A cell-free extract of yeast known to contain all of the enzymes required for al

Question

A cell-free extract of yeast known to contain all of the enzymes required for alcoholic fermentation is incubated anaerobically in 100 mL of a medium initially known to contain 200 mM glucose, 20 mM ADP, 40 mM ATP, 2 mM NADH, 2 mM NAD+, and 20 mM inorganic phosphate (Pi). Assuming that ethanol is removed continuously from the medium as soon as it is formed, what is the maximum amount of ethanol (in millimoles to the nearest ones) that can be formed if, in addition to the above components, an ATPase was added to the reaction mixture. Assume the ATPase catalyzes the following reaction: ATP + H2O --> ADP + Pi

Explanation / Answer

The maximum theoretical amount of ethanol production will be limited by the amount of free Pi available. There is enough ATP around to convert 20 mmoles of glucose into 40 mmoles of glyceraldehyde-3-P. However, to convert G3P into 1,3-bisphosphoglycerate requires NAD+ and Pi. The NAD+ will be regenerated when pyruvate is converted into ethanol and so is not limiting. However, there is only enough Pi in the mix to produce 20 mmoles of ethanol.

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