Bioc385-Spr2016-Homework3-KEY

Bioc385-Spr2016-Homework3-KEY - Bioc 385 – Spring 2016 Dr...

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Unformatted text preview: Bioc 385 – Spring 2016 Dr. Miesfeld Homework 3 1. Choose the ONE statement that BEST describes what the actual change in free energy (deltaG) tells you about an enzymatic reaction, which cannot be determined by the standard free energy (deltaGº') change. The direction of the reaction under steady state conditions. The direction of the reaction at 1M substrate concentration at pH7. The temperature of the reaction at 1 atmosphere pressure. The actual change in free energy = 0, so it does not tell you anything. The direction of the reaction at equilibrium. The spontaneity of the reaction in the presence of an inhibitor. 2. Here are five hypothetical reactions (numbered 1-­5). 1. A ↔ B ΔG°' = -­5 kJ/mol 2. B ↔ C + D ΔG°' = +8 kJ/mol 3. B ↔ E ΔG°' = -­10 kJ/mol 4. E ↔ F ΔG°' = +5 kJ/mol 5. D ↔ F ΔG°' = +1 kJ/mol From these five reactions it is possible to write TWO metabolic paths that could generate the metabolite F starting with metabolite A. Which of these two pathways will proceed spontaneously based on the overall ΔG°' value? 1, 3, 5 1, 2, 5 1, 5 1, 3, 4 3. Choose the true statement(s) referring to the following coupled reactions: Rxn1 Rxn2 Rxn3 Net Rxn deltaG°' = -­2 kJ/mol deltaG°' = +7 kJ/mol deltaG°' = -­8 kJ/mol deltaG°' = -­3 kJ/mol A↔B B↔C C↔D A↔D The net reaction of A→D is unfavorable because the δελταG°' value is less than 0. For Rxn2, if the δελταG°' value changes to +9 kJ/mol, the rate of the overall reaction A↔D will decrease. Rxn2 (B↔C) is unfavorable in the forward direction on its own, but if coupled to Rxn3 (C↔D), the overall conversion of B to D will occur. Rxn1 is a favorable reaction and will proceed in cells with any concentrations of reactants and products. 1 Bioc 385 – Spring 2016 Dr. Miesfeld 4. Put the following glycolytic reactions in the correct order. Only 8 of the 10 reactions are listed, so the answer should be the relative order of the reactions. Note that the answer is graded as an "all or nothing" question, meaning the order must be correct to earn 1 point. __1__ Phosphorylation reaction generating a hexose sugar. __4__ Cleavage reaction converting a diphosphate sugar into two monophosphate metabolites. __5__ A redox reaction utilizing inorganic phosphate and a coenzyme. __2__ An isomerization reaction converting an aldose sugar into a ketose sugar. __8__ Substrate level phosphorylation reaction generating a net yield in ATP for the glycolytic pathway. __6__ A substrate level phosphorylation reaction generating the ATP needed to replace the ATP investment in stage 1. __7__ A dehydration reaction generating a high energy phosphorylated compound. __3__ Phosphorylation reaction converting a hexose monophosphate into a hexose bisphosphate. 5. For the reaction: Fructose 1,6-­bisphosphate (F-­1,6-­BP) ↔ DHAP + GAP The standard free energy change, ΔG°', is +23.9 kJ/mol. (For this question, the biochemical standard state be defined to be at 37C.) Suppose that in a cell at 37 °C, the actual metabolite concentrations are as follows: [F-­1,6-­BP] = 5.3 x 10–5 M [DHAP] = 8.1 x 10–4 M [GAP] = 1.5 x 10–4 M Calculate the actual free energy change (ΔG) at 37 °C (310K). -­23.8 kJ/mol 8.2 kJ/mol -­56.2 kJ/mol -­8.6 x 104 kJ/mol 6. What is the metabolic advantage of having glucokinase activity limited to the liver and pancreas? There may be more than one correct answer. It provides a way to target glucose to tissues that use the highest amount of ATP during vigorous exercise. It changes the equilibrium constant for glucose phosphorylation in liver and pancreas cells. It prevents diabetes by increasing expression of insulin receptors on pancreatic beta cells. It provides a way to stimulate insulin release from liver cells by activating ATP synthesis and calcium-­mediated vesicle fusion. It provides a way to link blood glucose levels to insulin release in pancreatic beta cells. It provides a way to rebuild glycogen stores in liver cells, which is important to maintain safe blood glucose levels in between meals. It provides a way to stimulate glucagon release from pancreatic beta cells in response to low blood glucose levels (before breakfast). 