Deck 15: Principles of Metabolic Regulation

Question 1

Glycogen is converted to monosaccharide units by:

A) glucokinase.
B) glucose-6-phosphatase
C) glycogen phosphorylase.
D) glycogen synthase.
E) glycogenase.

Answer 1

C

Question 2

An increase in which compound leads to the dephosphorylation and activation of phosphofructokinase-2?

A) glucagon
B) xyulose-5-phosphate
C) pyruvate
D) citrate
E) ADP

Answer 2

B

Question 3

Which factor is NOT involved in up-regulating the transcription of glycolytic or gluconeogenic enzymes?

A) phosphofructokinase-2
B) carbohydrate response element binding protein
C) sterol response element binding protein
D) cAMP response element binding protein
E) FOXO1

Answer 3

A

Question 4

Reaction steps that are far from equilibrium are good control points in metabolic pathways because:

A) the net flux through those steps is easily reversed.
B) the rate differences between the forward and reverse steps are often small.
C) these reactions occur most frequently in the cell.
D) these reactions are highly endergonic.
E) these reactions are highly exergonic.

Question 5

For an enzyme to effectively change its activity in response to a change in substrate concentration, it is MOST favorable for:

A) K_m to be less than cellular substrate concentrations.
B) K_m to be equal to cellular substrate concentrations.
C) K_m to be greater than cellular substrate concentrations.
D) V_max to be at the diffusion limit.
E) the substrate to also be an allosteric effector.

Question 6

Which statement about gluconeogenesis in animal cells is TRUE?

A) A rise in the cellular level of fructose-2,6-bisphosphate stimulates the rate of gluconeogenesis.
B) An animal fed a large excess of fat in the diet will convert any fat not needed for energy production into glycogen to be stored for later use.
C) The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate is not catalyzed by phosphofructokinase-1, the enzyme involved in glycolysis.
D) The conversion of glucose 6-phosphate to glucose is catalyzed by hexokinase, the same enzyme involved in glycolysis.
E) The conversion of phosphoenol pyruvate to 2-phosphoglycerate occurs in two steps, including a carboxylation.

Question 7

Gluconeogenesis must use "bypass reactions" to circumvent three reactions in the glycolytic pathway that are highly exergonic and essentially irreversible. Reactions carried out by which three of the enzymes listed must be bypassed in the gluconeogenic pathway? 1) Hexokinase
2) Phosphoglycerate kinase
3) Phosphofructokinase-1
4) Pyruvate kinase
5) Triosephosphate isomerase

A) 1, 2, 3
B) 1, 2, 4
C) 1, 4, 5
D) 1, 3, 4
E) 2, 3, 4

Question 8

Aside from maintaining the integrity of its hereditary material, the MOST important general metabolic concern of a cell is:

A) keeping its glucose levels high.
B) maintaining a constant supply and concentration of ATP.
C) preserving its ability to carry out oxidative phosphorylation.
D) protecting its enzymes from rapid degradation.
E) running all its major metabolic pathways at maximum efficiency.

Question 9

Which factor is an example of flux regulation, not flux control, upon an increase in blood glucose levels?

A) induction of insulin
B) increased glucose transport into cells
C) induction of the synthesis of hexokinase
D) induction of the synthesis of phosphofructokinase-1
E) activation of glycogen synthase

Question 10

Cellular isozymes of pyruvate kinase are allosterically inhibited by:

A) high concentrations of AMP.
B) high concentrations of ATP.
C) high concentrations of citrate.
D) low concentrations of acetyl-CoA.
E) low concentrations of ATP.

Question 11

The enzyme glycogen phosphorylase:

A) catalyzes a cleavage of $\beta$ (1 $\rightarrow$ 4) bonds.
B) catalyzes a hydrolytic cleavage of $\alpha$ (1 $\rightarrow$ 4) bonds.
C) is a substrate for a kinase.
D) uses glucose 6-phosphate as a substrate.
E) uses glucose as a substrate.

