Figure Q14–1 rapid spectrophotometric analysis of the rates of oxidation of electron carriers in the respiratory chain (problem 14–7). Cytochromes a and a3 cannot be distinguished and thus are listed as cytochrome (a + a3).
the electron carriers in the respiratory chain? What is their (A) direction of rotation order?
14–8 Normally, the flow of electrons to O2 is tightly linked to the production of ATP via the electrochemical gradient. If ATP synthase is inhibited, for example, electrons do not flow down the electron-transport chain and respiration ceases. Since the 1940s, several substances— such as 2,4-dinitrophenol—have been known to uncouple electron flow from ATP synthesis. Dinitrophenol was once prescribed as a diet drug to aid in weight loss. How would an uncoupler of oxidative phosphorylation pro mote weight loss? Why do you suppose dinitrophenol is (B) no longer prescribed? 5 14–9 In actively respiring liver mitochondria, the pH in 4 the matrix is about half a pH unit higher than it is in the cytosol. Assuming that the cytosol is at pH 7 and the matrix is a sphere with a diameter of 1 μm [V = (4/3)πr3], calcu late the total number of protons in the matrix of a respiring liver mitochondrion. If the matrix began at pH 7 (equal to that in the cytosol), how many protons would have to be pumped out to establish a matrix pH of 7.5 (a difference of 0.5 pH units)?
14–10 ATP synthase is the world’s smallest rotary motor. Passage of H+ ions through the membrane-embedded portion of ATP synthase (the Fo component) causes rotation of the single, central, axle-like γ subunit inside the head group. The tripartite head is composed of the three αβ dimers, the β subunit of which is responsible for synthesis of ATP. The rotation of the γ subunit induces conformational changes in the αβ dimers that allow ADP and Pi to be converted into ATP. A variety of indirect evidence had suggested rotary catalysis by ATP synthase, but seeing is believing.
To demonstrate rotary motion, a modified form of the α3β3γ complex was used. The β subunits were modified so they could be firmly anchored to a solid support and the γ subunit was modified (on the end that normally inserts into the Fo component in the inner membrane) so that a fluorescently tagged, readily visible filament of actin could be attached (Figure Q14–2A). This arrangement allows rotations of the γ subunit to be visualized as revolutions of the long actin filament. In these experiments, ATP synthase was studied in the reverse of its normal mechanism by allowing it to hydrolyze ATP. At low ATP concentrations, the actin filament was observed to revolve in steps of 120° and then pause for variable lengths of time, as shown in Figure Q14–2B.
A. Why does the actin filament revolve in steps with pauses in between? What does this rotation correspond to in terms of the structure of the α3β3γ complex?
B. In its normal mode of operation inside the cell, how many ATP molecules do you suppose would be synthesized for each complete 360° rotation of the γ subunit? Explain your answer.
14–11 How much energy is available in visible light? How much energy does sunlight deliver to Earth? How efficient