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Unformatted text preview: BIOCHEMISTRY 441 Spring 2010 Instructor: David O. Peterson Homework 1
1. As we will learn later in the semester, glucose can be stored in several tissues as a polymer called glycogen. Glucose monomers are released from glycogen by the action of glycogen phosphorylase, which catalyzes the reaction shown below, where the subscript on the glycogen indicates the number of glucose monomers in the glycogen polymer. glycogenn + Pi → glucose-1-P + glycogenn-1 The glucose-1-P can then be converted to glucose-6-P by the action of phosphoglucomutase, which does not require ATP. How much ATP would you expect to be generated during glycolysis using the glucose-1-P derived from the breakdown of glycogen? Explain your answer. (Express your answer in moles of ATP generated per mole of glucose-1 -P.) Glucose-6-P generated from glycogen bypasses the hexokinase step in glycolysis. Consequently, only one ATP needs to be invested to get to the "payoff" phase. Thus, there will be a net synthesis of 3 ATPs per glucose-1-P generated by this pathway.
2. All the reactions of glycolysis can be shown to take place in crude cytoplasmic extracts of cells. If radioactive (32P) phosphate is added to such extracts (along with glucose) and incubated for a short time, what glycolytic intermediate(s) (aside from ATP) would you expect to become labeled first? Would the result change if the incubation was allowed to proceed for a longer time? After addition of the radioactive phosphate, the first labeled molecule will be 1,3 bisphosphoglycerate, with the label on the #1 carbon. In the very next step (the reaction catalyzed by phosphoglycerate kinase), the labeled phosphate will be transfered to ADP to make ATP. Thus, at early times in the incubation, the only glycolytic intermediate that will be labeled (aside from ATP) is 1,3 bisphosphoglycerate. After a longer incubation time, as radioactive ATP builds up due to the action of phosphoglycerate kinase, all the phosphates in all glycolytic intermediates will become labeled.
3. ∆G°' for the conversion of dihydroxyacetone-P to glyceraldehyde-3-P is +7.6 kJ/mol. If the intracellular concentrations of dihydroxyacetone-P and glyceraldehyde-3-P are 0.16 mM and 0.02 mM, respectively, what is ∆G for this reaction inside the cell? What is Keq' for this reaction? Show your work. (Assume the temperature is 25°C.) For the reaction dihydroxyacetone-P → glyceraldehyde-3-P ∆G = ∆G°' + 2.3RTlog([G3P]/[DHAP]) = 7.6 kJ/mol + (2.3)(8.314 J/mol°K)(298°K)log(0.02/0.16) = 7.6 + (5.7)log(0.125) = 7.6 - 5.1 = 2.5 kJ/mol Keq' = 10(-∆G°'/2.3RT) = 0.046 4. Starting with glucose containing a 14C label in the C2 carbon, draw the glycolytic intermediates from glucose to pyruvate showing the position of the labeled carbon in each molecule. ...
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