Question 12

The glycogen-branching enzyme catalyzes:

A) degradation of ( $\alpha$ 1 $\rightarrow$ 4) linkages in glycogen
B) formation of ( $\alpha$ 1 $\rightarrow$ 4) linkages in glycogen.
C) formation of ( $\alpha$ 1 $\rightarrow$ 6) linkages during glycogen synthesis.
D) glycogen degradation in tree branches.
E) removal of unneeded glucose residues at the ends of branches.

Question 13

Which factor does NOT contribute to the regulation of enzymatic activity?

A) protein phosphorylation
B) allosteric regulation
C) protein stability
D) mRNA stability
E) DNA stability

Question 14

The elasticity coefficient for an enzyme in a multistep pathway depends on:

A) the concentration of the enzyme itself.
B) the levels of regulatory molecules.
C) the amounts of substrate molecules present at each step.
D) both the concentration of the enzyme itself and the amounts of substrate molecules present at each step.
E) both the levels of regulatory molecules and the amounts of substrate molecules present at each step.

Question 15

Which statement about mammalian glycogen synthase is NOT correct?

A) It is especially predominant in liver and muscle.
B) The donor molecule is a sugar nucleotide.
C) The phosphorylated form of this enzyme is inactive.
D) This enzyme adds glucose units to the nonreducing end of glycogen branches.
E) This enzyme adds the initial glucose unit to a tyrosine residue in glycogenin.

Question 16

Which type of mechanism is NOT known to play a role in the reversible alteration of enzyme activity?

A) activation by cleavage of an inactive zymogen
B) allosteric response to a regulatory molecule
C) alteration of the synthesis or degradation rate of an enzyme
D) covalent modification of the enzyme
E) interactions between catalytic and regulatory subunits

Question 17

The flux control coefficient for an enzyme in a multistep pathway depends on the:

A) concentration of the enzyme itself.
B) concentration of other enzymes in the pathway.
C) levels of regulatory molecules.
D) amounts of substrate molecules present at each step.
E) All of the answers are correct.

Question 18

If the mass action ratio, Q, for a reaction under cellular conditions is larger than the equilibrium constant, K_eq, then the reaction will:

A) be at equilibrium.
B) go backward and be endergonic.
C) go backward and be exergonic.
D) go forward and be endergonic.
E) go forward and be exergonic.

Question 19

There is reciprocal regulation of glycolytic and gluconeogenic reactions interconverting fructose-6-phosphate and fructose-1,6-bisphosphate. Which statement about this regulation is NOT correct?

A) Fructose-2,6-bisphosphate activates phosphofructokinase-1.
B) Fructose-2,6-bisphosphate inhibits fructose-1,6-bisphosphatase.
C) The fructose-1,6-bisphosphatase reaction is exergonic.
D) The phosphofructokinase-1 reaction is endergonic.
E) This regulation allows control of the direction of net metabolite flow through the pathway.

Question 20

The MOST sensitive indicator of the energetic status of the cell is the concentration of:

A) AMP.
B) ADP.
C) ATP.
D) cAMP.
E) glucose.

Question 21

Glycogenin:

A) catalyzes the conversion of starch into glycogen.
B) is the enzyme responsible for forming branches in glycogen.
C) is the gene that encodes glycogen synthase.
D) is the primer on which new glycogen chains are initiated.
E) regulates the synthesis of glycogen.

Question 22

Glycogen phosphorylase a can be inhibited at an allosteric site by:

A) AMP.
B) calcium.
C) GDP.
D) glucagon.
E) glucose.

Question 23

Is glycogen phosphorylase activated, inhibited, or unaffected in response to an insulin signal in skeletal muscle?

A) activated
B) inhibited
C) unaffected

Question 24

Which factor directly results in the activation of glycogen synthase?

A) binding of glucose-6-phosphate
B) dephosphorylation of multiple residues by phosphoprotein phosphorylase-1 (PP1)
C) phosphorylation of specific residues by casein kinase II (CKII)
D) phosphorylation of specific residues by glycogen synthase kinase-3 (GSK-3)
E) the presence of insulin

Question 25

Which statement does NOT describe a characteristic of phosphoprotein phosphorylase-1 (PP1)?

