M2: Citric Acid Cycle L10 Flashcards
What are alternate names for the CAC?
TCA cycle or Krebb’s cycle.
What does it mean to be amphibolic?
Amphibolic = site of anabolism and catabolism
What is anabolism in the CAC? Give examples.
CAC intermediates are the starting point of anabolic pathways.
Ex: gluconeogenesis, fatty acid synthesis (to make membranes), and amino acid synthesis (to make proteins).
What type of reaction are the anabolic processes that stem from the CAC? Why?
The anabolic reactions are Cataplerotic reactions (cata = emptying) because they deplete the CAC intermediates.
What is catabolism in the CAC? Give examples.
CAC intermediates are the end point of catabolic pathways. The aerobic catabolism of carbohydrates, lipids and amino acids merge into the CAC.
Ex: oxaloacetate (from pyruvate carboxylase) and amino acid degradation.
What type of reactions are the catabolic processes involving the CAC? Why?
The catabolic rxns are Anaplerotic reactions (ana = filling up) because they replenish the depleted CAC intermediates.
What are the 3 main functions of the CAC?
- To produce reducing equivalents (3NADH and 1FADH2): electron carriers that donate their electrons to ETC.
- Produces intermediates for biosynthesis (citrate, fatty acids, amino acids).
- Removes carbons to form CO2.
Is the CAC anaerobic or aerobic?
Neither! It supports aerobic metabolism but doesn’t consume oxygen directly.
Does the CAC generate energy?
Yes, its harvested through the reducing equivalents and It makes one molecule of GTP (which is an ATP equivalent).
What 3 irreversible steps confer directionality to the CAC? Why?
Step 1: citrate synthase
Step 3: Isocitrate dehydrogenase
Step 4: Alpha-ketoglutarate dehydrogenase
Because they all have negative ∆G.
What are the high energy steps of the CAC? What are the recycling steps of the CAC?
High energy: first 5 steps.
Recycling: last 3 steps.
What is step 1 of the CAC? What is unique about this step?
Oxaloacetate (4 carbons) + acetyl-CoA (2 carbons) to Citrate (6 carbons) by citrate synthase.
Unique because it’s the only reaction where a C-C bond is formed.
How is Oxaloacetate turned into citrate? Why is this step (step 1) irreversible?
1) Enol of acetyl-CoA attacks carbonyl C in oxaloacetate
2) Citryl-CoA is an intermediate which contains a thioester (i.e. high energy bond)
3) Hydrolysis of Citryl-CoA is associated negative free E value (∆G˚’ = - 31.5 kJ/mol)
Driving force for the reaction. Irreversible.
Memorize the CAC.
Lecture 10.
What is the overall rxn of step 2 of the CAC? What are key points of the reaction?
Citrate to isocitrate reversibly. Enzyme = Aconitase.
Isomerization of citrate to isocitrate, with cis-aconitate as an intermediate.
1) Dehydration of Citrate
2) Re-hydration of cis-aconitate to make isocitrate (new isomer).
What is the overall rxn of step 3 of the CAC? What are key points of the reaction? Irreversible or reversible?
Step 3 is a decarboxylation via the enzyme isocitrate dehydrogenase.
Isocitrate (6 carbons) + NAD+ to a-ketoglutarate (5 carbons) + CO2 (1 carbon) + NADH + H+
Key points:
1) Dehydrogenase activity generates NADH (first time),
decarboxylase activity generates CO2 (first time).
2) CO2 comes from oxaloacetate, not from acetyl-CoA (exergonic, irreversible).
3) Rxn is irreversible.
What is the overall rxn of step 4 of the CAC? What are key points of the reaction? Reversible or irreversible?
Step 4 is a decraboxylation reaction that occurs via the enzyme a-Ketoglutarate Dehydrogenase and is irreversible.
a-ketoglutarate (5 carbons) + CoA + NAD+ to succinyl-CoA (4 carbons)+ CO2 (1 carbon) + NADH + H+.
Key points:
1) Oxidative decarboxylation that generates NADH
(second time) and CO2 (second time).
2) The decarboxylation reaction provides the energy to generate the high energy intermeidate: succinyl-CoA
3) Like Pyruvate dehydrogenase, alpha-KG
dehydrogenase is a multienzyme complex.
What are the enzymes in the ketoglutarate dehydrogenase complex?
E1 = ketoglutarate dehydrogenase; E2 = dihydrolipoyl transsuccinylase; E3 = dihydrolipoyl dehydrogenase
What is the overall rxn of step 5 of the CAC? What are key points of the reaction? Reversible or irreversible?
Step 5 is a reversible rxn that occurs via Succinyl-CoA Synthetase.
Succinyl-CoA + GDP + Pi to succinate + GTP
Key points:
1) Use of the high energy succinyl-CoA to generate GTP.
2) Energy of succinyl-CoA is conserved through succinyl-phosphate, 3-phospho-Histidine residue, then GTP.
3) At this point, 1 equivalent acetyl-CoA has been completely oxidized to 2 CO2
4) 2 NADH, and 1 GTP have been generated
What is the overall rxn of step 6 of the CAC? What are key points of the reaction? Reversible or irreversible?
Step 6 is a reversible oxidation reaction via the succinate dehydrogenase enzyme.
Succinate + E-FAD to fumarate + E-FADH2
Key points:
1) Succinate dehydrogenase is covalently bound to FAD therefore FAD cannot diffuse as a free metabolite.
2) Dehyrogenation of succinate produced Succinate dehydrogenase-FADH2
3) Succinate dehydrogenase-FADH2 restores its FAD by directly funneling electrons into ETC through complex 2.
What step of the CAC is directly involved in the ETC?
Step 6: succinate to fumarate via succinate dehydrogenase. Generates FADH2 that enters ETC through complex 2.
What is the overall rxn of step 7 of the CAC? What are key points of the reaction? Reversible or irreversible?
Step 7 is a reversible hydration reaction via the fumarase enzyme.
Fumarate + H2O to malate
Key point:
- Fumarase catalyzes the hydration of the double bond of fumarate to generate malate.
What is the overall rxn of step 8 of the CAC? What are key points of the reaction? Reversible or irreversible?
Step 8 is a reversible reaxtion that occurs via the malate dehydrogenase enzyme.
Malate + NAD+ to oxaloacetate + NADH + H+
Key points:
1) Although the reaction is endergonic (∆G˚’ = + 29.7 kJ/mol), the true ∆G ~ 0 kJ/mol because:
In vivo: at equilibrium [Malate]»_space;» [Oxaloacetate] so it pushes rxn towards oxaloacetate.
2) The next reaction (Citrate Synthase reaction) is highly exergonic (∆G˚’ = -31.5 kJ/mol), which allows the formation of citrate to be exergonic even at low [oxaloacetate]
3) Coupling of both reactions helps the CAC run forward.
How many ATP’s are made from one round of the CAC?
10 ATP per acetyl-coA. each glucose makes 2 pyruvates, which makes 2 acetyl-CoA, From each acetyl CoA: 3 NADH (2.5 ATP/NADH) = 7.5 ATP 1 FADH2 (1.5 ATP/FADH2) = 1.5 ATP 1 GTP (1ATP/GTP) = 1ATP Total: 10 ATP/ acetyl CoA which therefore generates a total of 20 ATP for 2 rounds of CAC.
