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Chapter 17: Mitochondrial Functions and Oxidative Phosphorylation

You are studying the effects of a compound on the respiratory activity of isolated mitochondria. Your experiments demonstrate that oxygen consumption is normal when pyruvate and malate are used as well as when succinate is added. However, you find that the production of ATP is severely impaired with addition of the compound. These results most closely resemble the effects that would be seen by the addition of the following electron transport inhibitor to your system?

A. antimycin A

B. azide

C. dinitrophenol (DNP)

D. oligomycin

E. rotenone

Answer C: The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP. If the process of proton movement is uncoupled from the normal pathway through the ATP synthase complex, oxygen can be consumed but no ATP will be synthesized. This is the effect of uncoupling agents such as DNP, which act to discharge the proton gradient.

A laboratory analysis of isolated mitochondria demonstrates that oxygen consumption is normal when succinate is added but extremely low when pyruvate and malate are used. The mitchondria are subsequently shown to have normal cytochromes but a reduced iron content. The reduced pyruvate/malate oxidation is due to a defect in which of the following respiratory components?

A. cytochrome c

B. cytochrome oxidase

C. NADH dehydrogenase

D. succinate dehydrogenase

E. ubiquinone

Answer C: During the process of oxidative phosphorylation, electrons from reduced electron carriers enter the pathway at either complex I or complex II. The reduced electron carrier, derived from the oxidation of pyruvate and malate (NADH), enters the oxidative phosphorylation machinery at complex I, NADH-coenzyme Q reductase (also identified as NADH-ubiquinone oxidoreductase). Whereas, the reduced electron carrier generated during the oxidation of succinate (FADH2) enters the oxidative phosphorylation machinery at complex II. Thus, a defect in the activity of complex I would inhibit oxygen consumption in the presence of pyruvate or malate but not succinate. The pathway of electron flow through the electron transport assembly have been determined through the uses of compounds termed antimetabolites. Some of these agents are inhibitors of electron transport at specific sites in the electron transport assembly, while others stimulate electron transport by discharging the proton gradient (uncouplers). An important example ...

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