and subunits form an arm that also links the Fo component to the aB3 oligomer. This a-b-6-a Ba unit is called a "stator." The passage of protons through the channel at the interface between the a and e subunits causes the rotor assembly to rotate in one direction, relative to the stator. The entire structure is often called a molecular motor. There are 10 to 14 e subunits in the membrane-associated c ring at the base of the rotor. The number of subunits depends on the species; yeast and E coli have a ring with 10 subunits, but plants and animals have up to 14 subunits. There are good reasons that the rotation of each c subunit by the stator is driven by the translocation of a single proton. The rotation of the subunit and within the F component is carried out in a staggered and agitated manner, where each step is 120 of rotation. When ring c rotates, it twists the y-axis until enough tension is built up to make it jump to the next position within hexamer aB. If the c ring has 10 subunits, then one complete turn requires the translocation of 10 protons and results in the production of three ATP molecules. The exact stoichiometry is still being deduced. The results of many experiments indicate that, on the average, three protons must be translocated for each molecule of ATP synthesized, and this is the value that will be used in the rest of this book.