Revised 1/7/06 - 1 - Chemistry 153C Answers to 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. 1. (a) These are conditions which favor glucose uptake and storage as glycogen and fatty acids. Glucokinase would be active under these conditions of high glucose and give favorable uptake of glucose even in the presence of relatively high glucose-6-P levels. The dephosphorylated forms, phosphorylase b and glycogen synthase I, would prevail under these conditions. Since this is the less active form of phosphorylase and the more active form of glycogen synthase, the synthesis of glycogen would be favored and breakdown inhibited. High glucose-6-P levels may also stimulate glycogen synthase and inhibit phosphorylase. 1. (b) These are conditions which favor glucose synthesis for release to the blood by gluconeogenesis and glycogen breakdown, and minimize glycogen synthesis and glycolysis. The phosphorylated forms of phosphorylase a and glycogen synthase would predominate due to high cyclic AMP levels and activation of protein kinase. Since this is the more active form of phosphorylase and the less active form of glycogen synthase, glycogen breakdown would be favored and glycogen synthesis inhibited. (Lower glucose-6-P levels may have an additional effect in this direction.)2 1. (c) continued (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.) 1.(c) This condition would favor glycolysis and glycogen breakdown and inhibit gluconeogenesis and glycogen synthesis. Phosphorylase and phosphofructokinase are R-type enzymes and would be stimulated in rate by the lower energy charge value (due to the decrease in inhibitory ATP and the increase in stimulatory AMP at the lower E.C.). This would increase the rate of glycogen breakdown and glycolysis. Glycogen synthase and F-1,6 BP phosphatase are U-type enzymes and would be inhibited by the lower than normal energy charge levels. This would tend to limit the activity of gluconeogenesis and glycogen synthesis. PiH2Ofructose-6-PATPfructose-1, 6-bis PADPORCO2, GDP (ADP)phosphoenolpyruvate (PEP) ADPGTP (ATP)oxaloacetatepyruvateATPADP, PiATP, CO2 , H2O3 2. continued (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. (b) ATP + H2O ∏ ADP + Pi or GTP (ATP) + H2O ∏ GDP (ADP) + Pi (c) For the interconversions of fructose-6-P and fructose 1,6-bisphosphate: High energy charge values would tend to favor the phosphatase activity while inhibiting the kinase. Low energy charge values would have the opposite effect. 3. Phosphorylase kinase is responsible for catalyzing the phosphorylation of both phosphorylase b and glycogen synthase I to their phosphorylated forms. This would therefore interrupt the regulatory cascade in liver triggered by the hormones glucagon and epinephrine. Such a liver cell would be stuck with its phosphorylase in the less active b form and glycogen synthase in the more active form. One might therefore expect glycogen to accumulate (a so-called glycogen storage disease). Local regulatory factors (such as low energy charge) might trigger some breakdown of glycogen via phosphorylase b, but the liver would be impaired in glycogenolysis and thus unable to respond to a hormonal demand for more blood glucose.4 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. 4. (a) EC(ATP)+1/2(ADP)(AMP) + (ADP) + (ATP)=State I EC=12 + 1=13=0.871+ 2+1215State II EC=10 + 1.5=11.5=0.7710 3 + 215+ (b) The decrease in E.C. in State II would be expected to increase the rate of glycogen breakdown and the ATP production. This would be expected because of the positive modulation of several regulatory enzymes of the R-type in the central metabolic pathways. These include: phosphorylase phosphofructokinase pyruvate kinase pyruvate dehydrogenase citrate synthase isocitrate dehydrogenase5 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