Question

The effect of inorganic phosphate on the activity of the citric acid cycle can be studied in mitochondrial

extracts by monitoring the rate of CO2 production. a. Why is the citric acid cycle dependent upon inorganic phosphate? b. Acetyl CoA is added to a rat liver mitochondrial extract containing excess NAD+, FAD, and GDP. The rate of CO2 production increases and then levels off very quickly; the addition of addition acetyl CoA has no effect. Explain why CO2 production begins but then quickly ceases. c. When potassium phosphate is added to the system, CO2 production resumes but eventually levels off. Explain why CO2 production resumed and why it ceased. d. The experiment in part (c) is repeated with 14C-acetyl CoA. After 2 hours, the production of CO2 has ceased. A sample of the extract (not the CO2 gas!) is analyzed by TLC and the majority of the 14C is found in a single organic molecule. Name this molecule, draw its structure, and explain why it accumulates.

Answer 1

Ans a) The citric acid cycle dependent on inorganic phosphate as it helps in catalyzing the reactions. It is required in the catalysis when there is succinyl CoA synthetase. The succinyl CoA is converted into succinate as the GTP+CoA is converted into GDP and inorganic phosphate or Pi is required.

Ans b) The carbon dioxide production begins very quickly and ceases due to aerobic metabolism. The acetyl CoA is further oxidized to carbon dioxide in citric acid cycle. Due to the aerobic metabolism, there is continuous evolution of carbon dioxide in the beginning.