Week 6: Citric Acid Cycle Flashcards
Is the CAC aerobic or anaerobic?
Aerobic
Where is the citric acid cycle located?
Mitochondria in muscle and liver cells
Where are the enzymes for the CAC located?
Mitochondrial matrix
How does the RBC the receive energy if they don’t have mitochondria?
RBCs rely on energy generated from glycolysis
What are the 2 coenzymes for CAC?
NADH and FADH2
What would the CAC link?
The chemical energy of nutrients and chemical energy of ATP
Why would CAC be considered an aerobic metabolic process?
It links the ETC and oxidative phosphorylation
Which enzyme located in the inner membrane of the mitochondria and not the matrix?
Succinate dehydrogenase
How does pyruvate enter the mitochandria?
Must be transported from the cytosol into the matrix through transporter in the inner membrane
Where would would oxidative phosphorylation occur in?
Mitochondrial inner membrane
What needs to happen for glycolysis to transition to CAC?
- Conversion of pyruvate into Acetyl-CoA
2. Acetyl-CoA is enters the CAC
What occurs during the conversion of pyruvate to acetyl-CoA?
The oxidative decarboxylation of pyruvate by pyruvate dehydrogenase complex into Acetyl-CoA
Is pyruvate to Acetyl-CoA reversible or irreversible?
Irreversible
What are the 5 coenzymes involved with the conversion of pyruvate to Acetyl-CoA?
- TPP
- Lipoyllysine
- FAD
- NAD+
- CoA-SH
What are the 3 enzymes in the pyruvate dehydrogenase complex?
- Pyruvate dehydrogenase (E1)
- Dihydrolipoyl transacetylase (E2)
- Dihydrolipoyl dehydrogenase (E3)
What are the advantages of a multi-enzyme complex?
- Shorter distance between catalytic sites channeling of substrates from one catalytic site to another
- Channeling minimizes side reactions
- The regulation of activity of one subunit affects the entire complex
What enzymes of the CAC cause substrate-level phosphorylation?
Succinyl-CoA synthetase
What occurs in Step 1 of CAC?
Acetyl-CoA and oxaloacetate is condensated by citrate synthase to make citrate and CoA-SH
Why is Step 1 of CAC considered favorable and irreversible?
- Regulated by substrate availability and product inhibition
- Inhibited by increased concentration of ATP, NADH, citrate, and succinyl-CoA
- Stimulated by ADP
What occurs during Step 2 of CAC?
- H2O is eliminated from citrate by aconitase to for cis-Aconitate
- H2O is added to cis-Aconitate by aconitase to form isocitrate
Why does Step 2 of CAC require 2 steps?
Citrate is a poor substrate for oxidation, isocitrate is good for oxidation
Why is Step 2 of CAC unfavorable/reversible?
Product concentration kept low to pull forward
What occurs during Step 3 of CAC?
- Isocitrate is oxidized to oxalosuccinate by NAD+ and Isocitrate Dehydrogenase
- Oxalosuccinate is decarboxylated by Isocitrate Dehydrogenase to a-Ketoglutarate releasing CO2
Why is Step 3 of CAC considered favorable/irreversible?
- If ATP and NADH is high, enzyme is inhibited (high energy signal)
- Stimulated by increased ADP and Ca2+ (low energy signal)
What occurs during Step 4 of CAC?
a-ketoglutarate is oxidized and decarboxilated by a-ketoglutarate dehydrogenase complex to succinyl-CoA, NADH, and CO2
What kind of bond is Succinyl-CoA?
High energy thioester bond
Why is Step 4 of CAC favorable/irreversible?
- Succinyl-CoA or NADH increase inhibits enzyme
2. Enzyme is stimulated by icnreased Ca+
What occurs during Step 5 of CAC?
Thioester energy bond from Succinyl-CoA is broken by GDP, Pi, and succinyl-CoA synthetase to produce Succinate
Why is Step 5 of CAC considered favorable/reversible?
Product concentration kept low to pull forward
What occurs during Step 6 of CAC?
Succinate is oxidated by FAD and succinate dehydrogenase to fumarate
Why is FAD used instead of NAD in Step 6 of CAC?
Reduction potential of C-H bond is too low for production of NADH
Why is succinate dehydrogenase bound to the mitochondrial inner membrane?
It also serves as Complex II in the ETC
What is the delta G of Step 6 of CAC?
0
What occurs during Step 7 of CAC?
Fumarate is converted to L-Malate by fumerase and addition of H2O
Why is Step 7 of CAC favorable/reversible?
Product concentration kept low to pull reaction forward
What occurs during Step 8 of CAC?
L-Malate is oxidized by L-malate dehydrogenase and NAD+ to form oxaloacetate
What occurs during Step 8 of CAC?
L-Malate is oxidized by L-malate dehydrogenase and NAD+ to form oxaloacetate
Why is Step 8 of CAC unfavorable/reversible?
Oxaloacetate concentration kept very low by citrate synthase pulling the reaction forward
What is the overall reaction of the pyruvate dehydrogenase complex?
Pyruvate + CoA-SH + NAD+ → Acetyl-CoA + NADH + CO2 +H+
What is the overall reaction of the CAC?
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O → 2CO2 + CoA-SH + 3NADH + 3H+ + FADH2 + GTP
What is the overall reaction of pyruvate oxidation?
Pyruvate + 4NAD+ + FAD + GDP + Pi + 2H2O → 3CO2 + 4NADH + FADH2 + GTP + 4H+
How much is ATP is produced from CAC?
12.5 ATP/pyruvate
25 ATP/glucose
Why does NADH have a greater reduction potential of FADH2?
E is more negative
What does it mean when E is more negative?
The greater the energy available for ATP generation when that compound passes it electron to oxygen
How does oxaloacetate leave the mitochondria?
Reduced to malate which exits the mitochondria into cytosol
How is NADH produced in the cytosol?
Malate is oxidized to oxaloacetate by malate dehydrogenase
What does it mean to be amphibiolic?
Reactions that are both catabolic and anabolic
What happens to the CAC when energy levels are high?
The CAC is cycled is slowed and the intermediates are diverted to other purposes
What reactions are used to replenish other biomolecules?
Anaplerotic reactions
What enzymes of the CAC are highly favorable and irreversible steps?
- Pyruvate dehydrogenase complex
- Citrate synthase
- Isocitrate dehydrogenase
- Ketogluterate dehydrogenase complex
What are the general regulatory mechanism?
- Activated by substrate availability
2. Inhibited by product accumulation
What are the inhibitors of the CAC?
NADH and ATP
What are the activators of CAC?
NAD+, Ca2+, AMP, ADP
Which actives and inactivates Pyruvate Dehydrogenase?
Activate: Dephosphorylation
Inactivate: Phosphorylation
Which PDHs are part of the mammalian complex?
Kinase and phosphorylase
What happens to kinase when ATP levels are high?
PDH is phosphorylated and less Acetyl-CoA is produced
What happens to kinase when ATP levels are low?
Kinase is less active and phosphorylase removes phosphate from PDH and more acetyl-CoA is produced
What is a strong activator of the phosphatase?
Ca2+
How is citrate synthase inhibited by succinyl-CoA?
- α-Ketoglutarate is an important branch point for amino acid metabolism.
- Succinyl-CoA communicates flow at this branch point to the start of the cycle.
How does regulation of isocitrate dehydrogenase control citrate levels?
- Aconitase is reversible.
- Inhibition of isocitrate dehydrogenase leads to accumulation of isocitrate and reverses acconitase.
- Accumulated citrate leaves mitochondria and inhibits phosphofructokinase (PFK) in glycolysis.