A) PP1 can be phosphorylated by protein kinase A (PKA).
B) PP1 can dephosphorylate glycogen phosphorylase, glycogen synthase, and phosphorylase kinase.
C) PP1 is allosterically activated by glucose-6-phosphate.
D) PP1 is inhibited by activated glycogen phosphorylase
E) PP1 is phosphorylated by glycogen synthase kinase-3 (GSK3).

Question 26

Which statement is TRUE of muscle glycogen phosphorylase?

A) It catalyzes phosphorolysis of the ( $\alpha$ 1 $\rightarrow$ 6) bonds at the branch points of glycogen.
B) It catalyzes the degradation of glycogen by hydrolysis of glycosidic bonds.
C) It degrades glycogen to form glucose 6-phosphate.
D) It exists in an active (a) form and an inactive (b) form that is allosterically regulated by AMP.
E) It removes glucose residues from the reducing ends of the glycogen chains.

Question 27

Which statement is TRUE of glycogen synthase?

A) Activation of the enzyme involves a phosphorylation.
B) It catalyzes addition of glucose residues to the nonreducing end of a glycogen chain by formation of ( $\alpha$ 1 $\rightarrow$ 4) bonds.
C) It uses glucose-6-phosphate as donor of glucose units
D) It catalyzes addition of glucose residues at branch points by formation of ( $\alpha$ 1 $\rightarrow$ 6) bonds.
E) The enzyme has measurable activity only in liver.

Question 28

After ingestion of a carbohydrate-rich meal, the increased concentration of glucose in the blood is associated with many changes in the metabolism of humans. Which change would NOT be expected?

A) a decrease in the secretion of glucagon from the pancreas
B) an increase in glycogen phosphorolysis
C) an increase in the levels of fructose 2,6-bisphosphate
D) activation of phosphorylase b kinase
E) All of these changes would be expected.

Question 29

Starting from glucose, what is the net number of NTP molecules used in the synthesis of glycogen?

A) zero
B) one
C) two
D) three
E) four

Question 30

Which process is NOT an event in the signaling pathway from insulin to glycogen synthase kinase 3 (GSK3) inactivation?

A) insulin binding to a tyrosine kinase receptor
B) activation of insulin receptor substrate-1 (IRS-1) by phosphorylation
C) phosphatidylinositol 3-kinase associating with IRS-1 generates phosphatidylinositol 3,4,5-triphosphate
D) protein kinase B being activated by a protein kinase known as PDK-1
E) protein kinase B phosphorylating GSK3, resulting in its activation

Question 31

Which statement is TRUE of glycogen synthesis and breakdown?

A) Phosphorylation activates the enzyme responsible for breakdown, and inactivates the synthetic enzyme.
B) Synthesis is catalyzed by the same enzyme that catalyzes breakdown.
C) The glycogen molecule "grows" at its reducing end.
D) The immediate product of glycogen breakdown is free glucose.
E) Under normal circumstances, glycogen synthesis and glycogen breakdown occur simultaneously and at high rates.

Question 32

Is glycogen phosphorylase activated, inhibited, or unaffected in response to a glucagon signal in skeletal muscle?

A) activated
B) inhibited
C) unaffected

Question 33

Which statement is TRUE for glycogen phosphorylase b?

A) It catalyzes a phosphorylytic cleavage of $\alpha$ (1 $\rightarrow$ 6) glycosidic bonds.
B) It catalyzes a hydrolytic cleavage of $\alpha$ (1 $\rightarrow$ 4) glycosidic bonds.
C) It is regulated both allosterically and via covalent modification.
D) It uses UDP-glucose as a substrate.
E) It is an important blood glucose sensor in myocytes.

Question 34

Which statement is TRUE for the reciprocal regulation of phosphofructokinase-1 (PFK-1) and fructose 1,6-bisphosphatase (FBPase-1)?

