40 Questions
What is the product of the reaction catalyzed by nucleoside diphosphate kinase?
UTP
What is the average distance between branches in glycogen?
Eight glucosyl residues
What is the result of branching in glycogen?
An increase in the number of nonreducing ends
What is the function of the branching enzyme?
To break α(1→4) bonds and form α(1→6) linkages
What is the primary product of glycogen degradation in liver and skeletal muscle?
Glucose 1-phosphate
How are additional branches synthesized in glycogen?
By removing terminal glucosyl residues and attaching them to non-terminal residues
What type of linkage is formed by the branching enzyme?
α(1→6) linkage
What is the fate of free glucose in glycogen degradation?
It is released from each α(1→6)-linked glucosyl residue
What percentage of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)-glucosidase?
1-3%
What is the purpose of the lysosomal degradation of glycogen?
Unknown.
What is the result of a deficiency of the lysosomal enzyme, α(1→4)-glucosidase?
Accumulation of glycogen in vacuoles in the lysosomes.
What is the result of the binding of glucagon or epinephrine to their specific hepatocyte GPCR?
Activation of cAMP-dependent protein kinase A (PKA)
What type of disease is caused by the deficiency of the lysosomal enzyme, α(1→4)-glucosidase?
Glycogen storage disease Type II: Pompe disease.
What is the effect of cAMP on PKA?
It binds to the regulatory subunit, releasing individual catalytic subunits
What is the effect of PKA on phosphorylase kinase?
It phosphorylates the inactive 'b' form of phosphorylase kinase, producing the active 'a' form
When does glycogenolysis accelerate in the liver?
During periods of fasting.
What is the primary function of glycogen synthesis and degradation in the liver?
To maintain blood glucose levels.
What is the effect of active phosphorylase kinase on glycogen phosphorylase?
It phosphorylates the inactive 'b' form of glycogen phosphorylase, producing the active 'a' form
What percentage of the fresh weight of a well-fed adult liver is made up of glycogen?
10%
What is the effect of insulin on cAMP levels?
It activates the phosphodiesterase that degrades cAMP
How are glycogen synthesis and degradation regulated in the liver?
Through both hormonal regulation and allosteric control.
What is the result of the binding of hormones, such as glucagon or epinephrine, to plasma membrane G protein-coupled receptors?
Activation of glycogen degradation.
What is the effect of protein phosphatase-1 on phosphorylase a?
It dephosphorylates phosphorylase a, re-converting it to phosphorylase b
What is the primary source of carbons for the gluconeogenic pathway when glycogen stores are depleted?
Amino acids from the body’s proteins
Why is the body's reliance on dietary glucose alone not sufficient for maintaining blood glucose levels?
Dietary glucose is not always a reliable source due to sporadic intake
What is the role of inhibitor proteins in regulating glycogen degradation?
They bind to and inhibit protein phosphatase-1
What is the primary function of muscle glycogen?
To serve as a fuel reserve for the synthesis of ATP during muscle contraction
What is the net result of the cascade of reactions involving PKA, phosphorylase kinase, and glycogen phosphorylase?
Glycogenolysis
What is the approximate amount of glycogen that makes up the fresh weight of resting muscle?
400 g
What is the primary source of glucose for the brain during fasting or starvation?
Glycogen degradation
What is the primary function of liver glycogen?
To maintain blood glucose concentration
What is the reason for the body's development of mechanisms for storing glucose in a rapidly mobilizable form, such as glycogen?
To ensure a constant supply of glucose for the brain
What is the product of glycogen phosphorylase activity?
Glucose 1-phosphate
What is the resulting structure after glycogen phosphorylase activity?
Limit dextrin
What is the enzyme responsible for removing branches in glycogen degradation?
Debranching enzyme
What is the function of the debranching enzyme?
Breaks α(1→4) bonds and forms α(1→4) bonds
What is the enzyme responsible for converting glucose 1-phosphate to glucose 6-phosphate?
Phosphoglucomutase
What is the intermediate formed during the conversion of glucose 1-phosphate to glucose 6-phosphate?
Glucose 1,6-bisphosphate
What is the enzyme responsible for dephosphorylating glucose 6-phosphate in the liver?
Glucose 6-phosphatase
Why can't glucose 6-phosphate be dephosphorylated in muscle?
Lack of glucose 6-phosphatase
Study Notes
Importance of Glycogen
- A constant source of blood glucose is essential for human life, particularly for the brain and exercising muscle.
- Blood glucose can be obtained from three primary sources: dietary intake, degradation of glycogen, and gluconeogenesis.
Structure and Function of Glycogen
- Glycogen is a rapidly mobilizable form of glucose storage.
- Muscle glycogen serves as a fuel reserve for the synthesis of adenosine triphosphate (ATP) during muscle contraction.
- Liver glycogen maintains blood glucose concentration, particularly during the early stages of fasting.
Amounts of Liver and Muscle Glycogen
- 400 g of glycogen make up 1–2% of the fresh weight of resting muscle.
- 100 g of glycogen make up to 10% of the fresh weight of a well-fed adult liver.
Formation of Branches in Glycogen
- Branching increases the number of nonreducing ends to which new glucosyl residues can be added, accelerating glycogen synthesis and increasing the size of the molecule.
- Branches are made by the action of the branching enzyme, amylo-α(1→4) → α(1→6)-transglucosidase.
Synthesis of Additional Branches
- After elongation, terminal six to eight glucosyl residues can be removed and used to make additional branches.
Degradation of Glycogen (Glycogenolysis)
- Glycogen phosphorylase sequentially cleaves α(1→4) glycosidic bonds, producing glucose 1-phosphate.
- Branches are removed by the debranching enzyme, α(1→4) bond is broken, and a new α(1→4) bond is made.
Shortening of Chains
- Glycogen phosphorylase cleaves α(1→4) glycosidic bonds until four glucosyl units remain on each chain before a branch point, resulting in a limit dextrin.
Removal of Branches
- The debranching enzyme breaks an α(1→4) bond and makes a new α(1→4) bond, and then hydrolytically removes the remaining single glucose residue.
Conversion of Glucose 1-Phosphate to Glucose 6-Phosphate
- Glucose 1-phosphate is converted to glucose 6-phosphate by phosphoglucomutase in the cytosol.
- In the liver, glucose 6-phosphate is transported into the endoplasmic reticulum, where it is converted to glucose by glucose 6-phosphatase.
Lysosomal Degradation of Glycogen
- A small amount (1–3%) of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)-glucosidase (acid maltase).
Regulation of Glycogen Synthesis and Degradation
- Regulation is accomplished on two levels: hormonal regulation to meet the needs of the body as a whole, and allosteric control to meet the needs of a particular tissue.
Activation of Glycogen Degradation by cAMP-Directed Pathway
- Hormones such as glucagon or epinephrine bind to G protein-coupled receptors, signaling the need for glycogen to be degraded.
- cAMP-dependent protein kinase A (PKA) is activated, phosphorylating and activating enzymes of glycogen metabolism.
This quiz covers the importance of maintaining a constant source of blood glucose in the human body and the three primary sources of blood glucose, including dietary intake, glycogen degradation, and gluconeogenesis.
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