S1B4 - TCA Cycle Flashcards
What metabolite from the citric acid cycle is used in…
- gluconeogenesis?
- fatty acid synthesis?
- oxaloacetate is used in gluconeogenesis
- citrate is involved in the pathway of fatty acid synthesis
How is the succinate dehydrogenase complex of the citric acid cycle related to the electron transport chain?
The succinate dehydrogenase complex catalyzes oxidation of succinate to fumarate.
Note that succinate dehydrogenase is embedded in the inner mitochondrial membrane as complex II of the electron transport chain.
What metabolic poison inhibits the enzyme aconitase?
Fluorocitrate, a metabolite of the pesticide fluoroacetate, inhibits the enzyme aconitase.
What reaction in the citric acid cycle (TCA cycle) produces GTP?
Succinyl-CoA synthetase converts succinyl-CoA to succinate and CoA. In the process, substrate-level phosphorylation produces GTP. The entire reaction is:
- Succinyl CoA + Pi + GDP ↔ Succinate + CoA + GTP
For each acetyl-CoA molecule that goes through the citric acid cycle and the electron transport chain, how many molecules of ATP are produced?
The net result of the oxidation (using both substrate level and oxidative phosphorylation) of one molecule of acetyl CoA is:
- 3 NADH x 3 ATP/NADH → 9 ATP
- 1 FADH2 x 2 ATP/FADH2 → 2 ATP
- 1 GTP → 1 ATP equivalent
For a total of 10-12 ATP molecules per acetyl CoA (12 is the ideal yield but the actual yield may be given as 10 is some texts because of imperfect efficiency.)
Which citric acid cycle reaction results in generation of FADH2?
FADH2 is produced in the conversion of succinate to fumarate. This reaction is catalyzed by succinate dehydrogenase.
What are some vitamins necessary for the α-ketoglutarate dehydrogenase complex to function?
The α-ketoglutarate dehydrogenase complex is one of the three regulated steps of the citric acid cycle. It requires many coenzymes, including:
- Vitamin B1
- Vitamin B2
- Vitamin B3
- CoA
- Lipoic acid
Note: these are the same cofactors as in the pyruvate dehydrogenase complex.
In the Krebs cycle, which enzyme catalyzes the conversion of citrate into isocitrate?
Aconitase catalyzes the isomerization of citrate into isocitrate.
Fluorocitrate, a metabolite of the pesticide fluoroacetate, inhibits the enzyme aconitase.
Which low energy molecule stimulates isocitrate dehydrogenase?
Which two high energy molecules inhibit isocitrate dehydrogenase?
Isocitrate dehydrogenase is stimulated by ADP (low energy state).
Isocitrate dehydrogenase is inhibited by ATP and NADH (high energy state).
What enzyme converts malate into oxaloacetate in the citric acid cycle?
Malate dehydrogenase oxidizes malate to oxaloacetate, and the cycle can begin anew.
In addition to the formation of alpha-ketoglutarate, what are the products of the reaction catalyzed by isocitrate dehydrogenase?
In the isocitrate dehydrogenase reaction, NAD+ → NADH, 1st molecule of CO2 is released.
What are the substrates and product of the enzyme citrate synthase?
Citrate synthase catalyzes the transfer of a 2-carbon acetyl group from acetyl-CoA to oxaloacetate, forming the 6-carbon molecule citrate.
What are some molecules that inhibit the enzyme α-ketoglutarate dehydrogenase?
α-ketoglutarate dehydrogenase is inhibited by:
- NADH
- Succinyl CoA
- ATP
- GTP
Which Krebs cycle reaction is catalyzed by the α-ketoglutarate dehydrogenase complex?
The α-ketoglutarate dehydrogenase complex converts α-ketoglutarate to succinyl-CoA.
The α-ketoglutarate dehydrogenase reaction catalyzes:
NAD+ → NADH, 2nd molecule of CO2 is released
In the second-to-last step of the Krebs cycle, fumarate is converted into ______________.
Mitochondrial fumarase converts fumarate to malate.
In which step of the citric acid cycle is the second molecule of CO2 released?
The α-ketoglutarate dehydrogenase reaction catalyzes:
NAD+ → NADH, 2nd molecule of CO2 is released
How many GTP, NADH, FADH2, and CO2 are produced per turn of the citric acid cycle?
For each turn, the citric acid cycle produces:
- 1 GTP
- 3 NADH
- 1 FADH2
- 2 CO2
Beginning with citrate, what are the substrates of the citric acid cycle?
The order of the citric acid cycle can be remembered with the following mnemonic:
“Citrate Is Krebs’ Starting Substrate For Making Oxaloacetate.”
- Citrate
- Isocitrate
- α-Ketoglutarate
- Succinyl CoA
- Succinate
- Fumarate
- Malate
- Oxaloacetate
In which cellular compartment does the citric acid cycle take place?
The citric acid cycle (tricarboxylic acid cycle or Krebs cycle) takes place in the mitochondrial matrix.
How is pyruvate transported into the mitochondria? What is the consequence of this transport on the energy yield from glucose metabolism?
Because the PDC is located in the mitochondrial matrix and the inner mitochondrial membrane is impermeable to pyruvate, pyruvate must be transported across the inner membrane by pyruvate translocase. The transport of pyruvate into mitochondria consumes energy, lowering the total ATP production of aerobic glucose metabolism.
Which two cofactors does pyruvate dehydrogenase complex E3 require?
