Bioch lec 10- citric acid cycle
What is the overall purpose of aerobic catabolism? (1 purpose)
To capture energy from fuel oxidation and use it to make ATP.
What happens to acetyl-CoA in the CAC? (3 outcomes)
Acetyl-CoA condenses with OAA, two carbons oxidized to CO₂, and OAA is regenerated.
What high-energy molecules are produced per acetyl-CoA? (3 products)
3 NADH, 1 FADH₂/QH₂, and 1 GTP.
Where does the CAC occur and why is it considered aerobic? (2 reasons)
It occurs in the mitochondrial matrix; it requires O₂ indirectly to reoxidize NADH/FADH₂ via the ETC.
Why is the CAC considered amphibolic? (1 explanation)
CAC intermediates feed anabolic reactions (amino acids, fats, carbohydrates, nucleotides).
What are the characteristics of the citrate synthase reaction? (4 features)
It is irreversible (ΔG ≪ 0), catalyzed by citrate synthase, forms citrate, and is not physiologically regulated.
What type of reaction converts citrate → isocitrate? (1 description)
An isomerization, reversible with ΔG ≈ 0.
What happens in the isocitrate dehydrogenase step? (4 facts)
Oxidative decarboxylation, irreversible, produces NADH, and is regulated.
What happens in the α-ketoglutarate dehydrogenase step? (5 facts)
Oxidative decarboxylation, irreversible, produces NADH, regulated, and mechanistically similar to PDH
Why is succinyl-CoA considered a high-energy intermediate? (1 reason)
It is a thioester, whose hydrolysis drives substrate-level phosphorylation.
What occurs in the succinyl-CoA synthetase reaction? (3 facts)
A reversible reaction (ΔG ~ 0), forms GTP, and is a substrate-level phosphorylation.
What three key features define the succinate dehydrogenase step? (3 features)
It is reversible, forms FADH₂/QH₂, and the enzyme is Complex II, an integral membrane protein.
How is FADH₂ reoxidized in the CAC? (2 steps)
FADH₂ transfers electrons to coenzyme Q (→ QH₂), which then enters the ETC.
What happens in the fumarase reaction? (2 events)
A reversible hydration converting fumarate → malate.
What occurs in the malate dehydrogenase step? (2 facts)
A reversible oxidation forming NADH and regenerating OAA.
Why is the CAC said to have no single rate-limiting enzyme? (1 reason)
Because it is cyclic, and flux depends on multiple steps and NAD⁺/NADH balance.
What two CAC enzymes are major regulatory points? (2 enzymes)
Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
What inhibits and activates the two regulated CAC enzymes? (4 regulators)
Inhibitors: NADH, ATP.
Activators: ADP, Ca²⁺
Why does the NAD⁺/NADH ratio strongly influence CAC reactions? (1 mechanism)
Because oxidation steps require NAD⁺ as substrate; high NADH inhibits by mass-action.
Is citrate synthase regulated physiologically? (1 answer)
No — citrate levels are not physiologically relevant to regulate flux.
What happens when ADP levels in the matrix decrease? (7-step flow)
↓ ADP → ↓ ATP synthase → ↑ H⁺ gradient → ↓ ETC rate → ↓ NADH oxidation → ↓ NAD⁺/NADH → slowed PDH + CAC.
(Flow diagram explicitly from slide.)
What happens when ADP levels increase? (7-step flow)
↑ ADP → ↑ ATP synthase → ↓ H⁺ gradient → ↑ ETC rate → ↑ NADH oxidation → ↑ NAD⁺/NADH → accelerated PDH + CAC
Why does Ca²⁺ stimulate CAC activity? (1 reason)
Ca²⁺ activates regulated enzymes (isocitrate DH, α-KG DH), signaling increased energy demand
What makes CAC amphibolic? (2 roles)
It provides precursors for biosynthesis and functions in energy generation.
Why are anaplerotic reactions necessary? (1 reason)
CAC intermediates are drained for biosynthesis, so they must be replenished.
What are the two main sources of anaplerotic intermediates? (2 sources)
Amino acid breakdown and pyruvate carboxylase forming oxaloacetate.
How is pyruvate carboxylase regulated? (2 regulators)
Activated by acetyl-CoA, inhibited by ADP.
How much ATP is produced from one acetyl-CoA? (1 number)
~10 ATP (7.5 from NADH, 1.5 from FADH₂, 1 from GTP).
What is the total ATP yield from a full oxidation of glucose? (1 approximate value)
Approximately 32 ATP under aerobic conditions
How many ATP are produced by anaerobic glycolysis alone? (1 number)
2 net ATP
What does substrate-level phosphorylation produce in the CAC? (1 product)
GTP, which can convert to ATP.
Which reaction links glycolysis to the CAC? (1 reaction)
The PDH reaction: pyruvate → acetyl-CoA + CO₂ + NADH.
Which two CAC reactions are oxidative decarboxylations? (2 reactions)
Isocitrate DH and α-KG DH.
Which CAC steps produce NADH, FADH₂, and GTP? (3 products)
NADH: isocitrate DH, α-KG DH, malate DH.
FADH₂: succinate DH.
GTP: succinyl-CoA synthetase.
Which CAC step is reversible hydration? (1 step)
Fumarase: fumarate → malate
Which enzyme is also Complex II of the ETC? (1 enzyme)
Succinate dehydrogenase.
Which steps are substrate-level phosphorylations across glycolysis + CAC? (2 steps)
Step 7 of glycolysis (PGK) and succinyl-CoA synthetase in the CAC.
Which CAC reactions are regulated? (2 key regulated steps)
Isocitrate DH and α-KG DH (inhibited by NADH/ATP; activated by ADP/Ca²⁺).
Why do yeast consume less glucose when exposed to oxygen? (1 explanation)
Because O₂ allows oxidative phosphorylation, which produces far more ATP, reducing the need for high glycolytic flux (Pasteur effect).
How can lactate be used as a metabolic fuel? (1 process)
Lactate → pyruvate → acetyl-CoA, then oxidized via the CAC.