TCA Cycle Flashcards
Main function of TCA cycle
Capture 8 electrons from Acetyl CoA
Acetyl CoA gets completely _____
Oxidized
TCA cycle chemistry
4 oxidation reduction reactions that transfer electrons
What type of bonds are important in the TCA cycle? Why?
Thioester bonds because they are high energy
Key regulatory enzyme are regulated by ?
ALL are regulated allosterically by energy charge
NONE are regulated hormonally
What can enter the TCA cycle?
All types of fuels- CHO, FA, amino acids, ethanol
Intermediates from the TCA cycle are also used for?
Biosynthesis
Ex. Making amino acids, substrates for purines and pyrimidines, protoporphyrin ring in heme
Total yield per 1 Acetyl CoA
2 CO2 3 NADH 1 FADH2 1 ATP (or GTP) 8 electrons
Catabolic role of TCA cycle? (4)
- Oxidizes acetyl CoA —> releases CO2
- Produces and delivers reducing equivalents (NADH and FADH2) to ETC
- Regenerates oxaloacetate (OAA)
- Generates a high energy molecule (ATP or GTP)
2 phases of the TCA cycle
Phase 1 = 4 reactions
- Oxidation of Acetyl CoA to yield 8 electrons and release 2 CO2
Phase 2 = 4 reactions
- Regenerate OAA
What needs to happen before acetyl CoA can be oxidized and why?
Have to combine acetyl CoA (2-C) with oxaloacetate (4-C) and carry out a beta cleavage
B/c acetyl CoA cannot be oxidized directly
BASIC summary of TCA cycle
Oxidize acetyl CoA —> release CO2 —> regenerate OAA —> capture all the energy as NADH and ATP
Step 1
Condensation reaction
Oxaloacetate + Acetyl CoA —> Citrate + CoA + H+
Enzyme: Citrate synthase
Ordered binding
- What step?
- What happens and why?
Step 1
OAA has to bind to citrate synthase first —> induce a conformational change to create a binding site for acetyl CoA
When is the thioester bond in acetyl CoA hydrolyzed?
Importance?
Step 1
Nrg release - ensures that reaction proceeds irreversibly in forward direction because OAA as a substrate in citrate synthase reaction is not sufficient enough to drive the cycle forward
Step 1 is a __________ reaction
Explain
First and last reaction of cycle are COUPLED
OAA + Acetyl CoA —> Citrate + CoA + H+ (first- favorable)
Enzyme: Citrate synthase
L-maleate + NAD+ —> OAA + NADH + H+ (last- unfavorable)
Enzyme: Malate dehydrogenase
How does step 1 proceed in the forward direction?
Combo of OAA being used as substrate in the reaction (product of last reaction) AND thioester bond in acetyl CoA being hydrolyzed
Step 2
Isomerization
Citrate Isocitrate
Importance:
Citrate is a poor substrate for oxidation
Isocitrate has a secondary -OH, which can be oxidized
Allows for oxidative decarboxylation in next step
Step 3
Oxidative Decarboxylation of Isocitrate
Isocitrate —> Oxalosuccinate —> alpha-ketoglutarate
RATE LIMITING STEP
Enzyme: Isocitrate dehydrogenase
Rate limiting step of TCA cycle
Why?
Step 3- Oxidative decarboxylation of Isocitrate
Why?
Because this is the first step in the cycle that produces NADH which a has to be regenerated to NAD+ to continue cycle
TCA cycle is dependent on the rate of ________ —> __________ is dependent on the rate of _______________
TCA cycle is dependent on the rate of the electron transport chain —> electron transport chain is dependent on the rate of ATP utilization
Use more ATP —> _______ electron transport chain
Faster
Faster regenerate NAD+ —> ____________ TCA Cycle
Faster
If ATP levels build up, how does this affect the TCA cycle and ETC?
If ATP builds up —> ETC slows down —> NADH builds up —> feeds back and inhibits isocitrate dehydrogenase
By what are ETC and TCA cycle chemically linked?
