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Questions and Answers
What is the purpose of the thioester bond in the citric acid cycle?
What is the purpose of the thioester bond in the citric acid cycle?
- To synthesize ATP
- To facilitate the breakdown of glucose
- To store energy for the cell (correct)
- To oxidize NADH
The carbons in CO2 originate from pyruvate in the citric acid cycle.
The carbons in CO2 originate from pyruvate in the citric acid cycle.
False (B)
What is the function of the enzyme succinyl-CoA synthetase?
What is the function of the enzyme succinyl-CoA synthetase?
Phosphoryl transfer to GDP, producing succinate plus GTP
The energy from the thioester bond is used to produce ______________ during substrate-level phosphorylation.
The energy from the thioester bond is used to produce ______________ during substrate-level phosphorylation.
What is the product of the succinate dehydrogenase reaction?
What is the product of the succinate dehydrogenase reaction?
Match the enzyme with its function:
Match the enzyme with its function:
GTP is not as good as ATP in the cell.
GTP is not as good as ATP in the cell.
What is the byproduct of the citric acid cycle after two pyruvates go through the cycle?
What is the byproduct of the citric acid cycle after two pyruvates go through the cycle?
What is the role of the iron-sulfur center in aconitase?
What is the role of the iron-sulfur center in aconitase?
The carbon lost as CO2 in the isocitrate dehydrogenase reaction is originally from carbons in acetyl-CoA.
The carbon lost as CO2 in the isocitrate dehydrogenase reaction is originally from carbons in acetyl-CoA.
What is the function of the manganese ion in the isocitrate dehydrogenase reaction?
What is the function of the manganese ion in the isocitrate dehydrogenase reaction?
Aconitase also regulates ___________ uptake and metabolism in the cell.
Aconitase also regulates ___________ uptake and metabolism in the cell.
What is the similarity between the α-ketoglutarate dehydrogenase complex and the pyruvate dehydrogenase complex?
What is the similarity between the α-ketoglutarate dehydrogenase complex and the pyruvate dehydrogenase complex?
What is the effect of inhibiting isocitrate dehydrogenase?
What is the effect of inhibiting isocitrate dehydrogenase?
The iron-sulfur center is only involved in substrate binding.
The iron-sulfur center is only involved in substrate binding.
Match the following enzymes with their respective cofactors:
Match the following enzymes with their respective cofactors:
Citrate synthase is activated by succinyl-CoA.
Citrate synthase is activated by succinyl-CoA.
What is the reaction catalyzed by isocitrate dehydrogenase?
What is the reaction catalyzed by isocitrate dehydrogenase?
What is the role of α-ketoglutarate in the citric acid cycle?
What is the role of α-ketoglutarate in the citric acid cycle?
The Aconitase reaction is a ______________ reaction.
The Aconitase reaction is a ______________ reaction.
What is the function of succinyl-CoA in the citric acid cycle?
What is the function of succinyl-CoA in the citric acid cycle?
The citric acid cycle and PDH complex are regulated independently.
The citric acid cycle and PDH complex are regulated independently.
Match the following enzymes with their functions:
Match the following enzymes with their functions:
What is the effect of accumulated citrate on phosphofructokinase-1 in glycolysis?
What is the effect of accumulated citrate on phosphofructokinase-1 in glycolysis?
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Study Notes
Iron-Sulfur Center in Aconitase
- Iron-sulfur center acts in both substrate binding and catalysis (dehydration and hydration)
- Aconitase regulates iron uptake and metabolism in the cell
Isocitrate Dehydrogenase
- Isocitrate dehydrogenase is an oxidation coupled to a hydride transfer to NAD+
- The enzyme uses a Mn2+ ion cofactor, enhancing the electron withdrawing power of the carbonyl and facilitating decarboxylation
- Carbon lost as CO2 is not originally from carbons in acetyl-CoA
α-KG Dehydrogenase Complex
- α-ketoglutarate dehydrogenase complex performs oxidative decarboxylation, splits the carbon-carbon bond, releases CO2, and reduces NAD+ to NADH
- Has a structure similar to PDH complex and also works similarly
- Has E1, E2, E3 and coenzymes TPP, lipoyllysine, CoA, FAD, and NAD+
- The energy of the oxidation is conserved in the form of a thioester bond
Similar Reactions
- Thioester bond is a high-energy bond with very negative free-energy of hydrolysis
- α-KG dehydrogenase complex and PDH complex share similarities
Origin of Carbon Atoms in CO2
- The carbons in CO2 originate from oxaloacetate, not from pyruvate
- Net complete oxidation of glucose occurs after two pyruvates go through the citric acid cycle
Succinyl-CoA Synthetase
- Succinyl-CoA synthetase performs phosphorylysis of the thioester bond, followed by phosphoryl transfer to GDP, producing succinate plus GTP
- Transfer of phosphate is mediated by His residue in the active site
- GTP is as good as ATP, with free conversion of NTPs in the cell
Succinate Dehydrogenase
- Succinate dehydrogenase converts succinate to fumarate, with reducing power transferred to FAD
- FADH2 yields 1.5 ATP
- Rate controlling enzymes: citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase
Regulation of Activity
- Regulation of activity: activated by substrate availability, inhibited by product accumulation
- Allosteric inhibition or activation by other intermediates
Other Regulation Mechanisms
- Citrate synthase is also inhibited by succinyl-CoA
- α-ketoglutarate is an important branch point for amino acid metabolism
- Succinyl-CoA communicates the flow at this branch point to the start of the cycle
- Inhibition of isocitrate dehydrogenase leads to accumulation of isocitrate, pushing the equilibrium towards citrate
- Accumulated citrate leaves mitochondria and inhibits phosphofructokinase-1 in glycolysis
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