Transcript module 2
Glycolysis Is it a good strategy to draw this out until you can produce it by memory? NO It is true that repeatedly testing recall is a way to assure that you have successfully memorized a thing. Most students do that by internal verbalization rather than actually drawing it out. BUT 1) Unless the information is linked to other things, you won’t have any long term memory of it. 2) Unless the specifics on that slide are linked to some learning objective that you have, you’d be memorizing the wrong information. The first thing you want to know about any metabolic pathway is: What’s it for? Glucose (food carbohydrate, blood sugar) A little ATP A little NADH Pyruvate If O2: TCA cycle Pyruvate -> CO2 + a lot of NADH Ox. Phos. NADH -> a lot of ATP. If can’t use O2: Pyruvate + NADH -> lactic acid. Now you tell me a physiologic situation where the information summarized on the previous slide is explanatory. Go to http://biochem.uthscsa.edu/hardies-bin/survey.pl When you’ve found something from the first presentation of today that this slide explains, check “True” for (a) and click “submit”. Then always know how the pathway is regulated, and be sure it makes sense to you with respect to what the pathway is for. Glucose (food carbohydrate, blood sugar) inhibited if the cell has enough ATP phosphofructokinase A little ATP A little NADH Pyruvate If O2: TCA cycle Pyruvate -> CO2 + a lot of NADH Ox. Phos. NADH -> a lot of ATP. If no O2: Pyruvate + NADH -> lactic acid. Since biochemists put a lot of emphasis on regulation, one should: 1) know the name of the regulated enzyme(s). 2) know the specifics of the regulation. phosphofructokinase stimulated by ADP, AMP inhibited by ATP, citrate At the moment we are considering the regulation of glycolysis in all cells for the primary purpose of keeping up the ATP levels (also called the “energy charge”). Inhibition by ATP and/or stimulation by AMP and ADP obviously makes sense with respect to that use of the pathway. How about citrate? If it’s not obvious, that would require looking up the role of citrate in energy metabolism to consolidate your understanding of the control of glycolysis. Now you do some research using the internet of the course manual and tell me which of the following statements about citrate might be reasonable explanations about why it would inhibit phosphofructokinase. a) Citrate is a high energy storage molecule, so excess ATP is converted to citrate. b) Citrate is an intermediate in breaking down pyruvate and sits before an enzyme inhibited by high energy charge. So citrate builds up when ATP is plentiful. c) Since citrate can shuttle between the mitochondrion and the cytoplasm, it can regulate glycolysis (in the cytoplasm) to keep pace with ATP production (in the mitochondrion). d) Under some circumstances glycolysis is driven not to make ATP but to convert excess glucose to fat. Citrate is an intermediate between glycolysis and fatty acid synthesis that can feed back to regulate glycolysis to match the rate of fatty acid synthesis. Go to http://biochem.uthscsa.edu/hardies-bin/survey.pl and give your opinion as to whether each of the above statements is true or false. Summary: Every time you encounter how an enzyme is regulated, look through all the other resources available to you to make sense of it. Every time there is information about what subcellular compartment a pathway occurs in, put that on your list of things to know. A review of past test questions from the chapter on glycolysis reveals a disportionate number of questions about the glycerol phosphate shuttle, which is described in the course manual as so: Why do you think that the professor is so interested in the glycerol phosphate shuttle? a) This is the only way to make glycerol-3-P, which is needed for fatty acid synthesis. b) This is the only way to make FADH2, which is a necessary co-factor for some enzymes. c) This allows NADH made by glycolysis to be reconverted to NAD+, allowing glycolysis to continue without having to divert the pyruvate to lactic acid. The mitochondrial FADH2 is used to make more ATP. d) At maximum muscular exertion, this pathway becomes limiting, glycolysis makes a little excess ATP at the cost of having to make lactic acid, and the lactic acid inhibits muscle function bringing the activity to an end. Summary: A review of past exam questions can reveal what issues are considered important by a given professor, and what level of detail that professor may expect of you. He will not repeat the same questions from year to year, however. So, simply memorizing the answers to last year’s exam will probably not be helpful.