Assume you have a solution containing the pyruvate dehydrogenase complex and all

Question

Assume you have a solution containing the pyruvate dehydrogenase complex and all the enzymes of the TCA cycle, but none of the metabolic intermediates. When you supplement this solution with 5 moles each of pyruvate, oxaloacetate, coenzyme A, NAD+, FAD, GDP, and Pi, you find that 5 moles of CO2 are evolved and then the reaction stops. When you add alcohol dehydrogenase and its substrate acetaldehyde, additional CO2 is produced.

How many moles of acetaldehyde are required to allow complete oxidation of the pyruvate to 15 moles of CO2? (Do not take into account the concentration that is already in the system.)

Explanation / Answer

10 micromoles of acetaldehyde are required to allow complete oxidation of the pyruvate to 15 moles of CO2.

This can be explained with the explanation that by adding additional alcohol dehydrogenase plus its substrate you are actually allowed regeneration of NAD+. That is the reason why due to the presence of oxidized NAD+ the additional TCA reactions will take place producing additional CO2. So you will be getting 15 micromoles of pyruvate oxidized for 10 micromoles of acetaldehyde.

As you can see, the limited supply of NAD+ will deplete the reaction initially causing the reaction to stop, but when you add the acetaldehyde along with ADH enzyme, you can see the formation of ethanol from acetaldehyde through NAD+ recycling, thus causing more oxidation of pyruvate, this shows the each mole of NAD+ will be recycled by a mole of acetaldehyde liberates 1 mole of CO2, by taking the above points into consideration, we can say 10 micromoles of acetaldehyde is required.

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