TCA Cycle Flashcards
Where does the citric acid cycle occur?
It takes place in the mitochondrial matrix
The citric acid cycle occurs under ________ (aerobic/anaerobic) conditions?
Aerobic
What is the carbon source for the citric acid cycle?
Acetyl CoA (via acetyl group)
Acetyl Coa is generated from what molecule prior to entry into the citric acid cycle?
It is generated from pyruvate (Remember: pyruvate is first decarboxylated)
Where in the mitochondira does the Citric acid cycle occur?
In the mitochondrial matrix
During the citric acid cycle the oxidation of 2 carbon units (via acetyl CoA) produce what other molecules?
2 CO2 molecules
1 GTP
Electrons in the form of NADH and FADH2
Summarize cellular respiration.
- High energy electorns are removed from carbon fuels
- Electrons reduce O2 - generating a proton gradient
- Gradient is used to synthesize ATP in oxidative phosphorylation
How are glycolysis and TCA cycle linked?
- Carbohydrates )glucose) can be can be converted to pyruvate in glycolysis
- Pyruvate from glycolysis can be converted to acetyl CoA
- Under aerobic conditions pyruvate is converted to lactate or ethanol
- Aerobic conditions result in pyruvate enter the mitochondria
What happens to pyruvate under anaerobic condtions?
it is converted into lactate or ethanol
What happens to pyruvate under aerobic conditions?
Pyruvate enters the mitochondria (TCA cycle)
Pyruvate dehydrogenase component: (abbreviation, number og chains, prosthetic group, reaction catalyzed)?
- Abbreviation - E1
- Number of chains - 24
- Prosthetic group - TPP
- Rxn catalyzed - oxidative decarboxylation of pyruvate
Dihydrolipoyl transacetylase: (abbreviation, number of chains, prosthetic group, reaction catalyzed)?
- Abbreviation - E2
- Number of chains - 24
- Prosthetic group - Lipoamide
- Rxn catalyzed - Transfer of acetyl group to CoA
Dihydrolipoyl dehydrogenase: (abbreviation, number of chains, prosthetic group, reaction catalyzed)?
- Abbreviation - E3
- Number of chains - 12
- Prosthetic group - FAD
- Rxn catalyzed - Regeneration of the oxidized form of lipoamide
The generation of acetyl CoA from pyruvate requires 3 enzymes and 5 coenzymes. What are the 3 enzymes?
Pyruvate dehydrogenase (E1)
Dihydrolipoyl transacetylase (E2)
Dihydrolipoyl Dehydrogenase (E3)
The generation of acetyl CoA from pyruvate requires 3 enzymes and 5 coenzymes. What are the 5 coenzymes?
Catalytic cofactors
- Thiamine Pyrophosphate (TPP)
- Lipoic Acid
- FAD
Stoichiometric cofactors
- CoA
- NAD+
What are the 3 steps/reactions that occur in order to generate acetyl CoA from pyruvate?
- Decarboxylation
- Oxidation
- Transfer of acetyl group to coenzyme A
** Reactions are coupled to preserve energy to drive formation of NADH and acetyl CoA
Decarboxylation of pyruvate: pyruvate is combined with TPP and then decarboxylated to form what molecule?
Hydroxyethyl-TPP
The decarboxylation of pyruvate is catalyzed by what enzyme?
Pyruvate dehydrogenase component (E1) using TPP as the prosthetic group
Summarize the mechanism behind the decarboxylation of pyruvate.
- H of TPP is very acidic
- Carbanion readily attacks carbonyl gorup of pyruvate (2) and decraboxylation of pyruvate occurs (3) before protonation
Summarize the oxidation reaction required to generate Acetyl CoA from pyruvate.
- Hydroxyethyl group attached to TPP is oxidized to an acetyl gorup
- Acetyl group is transferred to lipoamide (lipoic acid derivative) lilnked to a lysine residue of E1
- Creates an energy rich thioseter bond in acetyllipoamide
How is Acetyl CoA formed?
- Acetyl group transferred to CoA
- E2 catalyzes reaction
- Energy rich thioester bond remains
- Acetyl CoA used in TCA
Summarize the regeneration of lipoamide in the PDH reaction.
- Dihydrolipoamide must be reconverter to lipoamide to conduct another reaction
- Oxidized form of lipoamids regeneration by E3
- 2 electrons transferred to FAD then NAD+ (unique)
- Electron transfer potential of FAD uncreased because it is tightly associated with the enzyme
Summarize the pyruvate dehydrogenase reaction (6 steps)
- Pyruvate decarboxylation at E1
- Hydroxyethyl-TPP intermediate found
- CO2 is first product
- Lipoamide arm of E2 is inserted into a deep E1 channel containing the active site
- E1 catalyzes the transfer of acetyl group to lipoamide chain
- acetylated arm enters E2 active site
- Acetyl moeity transferred to CoA and second product, acetyl CoA leaves
- reduced lipoamide arm swings to active site of E3 FAD contianing protein
- At active site E3. lipoamide is oxidized by coenzyme FAD
- Reactive lipoamide ready for another cycle
- Final product, NADH is produced with reoxidation of FADH2 to FAD
** proximity of all enzymes increases the reaction rate and keeps all intermediates bound and are readily transferred as the fleixble arm of E2 calls on each active site in turn
How is pyruvate dehydrogenase regulated?
- Regulated allosterically and by reverse phosphorylation
- High [acetyl CoA] directly inhibits E2
- Products also increase phosphorylation of PDH E1, while acucmulation of ADP and pyruvate activate phosphatases
