T3: Metabolic Poisons Flashcards

1
Q

Q: Why is mitochondrial respiration dependent on ADP?

A

A: relates to physico-chemical coupling of mito

as substrates move along the electron transfer chain each component of the complex uses the energy released in the electron transfer to transport H+ across the inner mitochondrial membrane, so the [H+] is higher on the outer side of the membrane

This electrochemical gradient of H+ represents a temporary store of energy that is used by ATP synthase to phosphorylate ADP to ATP, the H+ flowing back across the membrane in the process

This “metabolic coupling” of the electron transport chain and oxidative phosphorylation ensures that substrates are only metabolised when there is a demand for ATP

In other words O2 consumption at the end of the chain only occurs in the presence of substrate and ADP

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2
Q

Q: Why is the rate of respiration in the presence of succinate and ADP greater than with glutamate plus malate and ADP?

A

A: NADH-linked substrates such as malate feed electrons into the electron transfer chain via NADH dehydrogenase (part of complex I) and so can generate 3 ATP for each oxygen atom reduced to H2O

In contrast, oxidation of succinate feeds electrons into the chain via FADH2 and succinate dehydrogenase (part of Complex II) thus utilising only two of the phosphorylation sites. So, to generate a given quantity of ATP, more succinate must be oxidised than malate and glutamate and hence more O2 used

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3
Q

Q: Explain the result from adding KCN?

A

A: By reacting covalently with the Fe3+ in cytochrome oxidase, the respiratory poison KCN inhibits the terminal step in the electron transport chain, so respiration ceases

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4
Q

Q: Under normal circumstances, what would happen after the addition of oligomycin?

A

A: Oligomycin interferes with and hence reduces the ability of the ATP synthase to utilise the H+ electrochemical gradient. So, in coupled mitochondria, oligomycin would be expected to inhibit respiration

antibiotic oligomycin which binds to ATPase (ATP synthase) preventing protons from passing back into the mitochondria and stopping the operation of the proton pumps as the gradients become too high for them to operate

Oligomycin prevents the increase in mitochondrial respiration induced by ADP without inhibiting uncoupler-stimulated respiration. NADH remains high and NAD+ is too low for the citric acid cycle to operate

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5
Q

Q: What does the data allow you to conclude regarding the metabolic effect of DNOC?

A

A: Aromatic weak acids such as DNOC and dinitrophenol are thought to pass readily across the mitochondrial inner membrane in their undissociated form thus dissipating the electrochemical gradient.

Outside Inside
DNOC- + H+ -> DNOC-H -> DNOC-H -> DNOC- + H+

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6
Q

Q: In the case report, why were the respiratory rate and body temperature elevated?

A

A: DNOC acts as a pesticide by uncoupling mitochondrial respiration in insects so that their electron transport chain runs uncontrollably and unproductively. The same has happened in the two agricultural workers. Large amounts of metabolic fuels are consumed with the released energy being wasted as heat

In other words the Kreb’s Cycle and oxidative phosphorylation are running maximally but the link to the generation of ATP is broken so the free energy released from substrates is lost as heat. Consistent with this is the large increase in mitochondrial O2 consumption after addition of DNOC

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7
Q

Q: Can you provide explanations for the post mortem findings and the circumstances of death? (absence of body fat, the effect on the femur, and the rapid onset of rigor mortis)

A

A: The principal fuels used for this uncontrolled respiration are fatty acids from the triglycerides stored in adipose tissues, thus depleting the body’s fat stores

The accompanying excessive oxygen consumption leads to tissue hypoxia, which the body attempts to alleviate by increased pulmonary respiration and by erythropoiesis in the bone marrow

The rigor mortis can be explained by considering muscle biochemistry. The power stroke moves actin filaments relative to the heads of the myosin, so shortening the muscle fibres. This involves ATP hydrolysis to ADP by myosin ATPase. To relax the fibre for the next power stroke, the ADP must be displaced by incoming ATP. Since DNOC poisoning greatly decreases the concentration of ATP, the contractile system is left in rigor

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