TCA/Krebs/Citric Acid Cycle Stage 2 Flashcards
The TCA cycle is
the catabolic pathway used for oxidizing all metabolic fuels
What 3 pathways does the TCA pathway affect for oxidizing all metabolic fuels
- Oxidation: By removing hydrogen (reduces NAD+ and FAD )
- Aerobic: Only when oxygen is present. Oxygen does not participate directly in the cycle, but as the ultimate acceptor of electrons in the electron transport chain, it allows the re-oxidation of the coenzymes reduced in the Krebs cycle and is necessary for the cycle to move forward.
- Acetyl-CoA: the point at which lipid, carbohydrate and protein catabolism converges.
TCA Cycle characteristics:
- 8 reactions
- 3 irreversible
For each acetyl-CoA:
1 GTP
3 NADH
1 FADH2
2 CO2
TCA cycle Step 1
- Condensation reaction of one acetyl-CoA and one oxaloacetate into citric acid (Citrate)
- Catalyzed by citrate synthase and CoA is released
- Hydrolysis of the thioester bound in acetyl-CoA provides the energy making this reaction favorable.
- Irreversible
TCA cycle step 2
-Isomerization of citrate into isocitrate
-Catalyzed by aconitase
-Reversible
-Done in 2 steps
TCA cycle step 3
-Decarboxylation of isocitrate into α-ketoglutarate
-Catalyzed by isocitrate dehydrogenase with coenzyme NAD+
-Done in 2 steps
-Produces one CO2 and one NADH+H+
-Irreversible
TCA Cycle step 4
-Oxidative decarboxylation of α-ketoglutarate into succinyl-CoA
-Catalyzed by α-ketoglutarate dehydrogenase
-Multi-enzymes complex of 3 enzymes and 4 coenzymes
-Consumes one CoA
-Produces one CO2 and one NADH
-Irreversible
TCA Cycle step 5
-Phosphorylation of a GDP by the transformation of succinyl-CoA into succinate.
-Catalyzed by succinyl-CoA synthetase
-Produces one GTP
-Reversible
TCA cycle step 6
-Dehydrogenation of succinate into fumarate.
-Catalyzed by succinate dehydrogenase using FAD as cofactor.
-Succinate dehydrogenase is located in the inner mitochondrial membrane (this enzyme belongs to the complex II of the respiratory chain)
-All others TCA cycle enzymes are found in the mitochondrial matrix.
-Produces FADH2
-Reversible
TCA Cycle Step 7
-Rehydration fumarate into L-malate.
-Catalyzed by fumarase.
-Consumes one H2O
-Reversible
TCA Cycle Step 8
-Dehydrogenation of L-malate into oxaloacetate.
-Catalyzed by malate dehydrogenase with coenzyme NAD+.
-Produces one NADH
-Reversible
NOTE:
-Oxaloacetate is reaction 1
3 of the first 4 steps are irreversible so it pushes the reaction in the forward direction
-Oxaloacetate in reaction 8 is being pushed by irreversible reaction 1
Why is reaction 8 reversible
If there is less acetyl coa (not much glucose in the blood) then oxalocetate cannot produce citrate so reaction 8 becomes irreversible and will be converted back to malate
NOTE: L-malate is linked to gluconeogenesis
Stage 2 TCA cycle is regulated by 4 things
Explain the regulation of Stage 2 TCA cycle through bioavailability of substrates
- Acetyl-CoA and oxaloacetate for citrate synthase
- NAD+ for the other two (If were in a situation where we produce a lot of energy in the cell (NADH), NAD+ concentration is lowered because we have enough NADH and the cell doesn’t need to produce anymore so TCA cycle is slowed)
- Drop in acetyl-CoA or oxaloacetate will slow down reaction 1 producing citrate
Explain the regulation of Stage 2 TCA cycle through inhibition by products accumulation
- Citrate inhibits citrate synthase
- Succinyl-CoA inhibits citrate synthase and α-ketoglutarate dehydrogenase
Describe how citrate (from TCA cycle) regulates glycolysis
Citrate signals for high energy so we don’t need to break down glucose (glycolyisis) so inhibits it
Explain the regulation of Stage 2 TCA cycle through inhibition by allosteric control
- ATP and NADH (have surplus of energy) are allosteric inhibitors whereas ADP and Ca2+ acts as allosteric activators (are energy deficit indicators).
- Can you explain why Ca2+ is an allosteric activator of these reactions? Accumulation of calcium means the muscles are working, so the muscle needs energy, so it activates the cycle to produce energy for the muscle
Exam Question: How can calcium regulate the TCA cycle
- ATP and NADH (have surplus of energy) are allosteric inhibitors whereas ADP and Ca2+ acts as allosteric activators (are energy deficit indicators).
- Can you explain why Ca2+ is an allosteric activator of these reactions? Accumulation of calcium means the muscles are working, so the muscle needs energy, so it activates the cycle to produce energy for the muscle
Exam Question: describe how reaction 8 delta g is affected (standard vs in cell)
oxaloacetate is always low because its driven by reaction 1 to form citrate this keeps the delta g in the cell low (instead of +29.7kj/mol), reaction 1 is highly exergonic and pushes it
Energy payoff from the TCA cycle
Practice converting this pathway into ATP
Remember NADH = 2.5 ATP and FADH = 1.5 ATP
** Will be 30-32 ATP depending on pathway used
Describe the difference in ATP between using the Malate-aspartate shuttle and the glycerol-3phosphate shuttle
Malate aspartate shuttle: Convert oxalacetate into malate, shuttle malate out, then convert back to oxaloacetate (gluconeogenesis)
Glycerol 3-phosphate: NADH converted to FADH2 (goes from 5 ATP to 3 ATP)
Why do we use the glycerol 3-phosphate shuttle if it produces less ATP
we sometimes use glycerol-3-phosphate shuttle because certain tissues use them (know the amount of ATP, NADH, FADH2 produced by each part of these cycles remember these numbers
Energy payoff from glucose oxidation (aerobic glycolysis
Name what happens in the 3 stages of the TCA cycle in regards to catabolism of proteins, fats and carbohydrates (cellular respiration)
Stage 1: Oxidation of fatty acids, glucose and some amino acids yields acetyl CoA and reducing equivalents (e-)
Stage 2: Oxidation of acetyl- CoA in the TCA cycle has four steps where reducing equivalents are further produced.
Stage 3: Reducing equivalents carried by NADH and FADH2 are funneled into the electron transfer chain and oxidative phosphorylation (producing H2O and ATP)