A) Fructose 2,6-bisphosphate inhibits FBPase-1 and activates PFK-1.
B) Fructose 2,6-bisphosphate inhibits PFK-1 and activates FBPase-1.
C) Fructose 1,6-bisphosphate inhibits FBPase-1 and activates PFK-1.
D) Fructose 1,6-bisphosphate inhibits PFK-1 and activates FBPase-1.

Question 35

Which isozyme of lactate dehydrogenase has the

Question 36

Which statement is TRUE of glycogen synthase?

A) It is activated by at least 11 different protein kinases.
B) It is activated by Ca²⁺ ions in skeletal muscle.
C) It catalyzes formation of ( $\alpha$ 1-4) glycosidic bonds.
D) It uses glucose-1-phosphate as a substrate.
E) All of the statements are true true.

Question 37

Which enzyme does NOT play a role in glycogen metabolism?

A) glucose-6-phosphatase
B) fructose 1,6-bisphosphatase
C) glycosyl-(4 $\rightarrow$ 6) transferase
D) protein phosphatase 1
E) phosphoglucomutase

Question 38

Is glycogen phosphorylase activated, inhibited, or unaffected in response to a glucagon signal in liver tissue?

A) activated
B) inhibited
C) unaffected

Question 39

To which enzyme in glycolysis is phosphoglucomutase MOST similar?

A) phosphohexose isomerase
B) triosphosphate isomerase
C) phosphoglycerate mutase
D) enolase
E) phosphoglycerate kinase

Question 40

Which substance activates glycogen synthase b in liver?

A) glucagon
B) insulin
C) glucose 6-phosphate
D) both insulin and glucose 6-phosphate
E) both glucagon and glucose 6-phosphate

Question 41

Which statement describes a possible fate for glucose 1-phosphate in skeletal muscle?

A) Glucose 1-phosphate can enter glycolysis after conversion to glucose 6-phosphate.
B) Glucose 1-phosphate cannot enter the pentose phosphate pathway after conversion to glucose 6-phosphate.
C) Glucose 1-phosphate can be used to replenish blood glucose.
D) Glucose 1-phosphate can be used to replenish blood glucose or enter glycolysis after conversion to glucose 6-phosphate.
E) All of the answers are correct.

Question 42

Which tissue has the FASTEST average rate of protein turnover?

A) the heart
B) the brain
C) the kidneys
D) skeletal muscle
E) the liver

Question 43

Which metabolic effect is associated with AMP-activated protein kinase (AMPK)?

A) When activated, AMPK stimulates insulin release from the pancreas.
B) When activated, AMPK activates fructose 2,6-bisphosphatase.
C) When activated, AMPK stimulates glucose uptake in cardiac and skeletal muscle.
D) When activated, AMPK both activates fructose 2,6-bisphosphatase and stimulates glucose uptake in cardiac and skeletal muscle.
E) None of the answers is correct.

Question 44

Which enzyme is used in both glycolysis and gluconeogenesis?

A) 3-phosphoglycerate kinase
B) hexokinase
C) glucose 6-phosphatase
D) phosphofructokinase-2
E) pyruvate kinase

Question 45

Which statement is FALSE with respect to near-equilibrium metabolic reactions?

A) Near-equilibrium reactions have a free-energy change near zero.
B) Near-equilibrium reactions operate nonenzymatically in vivo.
C) Near-equilibrium reactions are dependent on the concentration of the products.
D) Near-equilibrium reactions are not usually a control point in a metabolic pathway.
E) All of the statements are false.

Question 46

Which factor affects the activity of glucokinase in hepatocytes?

A) transcriptional regulation
B) subcellular localisation
C) association with a regulatory protein
D) transcriptional regulation and subcellular localisation
E) All of the answers are correct.

Question 47

Which enzyme is NOT regulated?

A) hexokinase
B) pyruvate kinase
C) phosphofructokinase-1
D) glyceraldehyde-3-phosphate dehydrogenase (GAPDH)
E) All these enzymes are regulated.

Question 48

Which statement is NOT valid with respect to the enzyme adenylate kinase?

