Title: BV2 / BV2 - 2 / Modul 5 / Citronsyracykeln

Q: What happens with the pyruvate molecules formed in glycolysis?
A: Once pyruvate molecules are formed via glycolysis, the pyruvate will then move into the mitochondria to undergo a series of oxidation reactions collectively called the citric acid cycle

Q: What is the TCA used as?
A: The citric acid cycle is used as the common metabolic pathway for the breakdown of fuel molecules, such as carbohydrates, amino acids or fatty acids

Q: WHat does the citric acid function to do?
A: 1. Provide a means by which any fuel molecule that contains an acetyl group can be oxidized to form ATP molecules
2. Provide the building blocks needed to form biological molecules such as nitrogenous bases, amino acids, glucose and porphyrin molecules

Q: Which 4 steps can Krebs be divided into?
A: The Krebs cycle divided into 4 steps:
1. The 2-carbon acetyl group of acetyl-CoA is combined with the 4-carbon molecule to form tricarboxylic acid
2. The 6-carbon tricarboxylic acid is decarboxylated to release CO2 and a hydride ion that is picked up by NAD+ to form NADH
3. The 5-carbon molecule is decarboxylated again to release CO2 and NADH
4. The 4-carbon molecule is converted back into oxaloacetate; in the process, an NADH, FADH2 and GTP are formed

Q: What is not formed in the TCA cycle?
A: Notice that no high-energy ATP molecules are actually formed in the citric acid cycle

Q: WHy are no high energy molecules formed in the TCA cycle?
A: This is because the citric acid cycle is actually the gateway to forming ATP, it harvests high-energy electrons that will be used by the electron transport chain to generate the proton gradient needed to form ATP molecules

Q: WHat happens in step 1 of the citric acid cycle?
A: 1. Synthesis (-31.4 kJ/mol)
  1. Citrate synthase catalyses the reaction of acetyl-CoA and oxaloacetate reacting to form citrate

Q: WHat happens in step 2 of the citric acid cycle?
A: 1. Isomerisation (+ 6.3 kJ/mol)
  1. Aconitase catalyses the isomerisation for citrate to become isocitrate

Q: What happens in step 3 of the citric acid cycle?
A: 1. Oxidative decarboxylation (-8.4 kJ/mol)
  1. Isocitrate dehydrogenase catalyses the oxidation of NAD+ to NADH + H+ and the decarboxylation that removes 1 CO2, this results in the formation of alfa-ketoglutarate

Q: What happens in step 4 of the citric acid cycle?
A: 1. Oxidative carboxylation (-30.1 kJ/mol)
  1. Alfa-ketoglutarate dehydrogenase complex catalyses the reaction in which alfa-ketoglutarate becomes succinyl-CoA via the oxidation of NAD+ to NADH + H+ and the decarboxylation removing 1 CO2, as well as the addition of CoA

Q: What happens in step 5 of the citric acid cycle?
A: 1. GTP synthesis (-3.3 kJ/mol)
  1. Succinyl-CoA becomes succinate through the catalysation by the enzyme succinyl-CoA synthetase as Pi and GDP reacts with succinyl-CoA and GTP and CoA are removed

Q: What happens in step 6 of the citric acid cycle?
A: 1. Redox reaction (0 kJ/mol)
  1. Succinate dehydrogenase catalyses the reaction where succinate becomes fumarate by dehydration and the help of the co-enzyme FAD, which reduces to form FADH2 as succinate is oxidised

Q: What happens in step 7 of the citric acid cycle?
A: 1. Hydration (-3.8 kJ/mol)
  1. Fumarase catalyses the reaction in which fumarate becomes malate through hydration of fumarate

Q: What happens in step 8 of the citric acid cycle?
A: 1. Redox reaction (+29.7 kJ/mol)
  1. Lastly, malate dehydrogenase catalyses the last step of the TCA cycle in which malate is oxidised through a dehydration reaction into oxaloacetate, where the co-factor NAD+ gets reduced into NADH + H+

Q: How many regulatory points does the TCA cycle have?
A: The TCA cycle has 2 regulatory points that are used to control its rate. Both of these points are allosteric enzymes used by the TCA cycle

Q: Which 2 enzymes regulate the TCA cycle?
A: - Isocitrate dehydrogenase
- Alfa-ketoglutarate dehydrogenase

Q: What does isocitrate dehydrogenase do?
A: - Isocitrate dehydrogenase catalyses the first oxidative decarboxylation step of the citric acid cycle
  - It's an allosteric enzyme that is affected by several allosteric effects

Q: How is isocitrate dehydrogenase activated?
A: When the cells needs to produce energy molecules, ADP will bind onto a regulatory site of the enzyme. This will increase the affinity of the enzyme for the isocitrate substrate, which will increase the rate of the citric acid cycle

Q: How is isocitrate dehydrogenase inhibited?
A: - If we have high levels of ATP, we will not need to generate any more.
  - In fact, ATP will act as an allosteric inhibitor of isocitrate dehydrogenase and decrease its affinity for the substrate.
  - In addition, NADH will also act as an allosteric inhibitor

Q: WHich substances affect the activity of isocitrate dehydrogenase?
A: Activate
- ADP
Inhibit
- ATP
- NADH

Q: WHat does alfa-ketoglutarate dehydrogenase catalyse?
A: Alfa-ketoglutarate dehydrogenase catalyses the second oxidative decarboxylation step

Q: WHat happens to alfa-ketoglutarate dehydrogenase at a high energy charge?
A: When the energy charge of the cell is high, ATP as well as the product molecules succinyl-CoA and NADH act as allosteric inhibitors

Q: What substance can also inhibit the TCA cycle?
A: Also, citrate inhibits glycolysis if there is a high concentration of it in the cell -> negative feedback loop

Q: What is required for the TCA cycle to keep up with the energy demands of the cell?
A: For the TCA cycle to keep up with the energy demands of the cell, the concentrations of the intermediate molecules such as oxaloacetate must be maintained at a minimal level.

Q: How can the levels of the oxaloacetate be
maintained when  our cells deplete oxaloacetate to form amino
acids, glucose and nitrogenous bases (reaction)?
A: By the following reaction:
Pyruvate + CO2 + ATP + H2O -> Oxaloacetate + ADP + Pi + 2H+

Q: Which enzyme is used to replenish oxaloacetate in the Krebs cycle?
A: Pyruvate
carboxylase

Q: What will happen to oxaloacetate under high energy conditions?
A: Under high energy conditions, the oxaloacetate will be converted into a variety of products such as glucose, amino acids and nitrogenous bases

Q: What will happen to oxaloacetate under low energy conditions?
A: Under low energy conditions, the oxaloacetate concentration will be replenished by action of pyruvate carboxylase

Q: What can the intermediate molecule citrate in the TCA cycle become?
A: Fats

Q: What can the intermediate molecule alfa-ketoglutarate in the TCA cycle become?
A: - Amino acids
- Purine bases

Q: What can the intermediate molecule succinyl-CoA in the TCA cycle become?
A: - Heme group
- Porphyrin group

Q: What can the intermediate molecule oxaloacetate in the TCA cycle become?
A: - Glucose
- Amino acids
- Purine bases
- Pyrimidine bases