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Friday, June 24, 2005

Brewer or Drinker

The Panda's Thumb talks about the evolution of alcohol synthesis, or Do Yeast Drink What They Brew ?

A: Yes. But they didn't always.

Or, as they put it:

    The authors compared the sequences of Adh1 [alcohol dehydrogenase - a yeast enzyme that produces ethanol] and Adh2 [a similar enzyme that can convert ethanol back into a form the yeast can consume] from S. cerevesiae [brewer's yeast] and from 15 other Adh homologs in other yeast species. They then calculated the maximum likelihood gene sequence for the last common ancestor of these enzymes, the primordial alcohol enzyme, which they called AdhA. They then took modern yeast, removed their Adh1 and Adh2 genes, and replaced them with AdhA.. VoilĂ , they have yeast from the Age of the Dinosaurs.

    They then analyzed the chemical kinetics of this enzyme. The question was whether it was more like Adh1, the enzyme that primarily makes ethanol, or whether it was more like Adh2, the enzyme that primarily consumes alcohol. Did yeast evolve this enzyme to make a byproduct to inhibit its competitors, or did it evolve it to eat this byproduct?

    The answer is that it was more like Adh1, and that early yeast were brewers, not drinkers.

Cool. And the conclusion:

    We can assemble a history of yeast fermentation from this information now. The first step was the gradual evolution of efficient alcohol-producing enzymes that allowed the yeast to colonize and exploit rotting fruit exclusively. This occurred a very long time ago, in the Cretaceous. Next, there was a gene duplication event that produced two copies of Adh; initially, both would have done exactly the same thing, just allowing the lucky duplicators to pump out alcohol even faster. With two copies, though, one would have more freedom to drift and change its enzymatic properties without serious consequence to the owner. One fortuitous change would be a shift in enzyme kinetics in one copy to better promote conversion of alcohol back to acetaldehyde and enter back into the citric acid cycle. So, first they learned how to make an environmental poison to give them exclusive access to a food source, and then that same machinery was adapted to better allow them to eat that poison, permitting them recover some of the energy lost in secreting it.


Oh, and by the way, Fuck Karl Rove.

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