A) The reversible reaction catalyzed by adenylate kinase is 2ADP $\nu$ ATP + AMP.
B) During a 100-meter sprint, adenylate kinase in myocytes catalyzes a biochemical reaction that provides some ATP.
C) Activated adenylate kinase catalyzes the reaction that generates the second messenger cAMP from ATP.
D) When cellular ATP is being used, adenylate kinase generates AMP, a signal of a low-energy state.
E) Adenylate kinase is a phosphotransferase enzyme that plays a critical role in cellular energy homeostasis.

Question 49

Glycogen storage disease (GSD) type 1a is associated with mutations in glucose 6-phosphatase. There are other forms of GSD type I. Which protein is likely mutated in these other forms of GSD?

A) G6P transporter (T1)
B) P_i transporter (T3)
C) glucose transporter (GLUT2)
D) T1 and T3
E) T1, T3, and GLUT2

Question 50

Which statement accurately describes the effect of insulin on the expression of the gene for glucose 6-phosphatase?

A) Insulin triggers the phosphorylation of PKB, which then phosphorylates FOXO-1. Phosphorylation of FOXO-1 causes a decrease in its degradation by the proteasome, which is associated with an increase in the expression of the gene for glucose 6-phosphatase.
B) Insulin activates PKB by phosphorylation, which then phosphorylates FOXO-1. Phosphorylation of FOXO-1 causes an increase in its degradation by the proteasome, which is associated with an increase in the expression of the gene for glucose 6-phosphatase.
C) Insulin leads to the activation of PKA, which then phosphorylates FOXO-1. Phosphorylation of FOXO-1 causing a decrease in its degradation by the proteasome, which is associated with a decrease in the expression of the gene for glucose 6-phosphatase.
D) Insulin leads to the activation of PKB, which then phosphorylates FOXO-1. Phosphorylation of FOXO-1 causing an increase in its degradation by the proteasome, which is associated with a decrease in the expression of the gene for glucose 6-phosphatase.
E) Insulin activates phosphoprotein phosphatase, which then dephosphorylates FOXO-1. Dephosphorylation of FOXO-1 causes a decrease in its degradation by the proteasome and an increase in the expression of the gene for glucose 6-phosphatase.

Question 51

Although in higher animals fats are the most efficient storage form of "free-energy," not all ingested carbohydrate is converted to fat. Some is stored in the form of glycogen. Which statement does NOT apply to the metabolic benefits of storing "free-energy" in the form of glycogen?

A) Glycogen can be rapidly catabolized because its structure allows for several catabolic enzymes to work in parallel.
B) Glycogen in skeletal muscle can be mobilized under anaerobic conditions, whereas products of fatty acid catabolism cannot.
C) In contrast to glycogen, the products of fatty acid catabolism are not gluconeogenic.
D) Erythrocytes have an absolute requirement for glucose and cannot use any product of fat metabolism for their energy needs.
E) Glycogen molecules are highly hydrated.

Question 52

Which method of directly regulates PFK-I?

A) feedback inhibition by fructose 2,6-bisphosphate
B) feed forward activation by phosphoenolpyruvate
C) allosteric activation by AMP
D) its phosphorylation in response to glucagon signaling
E) allosteric activation by citrate

Question 53

Which pairing CORRECTLY matches an enzyme with its effector?

A) phosphofructokinase-1/fructose 1,6-bisphosphate
B) phosphofructokinase-2/fructose 2,6-bisphosphate
C) fructose 1,6-bisphosphatase/fructose 1,6-bisphosphate
D) fructose 1,6-bisphosphatase/fructose 2,6-bisphosphate
E) fructose 2,6-bisphosphatase/fructose 2,6-bisphosphate

Question 54

What is the predominant source of glucose circulating in the bloodstream of a normal individual who is asleep at 3 AM?

A) catabolism of even-chained fatty acids
B) gluconeogenesis in the liver
C) phosphorolysis of muscle glycogen
D) glycogenolysis in the liver
E) dietary carbohydrates

Question 55

Metabolic pathways are said to operate in a steady state. Which process does NOT contribute to this?

A) Most biological reactions are reversible under normal cellular conditions.
B) Key reactions at the beginning and end of a pathway are often regulated synchronously.
C) Catabolic pathways are active when cellular energy levels are low.
D) Reversible reactions proceed to equilibrium.

