Use the information below to answer Question 23-25. You are studying a type of y

Question

Use the information below to answer Question 23-25. You are studying a type of yeast that can grow in both the presence and absence of oxygen using normal metabolic pathways (glycolysis, Krebs cycle, ETS, ATP synthase). You have generated a mutant that grows well in the presence of oxygen, but not in its absence. You do a study where you look at the relative levels of NADH in both the mutant and wild-type yeast in the presence and absence of oxygen, and you get the following data. Answer the questions below 40 35 30 25 Oxygen Oxygen 10 0 Wild-type Mutant Click YES below to indicate that you understand the instructions O Yes No

Explanation / Answer

1. Under aerobic condition, in case of both wildtype and mutant yeast, NADH that is produced during glycolysis is re-oxidized by the mitochondrial respiration to facilitate more ATP production. Under anaerobic condition, the two molecules of NADH produced during glycolysis is reutilized while glucose is reduced to glycerol, as in the other two pathway, such as alcoholic fermentation or ethanol formation do not need for the re-oxidization of NADH. The described mutant can grow in the presence of oxygen but in the absence of oxygen, and it exhibits higher level of NADH in the absence of oxygen, which indicate that this mutant strain bears mutation in the enzymes involved in glycerol synthesis pathway. Therefore, this strain failed to reutilized the NADH produced from the glycolysis in the absence of oxygen, which raise its NADH level compared to the wild-type strain.

2. Yeast genome encodes two paralogous enzymes Gpd1 and Gpd2 (Glycerol-3-phosphate dehydrogenase) that are the key enzymes that facilitate the formation of glycerol-3-phasphate from the dihydroxyacetone phosphate, an intermediate of glycolysis pathway. It is reported that under anaerobic condition Gpd2 maintains cellular redox balance, and while yeast cells are shifted from the aerobic to anaerobic condition, the expression of GPD2 is upregulated at 9 times. Under anaerobic condition, gpd2 also experienced growth retardation. Therefore, it can be predicted that this described mutant may bear mutation in the catalytic site of the GPD2, which renders its functional activity. As a result this mutant cells showed growth retardation in the absence of oxygen and also exhibits higher level of NADH, which cannot be re-utilized in glycerol synthesis pathway.

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