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The concentration of glucose in human blood plasma is maintained at about 5nM. The concentration of free glucose inside a myocyte is much lower. Why is the concentration so low in the cell? What happens to glucose after entry into the cell? Glucose is administered intravenously as a food source in certain clinical situations. Given that the transformation of glucose to glucose-6-phosphate consumes ATP, why not administer intravenous glucose-6-phosphate instead?
The phosphate group of glucose 6-phosphate is completely ionized at pH 7, giving the molecule an overall negative charge. Because membranes are generally impermeable to electrically charged molecules, glucose 6-phosphate can not pass from the bloodstream into cells and hence cannot enter the glycolytic pathway and generate ATP. (This is why glucose, once phosphorylate, cannot escape from the cell.)
The Vmax of the glycogen phopshorylase from skeletal muscle is much greater than the Vmax of the same enzyme from liver tissue.
a) What is the physiological function of glycogen phosphorylase in skeletal muscle? In liver tissue?
b) Why does the Vmax of the muscle enzyme need to be greater than that of the liver enzyme?
(a) In muscle: Glycogen breakdown supplies energy (ATP) via glycolysis. Glycogen phosphorylase catalyzes the conversion of stored glycogen to glucose 1-phosphate, an intermediate in glycolysis. During strenuous activity, skeletal muscle requires large quantities of glucose 6-phosphate.
In the liver: Glycogen breakdown maintains a steady level of blood glucose between meals (glucose 6-phosphate is converted to free glucose)
(b) In actively working muscle, ATP flux requirements are very high and glucose 1-phosphate must be produced rapidly; requiring a high Vmax.
In muscle tissue, the rate of conversion of glycogen to glucose-6-phosphate is determined by the ratio of phosphorylase a (active) to phosphorylase b (less active). Determine what happens to the rate of glycogen breakdown is a muscle preparation containing glycogen phosphorylase is treated with (a) phosphorylase kinase and ATP; (b) PP1; (c) epinephrine
For each setting, determine the relative levels of AMP, ATP, citrate, and acetyl-CoA and describe how these levels affect the flow of metabolites through glycolysis by regulating specific enzymes. In periods of stress, rabbit leg muscle produces much of its ATP by anaerobic glycolysis and very little by oxidation of acetyl-CoA derived from fat breakdown.
Resting: [ATP] high; [AMP] low; [acetyl-CoA] and [citrate] intermediate
Running: [ATP] intermediate; [AMP] high; [acetyl-CoA] and [citrate] low.
Glucose flux through glycolysis increases during the anaerobic sprint because (1) the ATP inhibition of glycogen phosphorylase and PFK-1 is partially relieved, (2) AMP stimulates both enzymes, and (3) lower citrate and acetyl-CoA levels relieve their inhibitory effects on PFK-1 and pyruvate kinase, respectively.
Compare the regulation of muscle glycolysis during short-term intense activity, as in the fleeing rabbit, and during extended activity, as in the migrating duck. What must the regulation in these two settings be different?
The migrating bird relies on the highly efficient aerobic oxidation of fats, rather than the anaerobic metabolism of glucose used by a sprinting rabbit. The bird reserves its muscle glycogen for short bursts of energy during emergencies.
Determine which enzyme is defective and designate the appropriate treatment from the lists provided. Justify your choices.
(a) Muscle PFK-1
(b) Phosphomannose isomerase
(c) Galactose 1-phosphate uridylyltransferase
(d) Liver glycogen phosphorylase
(e) Triose kinase
(f) Lactase in intestinal mucosa
(g) Maltase in intestinal mucosa
(h) Muscle debranching enzyme
1. Jogging 5 km each day
2. Fat-free diet
3. Low-lactose diet
4. Avoiding strenuous exercise
5. Large doses of niacin
6. Frequent feedings (smaller portions) of a normal diet
Case A: f, (3)
Case B: c, (3)
Case C: h, (4)
Case D: d, (6)
A man with insulin-dependent diabetes is brought to the ER near comatose. While vacationing, he lost his insulin mediation and has not taken any insulin for two weeks.
