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Chapter 12: Problem 4

(a) In muscle cells, insulin stimulates a protein kinase that catalyzes phosphorylation of protein phosphatase-1, thereby activating it. How does this affect glycogen synthesis and degradation in muscle cells? (b) Why does glucagon selectively regulate enzymes in the liver but not in other tissues? (c) How does glucose regulate the synthesis and degradation of liver glycogen via protein phosphatase-1?

Short Answer

Expert verified
Insulin stimulates glycogen synthesis and halts Glycogen degradation in muscle cells by activating protein phosphatase-1, which activates glycogen synthase and deactivates glycogen phosphorylase. Glucagon only regulates liver enzymes due to the selective expression of its receptors in this organ. Glucose regulates liver glycogen by activating protein phosphatase-1, promoting the synthesis of glycogen and inhibiting its degradation.

Step by step solution

01

Insulin and Glycogen Synthesis in Muscle Cells

Insulin stimulates a protein kinase that activates protein phosphatase-1 by phosphorylating it. Protein phosphatase-1, when active, will dephosphorylate (remove phosphate groups from) glycogen synthase, activating it. Active glycogen synthase promotes glycogen synthesis. On the flip side, protein phosphatase-1 also dephosphorylates glycogen phosphorylase, thus inactivating it and inhibiting glycogen degradation.
02

Glucagon Selective Regulation

Glucagon tends to selectively regulate enzymes in the liver and not other tissues due to the selective expression of glucagon receptors in the liver. Glucagon receptors are most heavily expressed in the liver, and to a lesser extent in the kidneys and adipose tissue, but are not present in muscle tissue. Therefore, glucagon's effects are most prevalent in the liver.
03

Glucose Regulation of Liver Glycogen

When glucose levels increase in the liver, this causes a chain of events leading to the deactivation of glycogen phosphorylase (which breaks down glycogen), and the activation of glycogen synthase (which builds glycogen). Glucose activates protein phosphatase-1, which dephosphorylates and therefore deactivates glycogen phosphorylase. At the same time, it activates glycogen synthase. Thus, the effect of glucose is to promote glycogen synthesis and prevent glycogen degradation.

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Most popular questions from this chapter

The polypeptide hormone glucagon is released from the pancreas in response to low blood glucose levels. In liver cells, glucagon plays a major role in regulating the rates of the opposing glycolysis and gluconeogenesis pathways by influencing the concentrations of fructose 2,6 bisphosphate (F2,6 BP). If glucagon causes a decrease in the concentrations of F2,6 BP, how does this result in an increase in blood glucose levels?

Glycogen storage diseases (GSDs) due to specific enzyme deficiencies can affect the balance between glycogen stores and blood glucose. Given the following diseases, predict the effects of each on (1) the amount of liver glycogen stored and (2) blood glucose levels. (a) Von Gierke disease (GSD-1a), defective enzyme: glucose 6-phosphatase. (b) Cori's disease (GSD III), defective enzyme: amylo1,6 glucosidase (debranching enzyme). (c) Hers's disease (GSD VI), defective enzyme: liver phosphorylase

Epinephrine promotes the utilization of stored glycogen for glycolysis and ATP production in muscles. How does epinephrine promote the use of liver glycogen stores for generating the energy needed by contracting muscles?

In many tissues, one of the earliest responses to cellular injury is a rapid increase in the levels of enzymes in the pentose phosphate pathway. Ten days after an injury, heart tissue has levels of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase that are 20 to 30 times higher than normal, whereas the levels of glycolytic enzymes are only \(10 \%\) to \(20 \%\) of normal. Suggest an explanation for this phenomenon.

After glycogen reserves are depleted what are the major gluconeogenic precursors of glucose under the conditions of (a) starvation and (b) intense exercise?
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