Question

Yeast are single-celled eukaryotes. Imagine you are a biologist studying cellular respiration in yeast. You BIO 113 Spring 2023 have discovered a drug that binds and inhibits the enzyme that catalyzes pyruvate oxidation.

a. What would happen to pyruvate levels in yeast cells that have just been treated with this drug?
i. Pyruvate levels would increase.
ii. Pyruvate levels would decrease.
iii. Pyruvate levels would stay the same.
b. Explain your answer to part a 1-3 sentences.
c. Very soon after being treated with this drug, the process of glycolysis slows down. Why does this happen?
d. Eventually, yeast cells treated with this drug begin doing fermentation. What are two inputs of the process of fermentation?
e. One of the outputs (products) of fermentation is NAD* . What would these drug-treated yeast cells likely be using this NAD* for? Explain your answer.
f. You noticed that if you grow yeast cells in a liquid culture and treat them with the drug, bubbles start forming in the liquid. Why does this happen?
g. Which molecule(s) could these drug-treated cells use as their sole energy source? Select all correct answers.
i. glucose
ii. pyruvate
iii. acetyl-CoA
h. Explain your answer to part g in 2−3 sentences.

Answer 1

The drug inhibits pyruvate oxidation in yeast cells, causing an increase in pyruvate levels, slowing down glycolysis due to NAD+ depletion, and triggering fermentation to regenerate NAD+. Yeast can use glucose but not pyruvate or acetyl-CoA as an energy source under these conditions.

Step-by-step explanation:

The Pyruvate levels would increase. Yeast cells could use glucose as their sole energy source.When an enzyme that catalyzes pyruvate oxidation is inhibited, pyruvate cannot be converted into acetyl-CoA, causing pyruvate levels to accumulate. As a result, glycolysis slows down because the high concentration of pyruvate could inhibit key enzymes involved in the glycolysis pathway or because NAD+ becomes limited, impeding the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.

Cells begin fermentation to regenerate NAD+ from NADH to allow glycolysis to continue producing ATP. During alcoholic fermentation, a drug-treated yeast will undergo glycolysis using glucose to produce pyruvate, which is then converted into ethanol and CO₂, releasing NAD+. These yeast cells would use the regenerated NAD+ to continue glycolysis. Bubbles form in the liquid culture due to the production of carbon dioxide during fermentation. For question g, the treated cells can use glucose for energy via glycolysis followed by fermentation since the inhibited enzyme prevents further metabolism of pyruvate and completely blocks the use of acetyl-CoA as a fuel source.