Question 56

Glucokinase (hexokinase IV) differs from hexokinase (hexokinase I):

A) by having a higher affinity for glucose and not being inhibited by high levels of glucose.
B) by requiring larger concentrations of glucose before reaching maximal activity.
C) to allow the liver to export free glucose to the bloodstream when blood sugar levels drop.
D) so that the liver does not compete with other tissues for glucose when glucose levels are high.
E) to allow the pancreas to trigger insulin release when blood sugar levels are low.

Question 57

Which statement regarding regulation of glycolysis and gluconeogenesis is FALSE?

A) Phosphofructokinase-1 and fructose 1,6-bisphosphatase are prevented from forming a futile cycle by reversible covalent modification.
B) Hexokinase IV and glucose 6-phosphatase are transcriptionally regulated in hepatocytes.
C) Phosphofructokinase-1 and fructose 1,6-bisphosphatase are reciprocally regulated by the allosteric regulator, fructose 2,6-bisphosphate.
D) Binding of fructose 2,6-bisphosphate to phosphofructokinase-1 reduces the affinity of this enzyme for citrate.
E) The cellular concentration of fructose 2,6-bisphosphate is determined by a bifunctional enzyme that is hormonally regulated.

Question 58

What will be the net yield of ATP (per glucose unit) starting from glycogen and ending at pyruvate?

A) 1 ATP
B) 2 ATP
C) 3 ATP
D) 4 ATP
E) 5 ATP

Question 59

Which statement is NOT associated with controlling the rate of glycogen synthesis in myocytes?

A) Insulin stimulates the translocation of GLUT 4 transporters to the plasma membrane.
B) Insulin induces the synthesis of hexokinase I.
C) Glucagon inhibits the activity of glycogen synthase via covalent modification.
D) Epinephrine activates PKA, resulting in an activation of glycogen phosphorylase kinase.
E) Insulin inhibits the activity of glycogen synthase kinase 3.

Question 60

The rate of glycolysis is tightly regulated in all cells. Regulation is important to ensure that:

A) cells energy needs met.
B) fuel is not wasted.
C) there are appropriate levels of intermediates required for other purposes.
D) fuel is not wasted and the cells energy needs are met.
E) All of the answers are correct.

Question 61

Explain the distinction between metabolic "regulation" and metabolic "control" in a multienzyme pathway.

Question 62

Which statement is TRUE of the glycogen branching enzyme?

A) It catalyzes glycogen degradation by removing glucose from the branch points.
B) It catalyzes the formation of an $\alpha$ 1-4 linkages during glycogen synthesis.
C) It catalyzes the addition of a glucose molecule to glycogenin.
D) It is allosterically regulated by fructose 2,6-bisphosphate.
E) It is an enzyme that belongs to a family of transferases.

Question 63

After a meal rich in carbohydrates, which change would NOT be expected in a normal human?

A) increased secretion of the pancreatic hormone, insulin
B) increased activity of the liver enzyme, glucokinase
C) increased glucose uptake in myocytes via GLUT4 transporters
D) decreased activity of GSK-3 in both myocytes and hepatocytes
E) increased activity of glucose 6-phosphatase in hepatocytes

Question 64

In the glycolytic path from glucose to pyruvate, three steps are practically irreversible. What are these steps, and how is each bypassed in gluconeogenesis? What advantages does an organism gain from having separate pathways for anabolic and catabolic metabolism? What are the disadvantages?

Question 65

Which compound is NOT associated (in some way) with glycogen granules in the cytosol of myocytes?

A) glycogenin
B) glucose
C) inhibitor 1
D) adenylate kinase
E) calmodulin

Question 66

How are the opposing pathways of glycolysis and gluconeogenesis prevented from extensive futile cycling?

A) At least seven of the enzymes in the two pathways are different.
B) Key enzymes are regulated in a coordinated and reciprocal manner.
C) Catabolic and anabolic pathways do not take place in the same cellular compartments.
D) The overall free-energy change always favors the catabolic pathway.
E) Gluconeogenesis involves three endergonic bypass reactions.

