PROBLEM SET 11-Stereochemistry of the Citric Acid Cycle


In 1940, Evans and Slotin carried out one of the most important experiments in biochemistry.  They incubated radioactive bicarbonate (prepared from radioactive carbon dioxide) and pyruvate with minced pigeon liver in the presence of sodium malonate and oxygen. They isolated  2-oxoglutarate and found it was labeled, but succinate was unlabeled. This was the first unequivocal demonstration that animal cells could fix carbon dioxide, but the results created questions about the Citric Acid. In the 1947 version of his textbook, The Dynamic Aspects of Biochemistry, Ernest Baldwin wrote: “While these results show clearly that Krebs was wrong in regarding citrate as lying directly on the catalytic cycle, they do, at the same time, provide a brilliant vindication of the scheme as a whole”.

  1. Show how CO2 is fixed.
  2. Which enzyme does malonate inhibit and why? (HINT: Look at structures)
  3. Why was malonate added?
  4. Explain why Baldwin, and just about everyone else, concluded that citrate could not be an intermediate in the Kreb’s cycle.
  5. Using your knowledge of stereo-chemistry, explain why these conclusions were incorrect.
  6. Design an experiment that will tell us how aconitase converts citrate to      cis-aconitate.
  7. Design an experiment that will tell us how succinate dehydrogenase works.
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