S/A Oxidative Phosphorylation Flashcards
As you read and answer this question, you are (presumably) consuming oxygen. What single reaction accounts for most of your oxygen consumption?
O2 is converted to H2O by electrons from the respiratory chain. The final step is the one catalyzed by cytochrome oxidase (Complex IV).
Show the path of electrons from ubiquinone (Q or coenzyme Q) to oxygen in the mitochondrial respiratory chain. One of the two compounds (Q and O2) has a standard reduction potential (E’°) of 0.82 V, and the other, 0.045 V. Which value belongs to each compound? How did you deduce this?
:QH2 →cytb→cytc1 →cytc→cyt(a+a3)→O2
E’° for O2 must be the larger positive value (+0.82) because electron flow occurs spontaneously to the electron acceptor with the more positive E’°.
Diagram the path of electron flow from NADH to the final electron acceptor during electron transport in mitochondria. For each electron carrier, indicate whether only electrons, or both electrons and protons, are accepted/donated by that carrier. Indicate with an arrow where electrons from succinate oxidation enter the chain of carriers.
NADH (both) → FP (both) → Q (both) → cyt b (e– only) → cyt c1 (e– only) → cyt c (e– only)
→ cyt (a + a3) (e– only) → O2 (both) Electrons from succinate enter at Q.
Describe, in simple diagrams and a few words, the chemiosmotic theory for coupling oxidation to phosphorylation in mitochondria.
There are three central elements in the chemiosmotic model:
(1) Electron flow through asymmetrically arranged membrane-bound carriers causes transmembrane flow of H+, creating a proton gradient (a proton motive force).
(2) The proton motive force drives protons back across the membrane via specific proton channels (composed of Fo).
(3) The energy released by “downhill” movement of protons is captured when ADP and Pi are condensed by ATP synthase (FoF1). (See Fig. 19-17, p. 705.)
The skunk cabbage (Symphocarpus foetidus) can maintain a temperature of 10–25 °C higher than the temperature of the surrounding air. Suggest a mechanism for this.
One possible heat-generating mechanism is partially uncoupled mitochondria in which some of the energy of electron flow is dissipated as heat.
When the F1 portion of the ATP synthetase complex is removed from the mitochondrial membrane
and studied in solution, it functions as an ATPase. Why does it not function as an ATP synthetase?
: Like all enzymes, the F1 subunit of the ATP synthase catalyzes a reaction in both directions:
ADP + Pi ↔ ATP + H2O
The standard free-energy change (∆G’°) for ATP hydrolysis is –30.5 kJ/mol. With no proton motive force to drive the reaction toward ATP synthesis, the hydrolysis (ATPase activity) occurs
Using a simple diagram of the chemiosmotic theory, explain why anything that makes the mitochondrial membrane leaky stops ATP synthesis in the mitochondria.
Ans: There are three central elements in the chemiosmotic model:
(1) Electron flow through asymmetrically arranged membrane-bound carriers causes transmembrane
flow of H+, creating a proton gradient (a proton motive force).
(2) The proton motive force drives protons back across the membrane via specific proton channels (composed of Fo).
(3) The energy released by downhill movement of protons is captured when ADP and Pi are condensed by ATP synthase (FoF1). Anything that makes the membrane leaky to protons (an
uncoupler such as 2,4-dinitrophenol, or mechanical breakage of the membrane) prevents formation of a proton gradient. With no proton gradient, there is no energy source for ATP synthesis by FoF1
(ATP synthase). (See Fig. 19-17, p. 705, and Fig. 19-30, p. 717.)
When the ∆G’° of the ATP synthesis reaction is measured on the surface of the ATP synthase enzyme, it was found to be close to zero. Describe briefly why this is so.
The enzyme binds ATP more tightly than ADP thus stabilizing the former (i.e., the product of the synthesis reaction) relative to the latter (i.e., the reactant in the synthesis reaction).
Explain briefly the current model for how the proton motive force that is generated by electron transport is used to drive the ATP synthesis reaction.
The tight binding of ATP by the enzyme stabilizes it and makes the ∆G’° of the synthetic reaction more favorable. Once the reaction has occurred, the ATP product must be released from the enzyme. The proton motive force causes protons to move across the inner mitochondrial membrane through the pore in the Fo complex. This movement leads to conformational changes that decrease
the affinity of the F1 portion of the synthase for ATP, resulting in its release from the enzyme.
What is respiratory control in mitochondria? What is accomplished by this control mechanism?
Respiratory control is the regulation of electron flow and ATP synthesis by [ADP]. When [ADP] is low, electron flow slows; when [ADP] is high, electron flow is stimulated. The overall effect is to keep the level of ATP nearly constant and to prevent oxidation of fuels when the demand for ATP is low.
Although molecular oxygen (O2) does not participate directly in any of the reactions of the citric acid cycle, the cycle operates only when O2 is present. Explain this observation.
The citric acid cycle produces NADH, which is normally reoxidized to NAD+ by the passage of electrons through the respiratory chain to O2. With no O2 to accept electrons, NADH accumulates,
NAD+ is depleted, and the citric acid cycle slows for lack of NAD+.
Cytochrome c plays two distinct and very important roles in mammalian cells: (1) in the mitochondrial electron transport chain, and (2) in apoptotic cell death. Describe the roles of cytochrome c in these two processes.
In the electron transport chain, cytochrome c accepts electrons from complex III and transfers them to complex IV. In the electron transport chain, cytochrome c is the only water soluble carrier. It operates in the space between the outer and inner mitochondrial membranes. In apoptotic celldeath, the permeability of the outer mitochondrial membrane increases dramatically allowing the escape of cytochrome c into the cytoplasm where it activates caspase 9, one of the main proteolytic enzymes active in apoptosis.
Photophosphorylation differs from oxidative phosphorylation in that the former requires the input of energy in the form of ___________ to create a good electron donor. In photophosphorylation, electrons flow through a series of membrane-bound carriers including ___________ , ___________ , and ___________ proteins, whereas ___________ are pumped across a membrane to create an ___________ potential.
Ans: Light, cytochromes, quinones, iron-sulfur, protons, electrochemical.
