I said I’d do this sometime back in response to a question from, um…someone. Now we are into the glorious long summer and I have more time, some words on alcohol dehydrogenase.
Its role on humans is to convert ethanol to less harmful substances. It does this by oxidising ethanol CH3CH2OH to the toxic chemical acetaldeyde CH3CHO (notice what has happened in the oxidation reaction, look at the enzyme name as well). This is then oxidised further into harmless acetic acid (bonus points for working out the name of that enzyme).
However, the place we first encountered alcohol dehydrogenase was in the anaerobic respiration pathway of yeast, where pyruvate was decarboxylated to ethanal, which was then reduced further to ethanol with hydrogen from reduced NAD. Remember that the advantage of this step is to re-oxidise NAD so that more H can be accepted during glycolysis. This presents a few questions.
1) what is the difference between acetaldehyde and ethanal?
Nothing, it is the same chemical. Ethanal is an internationally accepted name, whereas acetaldehyde is a name in common usage by some people. I’m afraid it’s an example of scientists from different areas using different names for the same thing. I’d suggest sticking with ethanal (if in doubt, go with the syllabus).
2) If enzymes are specific, how come it is doing two different things in two different places?
This is really the same reaction in reverse (although there are actually quite a lot of different versions of alcohol dehydrogenase, we’ll leave that aside). Enzymes speed up the rate of a reaction, but they do not alter the equlibrium. In simple terms, in the case of yeast and human liver we have different substrate concentrations, leading the equilibrium in one direction more than the other.
Ethanol production in yeast gives the fungus an advantage because it is toxic to other organisms. What is the advantage of alcohol dehydrogenase in humans? Ethanol occurs naturally when fruit begins to ferment. Our ancestors diets included fruits, so anyone with this enzyme would have had a natural advantage in removing the toxin.