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Chapter: Biochemistry : Bioenergetics and Oxidative Phosphorylation

The change in free energy (∆G) occurring during a reaction predicts the direction in which that reaction will spontaneously proceed.


The change in free energy (∆G) occurring during a reaction predicts the direction in which that reaction will spontaneously proceed. If ∆G is negative (that is, the product has a lower free energy than the substrate), the reaction goes spontaneously. If ∆G is positive, the reaction does not go spontaneously. If ∆G = 0, the reactions are in equilibrium. The ∆G of the forward reaction is equal in magnitude but opposite in sign to that of the back reaction. The ∆Gs are additive in any sequence of consecutive reactions, as are the standard free energy changes (∆Gos). Therefore, reactions or processes that have a large, positive ∆G are made possible by coupling with those that have a large, negative ∆G such as hydrolysis of adenosine triphosphate (ATP). The reduced coenzymes nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) each donate a pair of electrons to a specialized set of electron carriers, consisting of flavin mononucleotide (FMN) , iron-sulfur centers, coenzyme Q, and a series of cytochromes, collectively called the electron transport chain. This pathway is present in the inner mitochondrial membrane (impermeable to most substances) and is the final common pathway by which electrons derived from different fuels of the body flow to O2, reducing it to water. The terminal cytochrome, cytochrome oxidase, is the only cytochrome able to bind O2. Electron transport results in the pumping of protons across the inner mitochondrial membrane from the matrix to the intermembrane space. This process creates electrical and pH gradients across the inner mitochondrial membrane. After protons have been transferred to the cytosolic side of the membrane, they reenter the matrix by passing through the Fo proton channel in ATP synthase (Complex V), dissipating the pH and electrical gradients and causing conformational changes in the β subunits of F1 that result in the synthesis of ATP from adenosine diphosphate + inorganic phosphate. Electron transport and phosphorylation are tightly coupled in oxidative phosphorylation (OXPHOS, Figure 6.17). Inhibition of one process inhibits the other. These processes can be uncoupled by uncoupling protein-1 of the inner mitochondrial membrane of cells in brown fat and by synthetic compounds such as 2,4-dinitrophenol and aspirin, all of which dissipate the proton gradient. In uncoupled mitochondria, the energy produced by the transport of electrons is released as heat rather than being used to synthesize ATP. Mutations in mitochondrial DNA, which is maternally inherited, are responsible for some cases o f mitochondrial diseases such as Leber hereditary optic neuropathy. The release of cytochrome c into the cytoplasm and subsequent activation of proteolytic caspases results in apoptotic cell death.

Figure 6.17 Key concept map for oxidative phosphorylation (OXPHOS). [Note: Electron (e-) flow and ATP synthesis are envisioned as sets of interlocking gears to emphasize the idea of coupling.] TCA = tricarboxylic acid; NAD(H) = nicotinamide adenine dinucleotide; FAD(H2) = flavin adenine dinucleotide; FMN = flavin mononucleotide. 


Study Questions

Choose the ONE best answer.


6.1 2,4-Dinitrophenol, an uncoupler of oxidative phosphorylation, was used as a weight-loss agent in the 1930s. Reports of fatal overdoses led to its discontinuation in 1939. Which of the following would most likely be true concerning individuals taking 2,4-dinitrophenol?

A. Adenosine triphosphate levels in the mitochondria are greater than normal.

B. Body temperature is elevated as a result of hypermetabolism.

C. Cyanide has no effect on electron flow.

D. The proton gradient across the inner mitochondrial membrane is greater than normal.

E. The rate of electron transport is abnormally low.

Correct answer = B. When phosphorylation is uncoupled from electron flow, a decrease in the proton gradient across the inner mitochondrial membrane and, therefore, impaired ATP synthesis is expected. In an attempt to compensate for this defect in energy capture, metabolism and electron flow to oxygen is increased. This hypermetabolism will be accompanied by elevated body temperature because the energy in fuels is largely wasted, appearing as heat. The electron transport chain will still be inhibited by cyanide.


6.2 Which of the following has the strongest tendency to gain electrons?

A. Coenzyme Q

B. Cytochrome c

C. Flavin adenine dinucleotide

D. Nicotinamide adenine dinucleotide

E. Oxygen

Correct answer = E. Oxygen is the terminal acceptor of electrons in the electron transport chain (ETC). Electrons flow down the ETC to oxygen because it has the highest (most positive) reduction potential (E0). The other choices precede oxygen in the ETC and have lower E0 values.


6.3 Explain why and how the malate-aspartate shuttle moves nicotinamide adenine dinucleotide reducing equivalents from the cytosol to the mitochondrial matrix.

There is no transporter for nicotinamide adenine dinucleotide (NADH) in the inner mitochondrial membrane. However, NADH can be oxidized to NAD+ by the cytoplasmic isozyme of malate dehydrogenase as oxaloacetate is reduced to malate. The malate is transported across the inner membrane, and the mitochondrial isozyme of malate dehydrogenase oxidizes it to oxaloacetate as mitochondrial NAD+ is reduced to NADH. This NADH can be oxidized by Complex I of the electron transport chain, generating three ATP through the coupled processes of oxidative phosphorylation.


6.4 Carbon monoxide binds to and inhibits Complex IV of the electron transport chain. What effect, if any, should this respiratory inhibitor have on phosphorylation of adenosine diphosphate to adenosine triphosphate?

Inhibition of the electron transport chain by respiratory inhibitors such as carbon monoxide results in an inability to maintain the proton gradient. Phosphorylation of ADP to ATP is, therefore, inhibited, as are ancillary reactions such as calcium uptake by mitochondria, because they also require the proton gradient.

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