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The Pentose Shunt

    We have seen how microbes have tried to improve on EMP glycolysis by inventing the Entner-Douderoff Pathway. Another pathway that is extremely popular in nature is the pentose shunt, which is a necessity in most higher forms of life as it forms a crucial part of photosynthesis, and of the making of ribose for synthesis of DNA and RNA. Another name for the Pentose Shunt is the Hexose Monophosphate Shunt or simply the HMS.

    As you look at the Pentose Shunt (page 262), you don't see the nice straight line of glycolysis nor of the Entner-Douderoff, nor a neat circle of the Krebs Cycle, but rather a convoluted tangle that surely should be a challenge to balance. (Hint: Start with three glucoses!)

    One thing you see is that it starts in a way not dissimilar to the Entner-Douderoff: the glucose is phosphorylated (of course!) and is then immediately oxidized. Then all $&@#%! breaks loose!

    But why the HMS in the first place? There's no hexose-6-phosphate isomerase! If you strike that from glycolysis, it's Situation Disaster unless you can shunt around it. ("X" this off the pathway.)

  1. An Initial Grand Tour of the HMS without attempting balancing!

  2. The HMS in the absence of light

    1. This is the way your local tree runs its basic metabolism in the dark

    2. What portions of the HMS are major and what are minor?
      Don't be shy about scribbling all over your HMS pathway page. You can always print out new copies!



    3. Using 14C6 tracers - put some identifiers on your pathway's carbons.
      Does the placement of the C-14 differ between following EMP-glycolysis, Entner-Douderoff and the HMS?

  3. Photosynthesis: behind the scenes in the dark reactions.

    1. Now our goal is not catabolism, but rather anabolism

      Put an identifying "circle" around this part of the HMS. (Is there a C-C bond formed? If so, what do you do with that bit of information? Hint: page 805.)

      1. What are the coenzymes that drive this reaction?

      2. Where did they come from? (Hint: "")

  4. Lab Work: A study of photosynthesis.