Question 67

What are the regulatory implications for the cell with regard to ATP and AMP, given that the former are generally high, and the latter are low?

Question 68

An altered version of glycogen synthase is produced in the laboratory, with one of its amino acid residues substituted for another. Unlike normal glycogen synthase, the activity of the mutant protein is unaffected by glycogen synthase kinase-3. Which mutation is MOST likely to produce this effect?

A) Cys $\rightarrow$ Ser
B) Ile $\rightarrow$ Thr
C) Ser $\rightarrow$ Ala
D) Cys $\rightarrow$ Met
E) Thr $\rightarrow$ Ser

Question 69

Which enzyme-catalyzed reaction is LEAST sensitive to changes in the concentration of its reactants under physiological conditions?

A) phosphoglycerate kinase
B) GAP dehydrogenase
C) adenylate kinase
D) fructose 1-phosphate aldolase
E) phosphofructokinase-1

Question 70

Which statement is TRUE for "far from equilibrium" reactions in a cell?

A) They are irreversible under all conditions.
B) Their standard free-energy changes are typically large and positive.
C) They are common control points of regulation in a metabolic pathway.
D) All of the statements are true.
E) None of the statements is true.

Question 71

Epinephrine binding to $\beta$ -adrenergic receptors on hepatocytes causes the eventual activation of protein kinase A. Which enzyme is NOT a substrate for protein kinase A?

A) phosphorylase b
B) phosphorylase b kinase
C) glycogen synthase
D) phosphofructokinase-2
E) pyruvate kinase

Question 72

Explain the difference between homeostasis and equilibrium.

Question 73

Why is it important for proper cell function that proteins turn over rather than persisting indefinitely after being synthesized?

Question 74

Muscle contraction is associated with elevated levels of cytosolic calcium. Which statement describes a consequence of this increase in calcium?

A) Calcium is a divalent cation, which binds to ATP and reduces electrostatic repulsion between the phosphates. Glycogen is broken down to compensate for the decreased energy available from ATP hydrolysis.
B) An increased level of calcium is associated with an increase in glycogen breakdown, as calcium is an allosteric activator of glycogen phosphorylase.
C) Calcium activates phosphoprotein phosphatase 1, which in turn will stimulate glycogen synthesis by dephosphorylating phosphorylase b.
D) An increased level of calcium is associated with an increase in glycogen breakdown, as calcium is an allosteric activator of phosphorylase b kinase.
E) All the answers are correct.

Question 75

Which statement is TRUE for glycogen granules in hepatocytes?

A) Hepatic glycogen granules account for approximately 1% to 2% of the weight of the liver.
B) Each particle may contain up to 55,000 glucose molecules.
C) Glycogen granules cluster together, forming $\alpha$ -rosettes that become visible after a 24-hour fast.
D) All of the statements are true.
E) None of the statements is true.

Question 76

Mutating which proteins is unlikely to result in a glycogen storage disease?

A) pyruvate carboxylase
B) GLUT2
C) phosphorylase
D) PFK-1 in muscle
E) phosphorylase kinase

Question 77

Which statement is NOT associated with controlling the rate of glycogen synthesis in hepatocytes?

A) Insulin stimulates the translocation of GLUT 2 transporters to the plasma membrane.
B) Insulin induces the synthesis of hexokinase IV.
C) Glucagon inhibits the activity of glycogen synthase via covalent modification.
D) Insulin activates phosphorylase a phosphatase.
E) Epinephrine and insulin both result in the phosphorylation of the glycogen targeting protein G_M.

Question 78

Explain why reactions that are far from equilibrium need to be regulated.

Question 79

Which enzyme-catalyzed reaction is MOST sensitive to changes in the concentration of its reactants under physiological conditions?

A) hexokinase
B) pyruvate kinase
C) adenylate kinase
D) glucose-6-phosphatase
E) phosphofructokinase-1

Question 80

Briefly explain the differences between the flux control, elasticity, and response coefficients.