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Download BCHM 463 Supplemental Problems for Friday, April 2, 2004 1. Write
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BCHM 463 Supplemental Problems for Friday, April 2, 2004 1. Write the net reaction for glycolysis. Glucose + 2 ADP + 2 NAD+ + 2 Pi 2 Pyruvate + 2 ATP + 2 NADH + 4 H+ + 2 H2O 2. During glycolysis, how many ADP molecules are converted to ATP. Explain this answer with regard to your answer to #1. 4 ADP molecules are converted into ATP. There is a net gain of only 2 ATP molecules because 2 are consumed during the first stage of glycolysis. 3. What are the three metabolically irreversible steps of glycolysis? What general type of reaction is catalyzed by these enzymes? Why are these reactions irreversible? Rxn 1: hexokinase Rxn 3: phosphofructo kinase Rxn 10: pyruvate kinase All three reactions involve group transfer reactions. The reactions are irreversible because they have a large negative ∆G. Note: ∆G should not be confused with ∆G°´. See Table 14.1 for more information. 4. The following is an intermediate formed along the glycolysis pathway. This is a substrate or product of what enzymes? Provide the net reaction for each catalytic process that produces or consumes this intermediate. OPO3-2 HO HO phosphoglucose isomerase O OH O -2O PO 3 OH HO OH OH phosphofructose kinase OPO3-2 O -2O PO 3 OH OH HO OH PGI G6P F6P + ATP F6P PFK FBP + ADP + H+ Mg2+ 5. Name the compounds that serve as phosphoryl group donors in glycolysis and also name the enzymes that catalyze these phosphoryl transfer steps. 1,3-BPG ATP Glucose hexokinase phosphoglycerate kinase G6P 1,3-BPG F6P phosphofructose kinase ATP PEP Pi GAP ADP FBP GAPDH 1,3-BPG pyruvate kinase PEP Pyruvate ADP ATP 3PG BCHM 463 Supplemental Problems for Friday, April 2, 2004 6. Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2phosphoglycerate. 2,3-Bisphosphoglycerate (2,3-BPG) is formed as an intermediate. Describe the catalytic process that forms and consumes this intermediate. What cofactors, if any, are necessary for these transformations? Does 2,3-BPG serve any physiological function? 3PG 2,3-BPG 2PG Each step is a group transfer step. First a phosphoryl group is transferred from the enzyme to the substrate to generate the bisphospho intermediate. This is followed by transfer of the second phosphoryl group back to the enzyme. Although not mentioned in the book, this enzyme is Mg2+ dependent in some organisms and also occasionally requires 2,3-BPG as a cofactor to reactivate the enzyme if it becomes dephosphorylated. Recall that 2,3-BPG is an allosteric modulator of hemoglobin in erythrocytes although the 2,3-BPG produced by escape from phosphoglycerate mutase is neglible. This of course means that there is some other source of 2,3-BPG in erythrocytes. 7. In Greek mythology, the Three Fates, or Moirae, were the goddesses who controlled the destiny of everyone from the time they were born to the time they died. Pyruvate also has three fates: fill in the boxes below to indicate the fate of pyruvate produced during glycolysis. Glucose Glycolysis Pyruvate anaerobic conditions 2 EtOH + 2CO2 aerobic conditions CO2 2 AcetylCoA citric acid cycle 4 CO2 + 4 H2O anaerobic conditions 2 Lactate BCHM 463 Supplemental Problems for Friday, April 2, 2004 8. When lactate is converted to pyruvate, which of the following, if any, are true? a. NAD+ is converted to NADH b. ADP is converted to ATP c. NADH is converted to NAD+ d. ATP is converted to ADP e. FAD is converted to FADH A is the only answer that is true. Remember that the reaction catalyzed by lactate dehyrogenase is freely reversible. 9. Which enzymes in glycolysis utilize covalent catalysis during conversion of substrate into product? Write the enzyme names and the net reaction catalyzed by each. GAPDH 1,3-BPG + NADH + H+ GAP + Pi + NAD+ O Enz OPO3-2 S OH acylenzyme intermediate 10. Glycolysis is regulated by controlling which enzymes? Identify the mechanisms of control for each. (e.g. product inhibition or feedback inhibition) Rxn 1: hexokinase: product inhibition by Glc6P Rxn 3: phosphofructo kinase: ATP is an allosteric inhibitor; AMP, ADP, F2,6P, and other compounds overcome this inhibition and therefore serve as activators Rxn 10: pyruvate kinase: product inhibition by ATP (allosteric); FBP acts as an allosteric activator 11. Xylose has the same structure as glucose except that it has a hydrogen atom at C-5 in place of a hydroxymethyl group. The rate of ATP hydrolysis catalyzed by hexokinase is markedly enhanced by the addition of xylose. Why? Although xylose does NOT serve as a substrate for hexokinase, it does bind and induce the conformational change found with glucose binding. The xylose binding event positions a water molecule to serve as a nucleophile to hydrolyze ATP. .