Revised 1/7/06 1 Chemistry 153C Study Questions and Problems IV Winter 2006 1. Indicate how each of the following conditions would be expected to affect the partitioning of glucose-6-phosphate between its various metabolic fates in liver cells. Briefly indicate the basis for your answer by mentioning regulatory influences of these conditions on enzymes no more than two steps removed from glucose-6-phosphate. (a) A condition in which there are relatively high levels of circulating insulin. (b) A condition in which there are relatively high levels of circulating glucagon and intracellular cyclic AMP. (c) A condition in which there is a lower than normal adenylate energy charge (0.80) in liver cells. 2. (a) Illustrate one potential futile cycle in the operation of the reactions of glycolysis and gluconeogenesis between glucose-6-P and pyruvate which could conceivably occur in a tissue such as mammalian liver. (Word equations to indicate the reactions will suffice.) (b) What would be the net chemical consequence of the operation of the reaction cited in (a) as a futile cycle? (c) Describe briefly the actions on the enzymes of modifier metabolites which tend to minimize the operation of this futile cycle in vivo. 3. What would be the probable consequences for glycogen metabolism of a mutation which resulted in a catalytically inactive phosphorylase kinase in the liver? Assume that all other components involved in the regulation of glycogen metabolism are capable of normal function. In your answer consider both the immediate impact of this deficiency on glycogen metabolism and the probable broader implications for the functioning of the organism.2 4. Assume that the values given below represent the levels of adenylate derivatives present in the resting skeletal muscle (State I) and the same muscle after a period of exercise (State II). State I State II micromoles / g wet weight ATP 12 10 ADP 2 3 AMP 1 2 (a) Calculate the adenylate energy charge in State I and State II. (b) How might you expect this change in adenylate energy charge from State I to State II to affect the rate of utilization of glycogen for energy metabolism in muscle? You should identify specific steps which may be influenced as a basis for any predicted change in rate, but it is not necessary to outline the entire sequence of glycogen metabolism in your answer. 5. The enzyme threonine dehydratase (threonine deaminase) catalyzes the following reaction: CH3CHOHCNH3+HCOO-CH3CH2COCOO-+ NH4+ One type of threonine dehydratase isolated from certain bacteria grown under appropriate conditions is strongly influenced by AMP as a positive modifier. What does this fact suggest to you about the metabolic function of the reaction catalyzed by this enzyme? Indicate concisely the reason for your answer. 6. The liver enzyme hexokinase, which we considered in connection with glycolysis, has a Km for D-glucose of 1 x 10-5 M and is subject to inhibition by the higher levels of glucose-6-P that occur under conditions when the rate of glycolysis is relatively low. A second enzyme, called glucokinase, which catalyzes the same overall reaction as hexokinase, is also present in liver. This enzyme has the relatively high Km for D-glucose of 2 x 10-2 M and is not subject to inhibition by glucose-6-P. (a) Explain the implications of the different Km values for the functioning of these two enzymes. (b) What physiological advantage may be gained from the presence of glucokinase under conditions when the glycolytic and respiration rates in liver are relatively low?3 7. Under some circumstances liver cells are carrying out extensive oxidation of fatty acids, gluconeogenesis, and ketogenesis, and are not utilizing very much glucose. (a) What is the source of metabolic energy (ATP) that supports the biosynthetic pathways operating under these circumstances? (b) What is the source of carbon compounds for use in gluconeogenesis under these circumstances? (c) What is the source of carbon compounds for ketogenesis under these circumstances? (d) Illustrate the pathways for the interconversions of pyruvate and phosphoenolpyruvate in liver, and describe the actions of some modifiers on enzymes of these interconversions that are influencing the flow of carbon between pyruvate and phosphoenolpyruvate under the circumstances described above. 8. (1) Glycogen phosphorylase b catalyzes the phosphorolytic cleavage of glycogen as shown in the following reaction: (glycogen)n + Pi ∏ (glycogen)n-1 + glucose-1-P (2) The following kinetic features of glycogen phosphorylase b activity were observed: (i) Initial rates (v) were determined in incubation mixtures containing fixed amounts of enzyme and glycogen and variable amounts of Pi and additions as indicated. [P ]iincreasing concentrationsrelative ratesVplus 1 mM AMPplus 1 mM AMP and 2mM G-6-Pplus 1 mM AMP and 2 mM ATPno additions4 8. (2) (continued) (ii) Initial rates (v) were determined in incubation mixtures containing fixed amounts of enzyme, glycogen and Pi plus variable amounts of AMP as indicated. increasing concentrationsVrelative rates[AMP] (3) Xray crystallographic studies of the structures of crystalline phosphorylase b in the presence of various ligands revealed that AMP, ATP and glucose-6-P bound at the same site at a location some distance away from the substrate binding catalytic site on both subunits of the enzyme. Furthermore, it was found that the binding of AMP was associated with structural changes in the enzyme, whereas the binding of ATP and glucose-6-P were not. (a) For each of the three substances listed below, describe in qualitative terms their modes of action as regulatory agents for phosphorylase b based on the information given above. (1) AMP (2) ATP (3) glucose-6-P (b) Discuss the implications of the actions of AMP, ATP and glucose-6-P observed in vitro for the regulation of activity of phosphorylase b in vivo.5 9. The first step in the biosynthesis of the protein amino acid proline is as follows: OOC - CH - CH - CH - COO22NH3+ATP ADP2-O3P - O - C - CH2-CH2- CH - COONH3+O (a) If -glutamyl kinase were regulated by adenylate energy charge (EC), indicate on the graph to the right the type of response curve you would expect to obtain if the relative initial rates of the -glutamyl kinase reaction