E3 requires, as cofactors, FAD (vitamin B2) and NAD+ (vitamin B3).
How does phosphorylation regulate pyruvate dehydrogenase complex E1 activity?
The regulation of the pyruvate dehydrogenase complex can be allosteric or via phosphorylation/dephosphorylation.
Pyruvate dehydrogenase kinase (PDH kinase) and pyruvate dehydrogenase phosphatase are part of the PDC and act on E1. Phosphorylation by PDH kinase inhibits E1 (this can be remembered because there is abundant ATP in the high-energy state and ATP can be used to inhibit E1), while dephosphorylation through PDH phosphatase activates E1. Recall that the E1 subunit of the pyruvate dehydrogenase complex catalyzes the rate-limiting step of the PDC reaction.
What cofactor(s) does pyruvate dehydrogenase complex E2 require?
E2 requires, as cofactors, CoA ( CoA made from vitamin B5 aka pantothenate) and lipoic acid.
It is this step that produces acetyl-CoA (hence the need for CoA in this step)
Which pyruvate dehydrogenase complex enzyme is the rate limiting step?
E1 catalyzes the rate limiting step of the PDC reaction.
E1 requires, as a cofactor, thiamine pyrophosphate (TPP), a derivative of vitamin B1.
What metabolites activate the pyruvate dehydrogenase complex? What metabolites inhibit the pyruvate dehydrogenase complex?
The PDC is activated by pyruvate, NAD+, ADP, and Ca2+.
- This makes sense because pyruvate is a substrate for the PDC, NAD+ and ADP signal a low energy state in the cell, and Ca2+ influx during muscle contraction drives further energy production.
The PDC is inhibited by ATP, acetyl-CoA, and NADH.
- This makes sense because ATP and NADH signal a high energy state and acetyl-CoA is a product of the PDC reaction.
Which pyruvate dehydrogenase complex enzyme requires thiamine pyrophosphate as a cofactor?
E1 requires, as a cofactor, thiamine pyrophosphate (TPP), a derivative of vitamin B1.
Which enzyme in the pyruvate dehydrogenase complex leads to the production of NADH?
In reactions catalyzed by the E3 subunit of the pyruvate dehydrogenase complex, FAD oxidizes a lipoic acid intermediate back to lipoic acid so that it can participate in more reactions. In the process, FAD is reduced to FADH2. FADH2 is then used to reduce NAD+ to NADH, regenerating FAD. The NADH can then be used in oxidative phosphorylation by the electron transport chain.
What are the enzymes that make up the pyruvate dehydrogenase complex?
The PDC consists of multiple copies of three catalytic enzymes: E1 (pyruvate dehydrogenase), E2 (dihydrolipoyl transacetylase), and E3 (dihydrolipoyl dehydrogenase).
Two regulatory enzymes, pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase, are also part of the pyruvate dehydrogenase complex.
What reaction is catalyzed by the pyruvate dehydrogenase complex?
The following reaction is catalyzed by the pyruvate dehydrogenase complex:
Pyruvate + NAD+ + CoA → acetyl-CoA + CO2 + NADH
In the reactions catalyzed by the pyruvate dehydrogenase complex, which is not a cofactor?
A) Vitamin B12
B) Vitamin B2
C) Lipoic acid
D) Vitamin B3
E) VItamin B1
A) Vitamin B12
The PDC has 3 primary enzymes, each of which requires cofactor(s):
E1 uses vitamin B1 (thiamine pyrophosphate) as a cofactor
E2 uses CoA and lipoic acid as cofactors
E3 uses vitamin B2 (FAD) and vitamin B3 (NAD) as cofactors
Vitamin B12 (cobalamin) is a cofactor for the enzymes methylmalonyl Coenzyme A mutase and methionine synthase.
What is the entry point in the TCA cycle for metabolites from…
- fatty acid and glycerol metabolism?
- Polysaccharide metabolism?
- Protein metabolism?
They enter the TCA cycle as Acetyl-CoA. See picture.
What is the rate limiting step of the TCA cycle?
Oxitadive decarboxylation of isocitrate to form alpha-ketoglutarate is the rate limiting step of the TCA cycle.
This reaaction is catalyzed by isocitrate dehydrogenase.
This reaction produces the first NADH and first CO<strong>2</strong>
At what point in the TCA cycle does the only substrate leve phosphorylation occur?
The reaction from succinyl-CoA to succinate, catalyzed by Succinyl CoA synthetase/Succinyl thiokinase, is the only substrate level phosphorylation of the TCA cycle.
In the malate-aspartate shuttle, which molecule moves into the inter-membrane space in exchange for glutamate?
A glutamate-aspartate antiporter moves aspartate into the inter-membrane space in exchange for glutamate. Aspartate is first converted to oxaloacetate, which is then converted to malate.
What pump moves malate into the mitochondrial matrix?
A malate-α-ketoglutarate antiporter moves malate into the mitochondrial matrix.
What is the purpose of the malate-aspartate shuttle?
NADH produced during catabolism of glucose to pyruvate in the cytosol cannot cross the inner mitochondrial membrane → shuttle mechanisms like malate-aspartate shuttle allow NADH reducing equivalents to cross into the mitochondria to produce ATP.
What is the net ATP production from aerobic oxidation of one glucose?
The grand total of aerobic oxidation of one glucose is 38 ATP.
- 8 from glycolysis
- 6 from pyruvate dehydrogenase
- 24 from the TCA cycle
