24 Questions
What type of bond is formed during the oxidation process?
High-energy bond
The carbons in CO2 originate from pyruvate.
False
What is the function of the enzyme nucleoside diphosphate kinase?
To convert NTPs in the cell, making GTP as good as ATP.
During the Succinyl-CoA Synthetase reaction, the thioester bond is phosphorylyzed, followed by phosphoryl transfer to _______________________, producing succinate plus GTP.
GDP
Match the following enzymes with their respective functions:
Succinyl-CoA Synthetase = Phosphoryl transfer to GDP, producing succinate plus GTP. Nucleoside Diphosphate Kinase = Converts NTPs in the cell, making GTP as good as ATP. Succinate Dehydrogenase = Converts FADH to ATP, yielding 1.5 ATP.
What is the role of the His residue in the active site of Succinyl-CoA Synthetase?
Mediates phosphoryl transfer
The citric acid cycle produces a net complete oxidation of glucose after two pyruvates go through the cycle.
True
What is the result of the Succinate Dehydrogenase reaction?
Conversion of FADH to ATP, yielding 1.5 ATP.
What is the role of the iron-sulfur center in aconitase?
It acts in both substrate binding and catalysis
The carbon lost as CO2 in the isocitrate dehydrogenase reaction is originally from the carbons in acetyl-CoA.
False
What is the function of the Mn2+ ion cofactor in isocitrate dehydrogenase?
The Mn2+ ion cofactor enhances the electron-withdrawing power of the carbonyl, facilitating decarboxylation.
Aconitase also regulates ______________________ uptake and metabolism in the cell.
iron
What is the type of reaction that occurs in the α-ketoglutarate dehydrogenase complex?
Oxidative decarboxylation
The α-ketoglutarate dehydrogenase complex has a similar structure and function to the pyruvate dehydrogenase complex.
True
Match the following enzymes with their corresponding reactions:
Aconitase = Dehydration and hydration Isocitrate dehydrogenase = Oxidation coupled to hydride transfer to NAD+ α-Ketoglutarate dehydrogenase = Oxidative decarboxylation Enolase = Glycolysis
What is the role of the iron-sulfur center in the regulation of iron uptake and metabolism in the cell?
The iron-sulfur center regulates iron uptake and metabolism in the cell.
What is the effect of inhibiting isocitrate dehydrogenase on the citrate levels?
Increased citrate levels
Citrate synthase is activated by product accumulation.
False
What is the role of succinyl-CoA in the citric acid cycle?
Succinyl-CoA communicates the flow at the branch point between citric acid cycle and amino acid metabolism to the start of the cycle.
The aconitase reaction (Step #2) is _______________________
reversible
What is the function of the iron-sulfur center in the citric acid cycle?
Electron transfer
α-ketoglutarate dehydrogenase is a rate-controlling enzyme in the citric acid cycle.
True
What is the effect of accumulated citrate on phosphofructokinase-1 in glycolysis?
Inhibition
Match the following enzymes with their functions:
Citrate synthase = Controls citrate levels Isocitrate dehydrogenase = Regulates the citric acid cycle Aconitase = Dehydrates citrate to form isocitrate α-ketoglutarate dehydrogenase = Important branch point for amino acid metabolism
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
Quiz about the mechanisms of aconitase and isocitrate dehydrogenase enzymes, including iron-sulfur centers and cofactors.
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