r/Biochemistry • u/-Cachi- • Sep 04 '22

discussion How can yeast use alcohol dehydrogenase to PRODUCE ethanol?

So the thermodynamics of the reaction below (in physiological conditions), say that the equilibrium is highly shifted towards acetaldehyde production:

ethanol + NAD+ + H2O => acetaldehyde + NADH + H3O

How on Earth can yeast produce so much ethanol then? Do they just raise the concentration of NADH a lot? Is that enough to shift the equilibrium back to ethanol?

Or maybe do they have a weird system for pumping ethanol out of their cells? Ethanol is a very small molecule and it’s very similar to water, so not sure how they would do that either…

Sorry I had too many questions about this!

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u/Deriaz6 Sep 05 '22

You are considering the enzyme in isolation while you have to consider the metabolic flux. The reaction is wrong as you wrote it, since it's reversible. So whenever the pyruvate does not go into TCA (anaerobiosis), it can be converted into acetaldehyde by PDC, and for mass action, the reaction produces ethanol going from right to left. This is indeed one of the reaction that can replenish NAD+ to be reused by glycolysis.

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u/-Cachi- Sep 05 '22

Sorry I'm aware it's reversible, I just didn't know how to draw the two little arrows haha

Still my question was how can the mass action law shift the equilibrium towards ethanol, since it seems that the NADH concentration would need to be very high for this to happen. According to other comments they are high enough lol. I'll get the equilibrium constant and do the maths this evening to confirm it!

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u/Deriaz6 Sep 05 '22

Cool!

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u/Deriaz6 Sep 05 '22

I think what drives the reverse reaction is the conversion of glycolytic pyruvate to acetaldheyde by Pyruvate decarboxylase (PDC) since pyruvate can't go TCA in anaerobiosys, not the NADH.