Question

Identify ONE metabolic enzyme that would account for the increased levels of NADH observed in the mutant yeast in the absence of oxygen, and explain why a mutation in the enzyme would cause the observed results. *you don't need the graph*

Answer 1

Alcohol dehydrogenases

S. cerevisiae (yeast)is a facultative anaerobe: it is capable of generating energy in both the presence and the absence of O2 using aerobic metabolism (the citric acid cycle, the electron transport system, and oxidative phosphorylation) and fermentation, respectively.

Under anaerobic or hypoxic conditions, many organisms regenerate NAD by transferring electrons from NADH to pyruvate, forming lactate. Other organisms, such as yeast, regenerate NAD by reducing pyruvate to ethanol and CO2. In these anaerobic processes (fermentations), there is no net oxidation or reduction of the carbons of glucose.

Respiration-deficient yeast mutants can be produced from wild-type parents by treatment with mutagenic agents. The mutants lack cytochrome oxidase, a deficit that markedly affects their metabolic behavior. One striking effect is that fermentation is not suppressed by O2—that is, the mutants lack the Pasteur effect(When O2 is added to an anaerobic suspension of cells consuming glucose at a high rate, the rate of glucose consumption declines greatly as the O2 is usedup, and accumulation of lactate ceases)

When glucose levels are high, yeast cells shift into the “make, accumulate, consume” ethanol pathway. At first, glucose is converted into ethanol molecules to regenerate NAD+. Ethanol is then released into the environment where it kills competing microbes. Once glucose is depleted, glucose repression ends. Among the results of derepression is that ADH(Alcohol dehydrogenases), the enzyme that converts ethanol back into acetaldehyde, is synthesized. Acetaldehyde is subsequently converted into acetyl-CoA, the substrate for the citric acid cycle. Note that although the “make, accumulate, consume” strategy is expensive (i.e., the energy expended to synthesize the enzymes needed to convert ethanol into acetyl-CoA and the ATPs used to synthesize acetyl phosphate),