BCH 4053 THIRD EXAM November 5, 1999

Transcription

1 BCH 4053 THIRD EXAM November 5, 1999 I remind you that you are bound by the Academic Honor Code. This means (1) you will not give or receive information during this exam, nor will you consult with unauthorized sources of information; (2) you will not discuss the contents of this exam with other students until all students have finished the exam; and (3) you will not tolerate violations of academic integrity on the part of other students. Please Sign: 1

2 1. (10 Points) Provide an explanation or description of the following terms, using one or two sentences. If necessary, you may use a diagram or an equation. Be sure to provide a brief explanation for any diagrams or equations. A. Positive Heterotropic Effector. B.reaction catalyzed by lactase. C.reaction catalyzed by pyruvate decarboxylase D. Bohr effect E. Allosteric regulation 2

3 2. (10 points) Discuss three possible molecular mechanisms through which covalent modification by phosphorylation can regulate the activity of enzymes. 3. (10 points) Enzymes increase the rates of biochemical reactions by decreasing the free energy difference between the ES complex and the transition state. Briefly describe the mechanisms used by enzymes to do this. 3

4 4. (12 points) A. Glycogen phosphorylase is regulated by a combination of covalent and non-covalent modifications. Briefly describe how this enzyme is regulated by the allosteric effectors listed below. ATP ADP Glucose-6-P B. Sketch a Hill plot showing the effect of increasing phosphate concentration on glycogen phosphorylase activity in the presence of high ATP concentration. Be sure to label your axes appropriately, and indicate how one would determine the phosphate affinities for the high and low affinity states, and for the overall reaction. C. Glycogen phosphorylase is also regulated by covalent modification, specifically by phophorylation. Indicate how this regulation is accomplished. Indicate the names of any additional enzymes involved in this process. 4

5 5. (6 points) Briefly describe the biological significance (or rationale) for the following regulatory phenomena. A. activation of pyruvate carboxylase by acetyl-coa. B. activation of pyruvate dehydrogenase kinase by NADH. C. regulation of phosphofructokinase by F-2,6-bP 6. (15 points) Glycolytic activity in red blood cells is tightly coupled to oxygen transport. Under conditions where there is a temporary shortage of oxygen for use in oxidative phosphorylation, 2,3-BPG and lactic acid accumulate and effect additional release of oxygen. A. How is 2,3-BPG made? Draw the reaction and name the enzyme. If this enzyme is only active in the presence of elevated concentrations of its substrate (2,3-BPGs precursor), explain how this regulation might be affected by the K M or k cat value of this enzyme compared to phosphoglycerate kinase. 5

6 B. Discuss how lactic acid is generated. Draw the reaction involved, including any necessary co-factors. Name the enzyme. Indicate how lactic acid accumulation effects additional oxygen delivery. C. Sketch a Hill plot for Hb under normal glycolytic flux conditions and show how this curve would be altered under conditions where either 2,3-BPG or lactic acid are elevated. Indicate how the po 2,50 for Hb under these two conditions (normal versus slowed glycolytic flux due to slight anoxia) would be identified. 7. (6 Points) Draw the structure of nicotinamde adenine dinucleotide in the oxidized form. Draw the structure of the nicotinamide moiety in the reduced form. 6

7 8. (16 points) The following compounds appear as intermediates in glycolysis. For each of these compounds, draw its structure and the reaction in which it is formed. Include the enzyme name and specify any co-factors involved. For glucose labeled in the C4 position, indicate the position of the label in these compounds. A. Fructose-1,6-bisphosphate B. 3-phosphoglycerate C. phospho-enol pyruvate D. 1,3-bisphosphoglycerate 7

8 9. (5 points) Many enzymes exhibit bell-shaped curves when plotting enyme efficiency as a function of ph. What is the significance of this type of activity as a function of ph? What can be determined from these plots regarding the number or type of functional groups (i.e., amino acid sidechains) involved in the enzyme-catalyzed reactions? k cat / K M ph 9 (5 points). Discuss the various mechanisms used to regulate enzyme activity in metabolic pathways (5 points) Use free energy diagrams to show why the most potent inhibitors are often transition-state analogs rather than substrate analogs. 8