TCA Cycle Flashcards
The 2 pyruvate molecules produced in the cytosol in the glycolytic process retain 90% of the energy stored in glucose. How is this energy recovered? Where do these processes take place.
- by the TCA cycle and oxidative phosphorylation
- take place inside the mitochondrial of eukaryotic cells
Explain the structure of the mitochondrial membranes.
- mitochondria enclosed by two membranes – inner and outer separated by an inter membrane space
What does the inner membrane possess in terms of structure?
Inward folds called cristae
What is another word for inner mitochondria space?
Matrix
Define TCA.
Tricarboxylic Acid Cycle
How is mitochondria inherited?
Materanally
How is the permeability of the outer membrane?
has large permeable channels or porins that allow compounds <5, 000 Da to pass freely
How is the permeability of the inner membrane?
- Highly impermeable
- Contains transport proteins (carriers) that transport specific substrates
What does the inner membrane contain?
succinate dehydrogenase complex NADH dehydrogenase complex electron transport chain components ATPase
What does the matrix contain?
soluble enzymes of TCA cycle
Pyruvate dehydrogenase complex DNA & ribosomes
What does the intermembrane space contain?
Enzymes
- Ex: adenylate kinase (ATP + AMP -> 2ADP)
Where is ATP produced during TCA and oxidative phosphorylation? Where is it exporter? How?
- Produced in mitochondrial matrix
- Exported to the cytosolic space by exchangers (membrane transporters)
What are porins?
Channels that allow small compounds to pass through (ex: NADH and ATPase)
What is the inner membrane impermeable to? What does it less pass?
- Impermeable to charged compounds
- Lets small compounds like O2 pass through
How does pyruvate go through into the matrix?
Pyruvate is charged, will diffuse into intermembrane space, transporter carrier will transport it to the matrix (pyruvate translocase)
What are 2 other names for the TCA cycle?
Citric acid cycle or Krebs cycle
What goes into the TCA cycle?
Pyruvate in the mitochondria
What happens to pyruvate when it enters the mitochondria?
- Is decarboxylated and coupled to Coenzyme A (Co-A)
- Produces NADH
What is the conversion of pyruvate to acetyl-CoA catalyzed by? Is the delta G^o’ favourable?
- Pyruvate dehydrogenase complex
- Yes
Where is pyruvate dehydrogenase complex located?
In the inner membrane of mitochondria
What is the overall equation for the oxidative decarboxylation of pyruvate to form acetyl-CoA
pyruvate + NAD+ + CoASH → acetyl-CoA + CO2 + NADH + H+
What is pyruvate dehydrogenase formed of?
- Multi-subunit with 3 activities
- E1, E2, E3
What is the activity of E1, E2, E3?
E1: pyruvate decarboxylation
E2: acetyl CoASH transfer
E3: re-oxidize dihydrolipoamide
What are the coenzymes for E1, E2, E3?
E1: TPP
E2: lipoic acid and CoASH
E3: NAD+ FAD
What is the overall TCA reaction?
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi +2H2O → 2CO2 + 3NADH + FADH2 + Co-ASH + GTP + 2H+
How many CO2 molecules does 1 glucose molecule make?
1 Glucose = 6 CO2
Explain the structure of the pyruvate dehydrogenase complex.
Outer: E1
Middle: E2
Centre: E3
Why is the pyruvate dehydrogenase complex efficient? What can this structure be called?
- Substrate never released from enzyme complex, passes substrate on the the next step directly
- Substrate channel
Name the 3 coenzymes in the TCA cycle
- Thiamine pyrophosphate (TPP)
- Lipoic acid
- Coenzyme A
What does TPP do?
- Thiazolium ring has a very acidic H
- Will leave a - charge on the carbon -> generates carbanion
- Which attracts pyruvate for decarboxylation
What does lipoic acid do?
Disulfide bond is easily interconverted between SH and SS
- Adds or removes 2e- and 2p+
What does coenzyme A do?
forms high energy thioesther because of the SH
NADH and FADH2 are both high energy electron carriers. What do they carry?
NADH: 2e- and 2p+
FADH2: 2e- and 2p+
What goes in/comes out of the mechanism for pyruvate dehydrogenase?
In: pyruvate, acetyl CoA, NAD+
Out: CO2, NADH, acetyl CoA
What does TPP do in the mechanism for pyruvate dehydrogenase?
TPP attacks pyruvate and forms a covalent C-C bond, which breaks another C-bond and releases a molecule of CO2
What does HETPP do in the mechanism for pyruvate dehydrogenase?
- Covalent intermediate (needs to be taken off)
- Nucleophile attaks lipoic acid, forms intermediate with a covalent esther bond
Why does arsenic kill?
Because it reacts with hydrolipoic acid, can’t regenerate lipoic acid, no more TCA cycle
What intermediates can be pulled out of the TCA cycle to make other compounds?
Protein and glucose
TCA cycle is not just for energy production. What does it also do?
A reservoir for making precursors
What is the anaplerotic reaction? What is it catalyzed by?
- Mechanism that keeps supplies of oxaloacetate very high
- Catalyzed by pyruvate carboxylase
Name the 3 enzymes that regulate the TCA cycle.
1) Citrate synthase
2) Isocitrate dehydrogenase
3) a ketoglutarate dehydrogenase
What reaction is citrate synthase responsible for?
Oxaloacetate + acetyl -> citrate
What is citrate synthase inhibited by?
succinyl-CoA, citrate , ATP and NADH
What ratios are high in resting cells?
NADH/NAD+ and ATP/ADP
What reaction is isocitrate dehydrogenase responsible for?
citrate↔isocitrate → α−ketoglutarate
What is isocitrate dehydrogenase inhibited by?
inhibited by high levels of ATP & NADH
What is isocitrate dehydrogenase activated by?
activated by high level of ADP and NAD+
What reaction is a-ketoglutarate dehydrogenase responsible for?
α ketoglutarate → succinyl-CoA
What is a-ketoglutarate inhibited by?
high levels of NADH & succinyl-CoA
What is a-ketoglutarate activated by?
activated by high level of AMP
How many molecules of ATP does one pyruvate generate? One molecule of glucose?
12.5 ATP per pyruvate
25 ATP per glucose
What is produced in the TCA cycle?
4 NADH, 1 FADH2, 1 GTP
How many molecules of ATP does glycolysis generate?
- 2 ATP + 2 NADH (2.5 ATP per NADH) -> 7 molecules of ATP
What is the net yield of ATP for one glucose molecule from glycolysis through oxidative phosphorylation?
30-32 ATP
Where is the pyruvate dehydrogenase complex located?
Matrix
Where is the NADH dehydrogenase complex located?
Inner membrane
Where are DNA and ribosomes located?
matrix
Where is succinate dehydrogenase complex located?
Inner membrane
Where are the electron transport chain components located?
Inner membrane
Where is ATPase located?
Inner membrane
Which steps in the TCA cycle are not favourable?
Cis-Aconitase (13.3 kJ/mol) Malate dehydrogenase (29.7 kJ/mol)
Which step in the TCA cycle is at equilibrium (0 kJ/mol)?
Succinate dehydrogenase
What are three properties of O2 that favour energy extraction?
- O2 abundant (21% of air)
- O2 diffuses through membranes
- O2 very reactive and easily accepts electrons