Energy Charge
Step 3 “Oxidative”
NAD+ —> NADH
Oxidation of alcohol group to keto group (C=O)
Step 3 “Decarboxylation”
Get rid of a CO2
Step 4
Second Oxidative Decarboxylation
Alpha-ketoglutarate + NAD+ + CoA —> Succinylcholine CoA + CO2 + NADH
Enzyme: alpha-ketoglutarate dehydrogenase
- Produces a thioester bond in succinyl CoA
The enzyme alpha-ketoglutarate dehydrogenase is identically to _____ mechanistically and structurally
Pyruvate dehydrogenase complex
Importance of succinyl CoA
Has a thioester bond
Step 5
Substrate level Phosphorylation
Succinyl CoA + Pi + GDP (or ADP) —> Succinate + CoA + GTP (or ATP)
Explain substrate level phosphorylation
Transfer an inorganic phosphate to either GDP or ADP using energy from the hydrolysis of a CoA thioester bond
Step 6
Dehydrogenation
Succinate + FAD —> fumerate + FADH2
Enzyme: Succinate dehydrogenase
Importance of Succinate dehydrogenase
Bound to the inner mitochondrial membrane
Part of complex II
______ is covalently bound to Succinate dehydrogenase.
Why?
FAD
To reduce the activity of the molecule because the FADH radical can damage. So ensure the full reduction to FADH2 by accepting 1 e-/1H+ and 1e-/1H+ in succession
Step 7
Hydration
Fumerate —> Malate
Add a water molecule across the double bond
Step 8
NAD+ Dependent Oxidation
L-Malate + NAD+ —> OAA + NADH + H+
Enzyme: Malate dehydrogenase
3 enzymes that are regulated
- Citrate synthase
- Isocitrate dehydrogenase
- Alpha-keto dehydrogenase
Regulation of citrate synthase
- [OAA] - very limiting
2. Feedback inhibited by product, citrate
concentration of ___ and ___ fluctuate a lot in cell depending on metabolic needs
OAA and acetyl CoA
Regulation of alpha-ketoglutarate dehydrogenase
ATP, NADH Inhibit
Succinyl CoA feedback Inhibit
ATP plays 2 roles
Allosteric modifier of many key enzymes and indirect feedback inhibits
Usually…..
ATP and _____ go together
ADP and _____ go together
ATP & NADH
ADP & NAD+
Is molecular oxygen consumed in TCA cycle?
NO!
Why is TCA cycle still considered part of aerobic respiration?
Even though molecular oxygen is not consumed, oxygen is required as the final electron acceptor in the ETC. It is needed for the regeneration of NAD+ and FAD
ATP controls its ________.
How?
Own rate of synthesis
By acting as negative allosteric modifier on enzymes in glycolysis, on PDC, and TCA cycle
Why do you need hormonal regulation at PDC and pyruvate kinase?
Critical when pathways share reactions
Ex. Glycolysis and gluconeogenesis
Cells in resting metabolic state
High ATP/ADP
Hight NADH/NAD+
Cells in highly active metabolic state
Low ATP/ADP
Low NADH/NAD+
3 molecules that DO NOT leave the mitochondria directly
- Acetyl CoA
- OAA
- NADH
Citrate in other pathways
- Exits TCA cycle in fed state
- Used as carrier for 2-C of acetyl CoA
- Synthesis of fatty acids/sterols
Alpha-ketoglutarate in other pathways
- Glutamic acid synthesis by adding amino group
- Make other amino acids and purines
- Reaction can go both ways
Succinyl CoA in other pathways
- Building block for porphyrin ring structure
Ex. Home
Oxaloacetate in other pathways
- keep TCA cycle going
- gluconeogenesis
- asparatic acid synthesis by adding amino acid to make other amino acids, purines, pyrimidines (goes both ways)
Pyruvate carboxylase function
Type of reaction? How?
Can generate OAA when TCA cycle intermediates are used in other reactions
“Anapleurotic reaction”
Use pyruvate as a substrate —> Add CO2 —> yield 4-C OAA
Why is pyruvate carboxylase important (2)
- Maintains TCA cycle in fed AND fasted states
2. Supplies OAA in liver for gluconeogenesis
Regulation of Isocitrate dehydrogenase
RATE LIMITING STEP - 1st reaction that produces NADH —> which must be oxidized to NAD+ in ETC for TCA cycle to continue in this step
ADP and NAD+ = positive allosteric modifier (activate)
(Low energy charge stimulates)
NADH and ATP = negative allosteric modifier (inhibit)