2 Bioc 385 – Spring 2016 Dr. Miesfeld 7. 2-­phosphoglycerate(2PG) is converted to phosphoenolpyruvate (PEP) by the enzyme enolase. The standard free energy change(deltaGo’) for this reaction is +1.7 kJ/mol. If the cellular concentrations are 2PG = 0.25 mM and PEP = 0.05 mM, what is the free energy change at 37 oC for the reaction 2PG ↔ PEP? +4146.4 kJ/mol +2.4 kJ/mol -­5.8 kJ/mol -­2.4 kJ/mol -­4146.4 kJ/mol 5.8 kJ/mol 8. Find ALL of the conditions listed on the left side that either stimulate or inhibit liver PFK-­1 activity as listed on the right. Note that every condition should lead to stimulated or inhibited PFK-­1 activity. 1. Liver PFK-­1 activity is stimulated. 2. Liver PFK-­1 activity is inhibited. __2__ The conformational equilibrium is shifted toward increased amounts of the T state structure. __2__ ATP is bound to both the catalytic site and the allosteric effector site. __1__ AMP is bound to the allosteric effector site and ATP is bound to the catalytic site. __1__ Low energy charge in the cell. __1__ The conformational equilibrium is shifted toward increased amounts of the R state structure. __2__ Decreased fructose-­2,6-­bisphosphate levels. __2__ High energy charge in the cell. __2__ Increased citrate levels in the cell. __1__ Increased fructose-­2,6-­bisphosphate levels. __1__ Increased levels of both ADP and AMP. 9. The equilibrium constant, Keq, for the reaction A↔B is 1 x 105 at 25 °C. If you started with a solution containing 1.000 M A and 1 mM B, and let the reaction proceed to equilibrium, what would be the equilibrium concentrations of A and B? [A] = 1.000989 M [B] = 1.000989 X 10-­5 M [A] = 100000 M [B] = 1 M [A] = 1 M [B] = 100000 M [A] = 1.000989 X 10-­5 M [B] = 1.000989 M 3 Bioc 385 – Spring 2016 Dr. Miesfeld 10. What properties of glucokinase listed below BEST explain why it is the glucose sensor in humans and not hexokinase. If there are three correct answers, then choose all three, otherwise choose "three correct answers are not provided." Three correct answers are not provided. Individuals with a deficiency in glucokinase get type 2 diabetes and secrete ten times the normal level of insulin, whereas defects in hexokinase are lethal to developing embryos. Glucokinase expression, but not hexokinase expression, in pancreatic cells is required to stimulate a signaling pathway that leads to fusion of insulin containing vesicles with the plasma membrane and subsequent release of insulin. The activity of glucokinase in pancreatic cells is directly proportional to the level of blood glucose across normal physiological ranges, whereas hexokinase activity is maximal at even very low blood glucose levels. Hexokinase phosphorylates fructose and glucose, whereas glucokinase is specific for glucose and thereby a more specific glucose sensor. Glucokinase expression in liver cells is much lower than hexokinase in liver cells, which decreases its affinity for glucose binding. Since hexokinase is expressed in every cell type, whereas glucokinase is expressed primarily in liver and pancreatic cells, glucokinase is the glucose sensor and hexokinase is not. Activation of glycolytic flux by increased glucokinase activity in pancreatic alpha cells results in calcium influx through inhibition of potassium channels, hexokinase has no effect on calcium or potassium levels in this cell type. 4 ...
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