Intermediary metabolism

ysk1
Mon, 02 Feb 2009 23:19:05 +0000

The prof is a bit demanding for an undergrad course in that she also gives us such medically-related or application questions on problem sets and exam in addition to memorization questions (I had my first midterm today). For example, one question on problem sets asked why giving a patient a glucose 6-phosphate instead of glucose as food source is useless. It’s because GLUT transporters in plasma membrane can only let unphosphorylated intermediates into cytosol for glycolysis.
If you radio-labelled carbon 5 in original glucose, what carbon would be labelled in lactate? Carbon 2 (This has to do with flipping of the carbon numbering caused by aldolase reaction on fructose 1,6-bisphosphate.) Also, today’s midterm had a question asking about the consequence of giving a cell an arsenate, a molecule structurally similar to phosphate but much less stable, to glyceraldehyde 3-phosphate in the presence of glyceraldehyde 3-phosphate dehydrogenase. What would happen is that the arsenate would initially latch onto glyceraldehyde 3-phosphate to form 1-arsenate 3-phosphate glycerate (or something like that, I don’t remember the actual name), but then it’s much more unstable than 1,3-bisphosphate glycerate so it would prefer to get hydrolyzed and stay dissociated as arsenate and glycerate 3-phosphate. The net result is that phosphoglycerate kinase that follows has no substrate (1,3-bisphosphate glycerate) that it can act on to make 2 ATPs. So basically, we would go from glyceraldehyde 3-phosphate to 3-phosphate glycerate and skip this ATP-making step. 2 ATPs would still be made at the final step catalyzed by pyruvate kinase, but then the net ATP will be 0 because 2 ATPs were previously used up in preparatory phase.