1. Citric acid cycle: A cell has a high demand for ATP and the energy charge in
ID: 93023 • Letter: 1
Question
1. Citric acid cycle: A cell has a high demand for ATP and the energy charge in the cell is low. The rate of oxidation of acetyl-CoA, and hence the production of ATP by oxidative phosphorylation, is affected by the overall concentration of the intermediates present in the citric acid cycle. Under these conditions:
a. What effect would a HIGH concentration of intermediates have on the rate of entry of acetylCoA into the cycle? (1 mark)
b. What effect would a LOW concentration of intermediates have on the rate of entry of acetylCoA into the cycle? (1 mark)
c. How can the concentration of the citric acid cycle intermediates be increased? Give one instance (3 marks)
Explanation / Answer
High concentration of intermediates indicates the energy sufficient metabolic state of the cell. It turns off the activity of pyruvate dehydrogenase complex and hence inhibits the conversion of pyruvate to Acetyl CoA. Low concentration of intermediates shows there is shortage of energy in the cell. It turns on the activity of Pyruvate dehydrogenase enzyme which increases the conversion of pyruvate to Acetyl CoA. The concentration of the citric acid cycle intermediates can be increased by increasing the concentration of ADP and Ca2+ ions. ADP is an allosteric activator of enzyme citrate synthase. Calcium ions activate both isocitrate dehydrogenase, alpha ketoglutarate dehydrogenase and also pyruvate dehydrogenase complex.
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