Talmudic Question #108

 

Why do you suppose that, to our knowledge, animals have not evolved the ability to break down cellulose without the help of microbes?

 
 

 
 

Comments

[Moselio Schaechter]

This is a perfect example of a Talmudic Question, Elio! When I teach introductory biochemistry, I often get the "so why does the enzyme do it that way?" kind of question. Why does Rubisco mistake carbon dioxide for oxygen? What does triose phosphate isomerase produce 96% DHAP (which is not usable, and can be toxic) to 4% GA3P at equilibrium (though to be fair, that latter example is a good illustration and reminder that *life* is not at equilibrium)?

I sometimes reply that these are more theological than biochemical questions. It's not always a satisfying answer, sort of like "what controls transcription factors" being answered by "other transcription factors."

Getting back to cellulose, I hammer on my freshman to consider the "shape" of ligands and bonds---I call it a made up word, "stericity"---are central to all of biology. So the beta 1->4 glucosidic bonds are not well recognized by eukaryotic hydrolases? And it appears that microbes and macrobes look at this as a metabolic problem with a "crowdsourced" solution (as observed in the microbial universe within a milliliter of rumen fluid, to borrow from William Blake).
Great opportunities for classroom discussion, in any event. I am going to pick and choose among your Talmudic Questions and make it a weekly feature in my Fall microbiology course (and I still think the collection would make one heck of a book!).

Thanks again, Mark Martin