An increase in which of the following would lead to decreased ethanol production in yeast?

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11ecba2d_0eef_bd7a_a3bf_e9ddb1277478_MD0008_00 Glycolysis (the breakdown of glucose) consumes NAD⁺ to produce NADH, and for glycolysis to continue, NAD⁺ must be regenerated. Under aerobic (high-oxygen) conditions, the NAD⁺ used in glycolysis is regenerated by the electron transport chain (ETC) in a process known as respiration. Because oxygen is required as the final electron acceptor in the ETC, insufficient oxygen (in anaerobic conditions) will prevent the ETC from functioning and inhibit the regeneration of NAD⁺. Accordingly, in anaerobic conditions, an alternate process called fermentation is necessary to regenerate NAD⁺. In higher eukaryotes, fermentation occurs by reduction of pyruvate to lactate. However, in bacteria and lower eukaryotes such as yeast, fermentation occurs through decarboxylation of pyruvate to form acetaldehyde, followed by reduction of acetaldehyde to form ethanol. If the partial pressure of oxygen in the environment were increased, yeast cells would exhibit increased function of the ETC and the need for fermentation to regenerate NAD⁺ would decrease. Because fermentation in yeast generates ethanol, increased partial pressure of oxygen (and decreased fermentation) would most likely result in reduced ethanol production. (Choice B) Pyruvate kinase catalyzes the transfer of a phosphate group from phosphoenolpyruvate to ADP, forming pyruvate and ATP. If pyruvate kinase activity were increased, more pyruvate would be produced. At a given O₂ partial pressure, fermentation of this pyruvate would likely increase (not decrease) ethanol production. (Choice C) The carbon atoms in ethanol originate in glucose. Therefore, increased glucose concentration would make more carbon available, likely increasing ethanol production. (Choice D) The Krebs cycle (citric acid cycle) uses acetyl-CoA, synthesized from pyruvate, to produce NADH and FADH₂ for the ETC. Pyruvate that is converted to acetyl-CoA cannot undergo fermentation. Krebs cycle inhibition would increase the amount of pyruvate available for fermentation and would most likely increase ethanol production. Educational objective: Glycolysis consumes NAD⁺ as a substrate. NAD⁺ can be regenerated by oxidizing NADH in the electron transport chain in the presence of oxygen, or by fermentation in the absence of oxygen. Fermentation converts pyruvate to lactate in higher eukaryotes, and to ethanol in bacteria and lower eukaryotes such as yeast.

An Increase in Which of the Following Would Lead to Decreased