(a) For each tissue listed below, is each pathway faster, slower, or unchanged in this patient, compared with the normal level when he is getting appropriate amounts of insulin?
(b) For each pathway, describe at least one control mechanism responsible for the change you predict.
Tissue and Pathways
1. Adipose: FA synthesis
2. Muscle: glycolysis, FA synthesis, glycogen synthesis
3. Liver: glycolysis, gluconeogenesis; glycogen synthesis; FA synthesis; pentose phosphate pathway
(a) (1) Adipose: FA synthesis slower. (2) Muscle: glycolysis, FA synthesis, and glycogen synthesis slower. (3) Liver: glycolysis faster, gluconeogenesis, glycogen synthesis, and FA synthesis slower; pentose phosphate pathway unchanged.
(b) (1) Adipose and (3) Liver: FA synthesis slower because lack of insulin results in inactive acetyl-CoA carboxylase, the first enzyme of FA synthesis. Glycogen synthesis inhibited by cAMP-dependent phosphorylation of glycogen synthase. (2) Muscle: glycolysis slower because GLUT4 is inactive, so glucose uptake is inhibited. (3) Liver: glycolysis slower because the bifunctional PFK-2/FBPase-2 is converted to the form with active FBPase-2 decreasing [F26P] which allosterically stimulates PFK and inhibits FBPase-1; also accounts for stimulation of gluconeogenesis
For the patient, predict the levels of the following metabolites in his blood before treatment in the ER, relative to the levels maintained during adequate insulin treatment: (a) glucose, (b) ketone bodies; (c) free FA.
Between your evening meal and breakfast, your blood glucose drops and your liver becomes a net producer rather than consumer of glucose. Describe the hormonal change triggers glucose production by the liver.
The drop in blood glucose triggers release of glucagon by the pancreas. In the liver, glucagon activates glycogen phosphorylase by stimulating its cAMP-dependent phosphorylation and stimulates gluconeogenesis by lowering [fructose 2,6-bisphosphate], thus stimulating FBPase-1.
Researchers can manipulate the genes of a mouse so that a single gene in a single tissue either produces an inactive protein (a "knockout" mouse) or produces an inactive protein (constitutively) active. What effects on metabolism would you predict for mice with the following genetic changes: (a) knockout of glycogen deb ranching enzyme in the liver; (b) knockout of hexokinase IV in liver; (c) knockout of FBPase-2 in liver; (d) constitutively active AMPK in muscle; (f) constitutively active ChREBP in liver?
(a) Reduced capacity to mobilize glycogen; lowered blood glucose between meals
(b) Reduced capacity to lower blood glucose after a carbohydrate meal; elevated blood glucose
(c) Reduced F26BP in liver, stimulating glycolysis and inhibiting gluconeogenesis
(d) Reduced F26BP, stimulating gluconeogenesis and inhibiting glycolysis
(e) Increased uptake of FA and glucose; increased oxidation of both
(f) Increased conversion of pyruvate to acetyl-CoA; increased FA synthesis
For a given concentration of fructose 6-phosphate, the PFK-1 activity increases with increasing concentrations of ATP, but a point is reached beyond which increasing the concentration of ATP inhibits the enzyme.
(a)Explain how ATP can be both a substrate and an inhibitor of PFK-1. How is the enzyme regulated by ATP?
(b) In what ways is glycolysis regulated by ATP levels?
(c) The inhibition of PFK-1 by ATP is diminished when ADP concentration is high. How can this observation be explained?
(a) There are two binding sites for ATP: a catalytic and regulatory site. Binding of ATP to a regulatory site inhibits PFK-1, by reducing Vmax or increasing Km for ATP at the catalytic site.
(b) Glycolytic flux is reduced when ATP is plentiful.
(c) The graph indicates that increased [ADP] suppresses the inhibition by ATP. Because the adenine nucleotide pool is fairly constant, consumption of ATP leads to an increase in [ADP]. The data show that the activity of PFK-1 may be regulated by the [ATP]/[ADP] ratio.
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