c340 Exercise on Carbohydrate Regulation

Key to Exercise on Regulation of Carbohydrate Metabolism
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Key to Exercise on Regulation of Carbohydrate Metabolism

1 and 3. See diagram.

2. a. glucose-6-P;
b. hexokinase is inhibited;
c. fructose-1,6-bis-P (which activates pyruvate kinase).

4. PKA doesn't modify pyruvate kinase in skeletal muscle cells.

5. a. fructose-2,6-bis-P; extracellular conditions; b. intracellular conditions.

7. Describe regulation of PFK-2 and FBPase-2 activity in the liver using Table 1.

Table 1: PFK-2 and FBPase-2
enzyme	PFK-2	FBPase-2
substrate	fructose-6-phosphate	fructose-2,6-bis-phosphate
product	fructose-2,7-bis-phosphate	fructose-6-phosphate
activated by*	PP1	PKA
inactivated by*	PKA	PP1
effect of activation^†	glycolysis increases	gluconeogenesis increases

*Name the enzyme.
^†Which pathway–glycolysis or gluconeogenesis–is more active?

8. Fill in Table 2 to summarize the effects of these enzymes on glycogen phosphorylase.

Table 2: Covalent Regulation of Glycogen Phosphorylase
enzyme	phosphorylase kinase	PP1
Does this act on phosphorylase a or phosphorylase b?	phosphorylase b	phosphorylase a
What other substrate is required?	ATP	H₂O
Does this increase or decrease phosphorylase activity?	increase	decrease
Is glycogen synthesized or degraded as a result?	degraded	synthesized

9. Fill in Table 3 to summarize the effects of PKA and PP1 on glycogen synthase.

Table 3: Covalent Regulation of Glycogen Synthase
enzyme	PKA	PP1
Does this act on glycogen synthase a or glycogen synthase b?	glycogen synthase a	glycogen synthase b
What other substrate is required?	ATP	H₂O
Does this increase or decrease synthase activity?	decrease	increase
Is glycogen synthesized or degraded as a result?	degraded	synthesized

10. Fill in Table 4 to list the effects of these modulators on glycogen degradation.

Table 4: Modulators that Affect Skeletal Muscle Glycogen Degradation
molecule	enzyme the modulator binds	effect on enzyme activity	effect on glycogen degradation
G_sα	adenylyl cyclase	increases	increases
Ca²⁺	phosphorylase kinase	increases	increases
glucose-6-P or ATP	glycogen phosphorylase	decreases	decreases
AMP	glycogen phosphorylase	increases	increases

11. a. G_sα and Ca²⁺
b. Omit ATP and AMP; change glucose-6-P to glucose

12.

Table 5: Kinase, Phosphatase, and Phosphorylase
Enzyme	Kinase	Phosphatase	Phosphorylase
Is ATP required?	yes	no	no
Is P_i a substrate?	no	no	yes
Is P_i a product?	no	yes	no

13.

Table 6: Conditions that Affect Glycogen Metabolism
condition	hormone secreted	affects muscle, liver, or both?	glycogen phosphorylase activity	glycogen synthase activity	Is glycogen degraded or synthesized?
high blood sugar	insulin	both	decreases	increases	synthesized
low blood sugar	glucagon	liver only	increases	decreases	degraded
exercise	epinephrine	both	increases	decreases	degraded

14. a. High energy favors glycogen synthesis and low energy favors glycolysis.
b. PFK-1 is affected by allosteric regulation. ATP and citrate inhibit PFK-1, and
     AMP, ADP, and fructose-2,6-bis-phosphate activate PFK-1.
c. Glycogen synthase is regulated by covalent modification. PKA phosphorylates
     and inhibits glycogen synthase, and PP1 removes the phosphate and activates
     glycogen synthase.

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