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MCQ1. Pyruvate (1) is converted to acetyl-CoA, which is then further catabolized in the Krebs cycle
to produce 2 CO2, 3 NADH, 1 FADH2 and 1 GTP molecules.
True
2. The Krebs cycle is amphibolic, which means:
The cycle plays an important function in both anabolism and catabolism
3. In which cell compartment does the Krebs cycle function?
Mitochondrion
4. Clarify how pyruvate is incorporated into the Krebs cycle by matching the correct word to
each sentence.
MPC transports pyruvate into the mitochondrion.
When energy levels are low, pyruvate are converted to oxaloacetate by the pyruvate
dehydrogenase complex.
Citrate synthase synthesizes citrate by combining acetyl-CoA with acetate (from acetyl-CoA).
5. Pyruvate can be directly converted to oxaloacetate. When will this happen? (When is
pyruvate dehydrogenase activity inhibited?)
When energy levels are high
6. Which enzyme converts pyruvate to oxaloacetate?
Pyruvate carboxylase
7. A biotin deficiency can result in a defect of pyruvate carboxylase, which is characterized by
elevated pyruvate and lactate, with lower levels of acetyl-CoA and ketone bodies.
False
8. Which compounds will accumulate when pyruvate dehydrogenase is defective?
Pyruvate
Lactate
9. Match correct compound names with numbers 1 - 5 in figure.
, Citrate
2-ketoglutarate
Succinate
Malate
Oxaloacetate
10. Which factors play an important role in regulating the Krebs cycle?
Allosteric regulation by specific products
NAD availability
Energy levels
11. Although propionyl-CoA is one methyl group "longer" than acetyl-CoA, it often replaces
acetyl-CoA in the Krebs cycle. This is especially the case with disorders like propionic
acidemia (where propionyl-CoA accumulates). Which of the following markers can be used
to help diagnose this disorder?
Methylcitrate
12. Match correct word to each sentence.
Metabolites are oxidized by dehydrogenase enzymes in numerous catabolic pathways.
In the process, coenzymes like NAD+ are reduced by the dehydorgenase.
13. All the components of the electron transport chain are coded for by the mitochondrial DNA
(and is therefore independent of nuclear DNA). This is why mtDNA are only received from
the mother.
False
Note: Remember that the complexes consist of numerous proteins that “stick together”.
That is why we call them complexes. Only some of these proteins are coded for by
, mitochondrial DNA while the rest is coded for by nuclear DNA. Hence, we require both DNAs
to produce a “whole” electron transport chain.
14. Where does the energy come from that drives ATP synthesis?
The proton gradient
15. Complex I of the electron transport chain accepts electrons from _____
NADH
16. That is why Complex I is also called ________
NADH dehydrogenase
17. As Complex I transfers the electrons to the next component, it pumps protons out of the
mitochondrion. What is the 'next component' in the chain?
CoQ (Ubiquinone)
18. Complex III receives electrons from _____ and transfers the electrons to ______?
CoQ (Ubiquinone) - Cytochrome c
19. Complex IV receives electrons from _____ and transfers the electrons to ______?
Cytochrome c – oxygen
20. Match the name to the correct complex.
Complex II = Succinate dehydrogenase
Complex III = Cytochrome c reductase
Complex IV = Cytochrome c oxidase
Complex V = F0F1-ATP synthase
21. FADH2 generates less energy than NADH. Why?
Since Complex II does not pump protons out of the mitochondrion, the proton gradient is
not as "strong" as when NADH is used.
22. Why are inhibitors of Complex I (e.g. rotenone) not as damaging as inhibitors of Complex IV?
Because electrons can still enter the electron transport chain via Complex II
23. Cytosolic NADH cannot provide electrons directly to Complex I.
True
24. What happens when oxidative phosphorylation is uncoupled? (You can select multiple
options).
The proton gradient weakens
ATP synthesis is reduced
ATP synthesis is reduced
25. Why is lactate produced during strenuous exercise?
To convert the accumulating NADH to NAD so that glycolysis keeps functioning
26. Patients with the same mitochondrial mutation (m.3243A>G) often have different
phenotypes (clinical symptoms).
True
27. Each mitochondrion contains numerous copies of mtDNA.
True
28. Heteroplasmy means that all the mtDNA copies in the mitochondrion are the same.
False
29. With mitochondrial disorders, many metabolites in other pathways accumulate due to lower
levels of ______ and _____.
ATP & NAD
30. Malate dehydrogenase oxidizes malate to produce oxaloacetate in the Krebs cycle. This
reaction can however be reversed (in the liver) to produce malate with the reduction of
oxaloacetate during gluconeogenesis.
True
, 31. Fluoroacetate is a potent inhibitor of the TCA cycle. Which enzyme of the TCA cycle does it
inhibit?
Aconitase
32. Glucose (via pyruvate) is the only carbon source feeding the TCA cycle.
False
Note: One of the figures I repeatedly show in the videos is the one explaining the amphibolic
nature of the TCA cycle. This figure shows a number of “arrows entering and exiting” the
cycle. Hence, the “entering arrows” indicate all the compounds that can feed into the TCA
cycle. Hence, it is not only pyruvate. You will later see that amino acids can feed into the
cycle at different places (like 2-ketoglutarate and succinate).
33. The P/O ratio is defined as:
Molecules of ATP formed per two electrons flowing through electron transport chain
34. Reactive oxygen species form when ....
Electrons leak to oxygen from several places in the chain
35. Cytosolic NADH gives electrons over to cytosolic oxaloacetate, producing malate. This malate
is then transported into the mitochondrion where it gives electrons over to mitochondrial
NAD to produce NADH; which in turn gives the electrons over to the electron transport
chain.
True
36. In which direction will the reaction in the figure run when the electron transport chain's
activity is slowed by oxygen shortage or a defect?
From left to right (AA>3HB)
Note: The effect of oxygen shortage and defective ETC are the same = redox imbalance. This
means that the amount of NADH accumulate while the “free” NAD concentration drop. This
redox imbalance can then favour certain reactions like the production of lactate (from
pyruvate) which uses NADH and produces NAD. Likewise, the high NADH concentrations will
favour the conversion of acetoacetate to 3-hydroxybutyrate to “free-up” NAD – which is
required in essential energy producing pathways such as glycolysis (the glyceraldehyde-3-P
dehydrogenase reaction).
37. What are carbohydrates?
Hydrated carbon
38. Match the structures and names
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