Flashcards in Biochemistry- Glycogen synthesis and breakdown Deck (31):
What is glycogenesis?
The synthesis of glycogen from freely available glucose
What is glycogenolysis?
The breakdown of glycogen to form glucose
Why is glycogen important?
It is the major storage molecule for carbohydrates in liver and muscle cells
How is glycogenolysis accomplished?
What is the function of liver glycogen?
It is broken down between meals to maintain blood glucose levels for red blood cells and the brain.
What is the function of muscle glycogen?
It is not available for the maintenance of blood glucose levels.
The energy can only be consumed by muscle cells.
It provides energy via glycolysis and TCA during bursts of physical activity
What is the primary source of glucose overnight?
Because at this time hepatic glycogen is depleted
What is gluconeogenesis?
The generation of new glucose from non-carbohydrate precursors.
What contributes most blood glucose straight after a meal?
What keeps the blood glucose level constant between meal?
Glycogenolysis of hepatic glycogen
Describe the structure of glycogen
It is a polymer consisting of glucose molecules joined by alpha-1,4-glycosidic links
Branches are introduced by alpha-1,6-glycosidic links
How is glycogen built up?
Glucose residues can only be added to an existing glycogen chain.
A glycogen primer containing at least 4 glucose residues is required.
The primer is covalently attached to a protein called glycogenin, which has enzymatic activity and can add these residues to itself.
What enzyme synthesizes glycogen?
Why is glycogenin required?
Because glycogen synthase cannot start a glycogen from start- it can only add glucose onto existing glycogen.
What must happen to glucose before it can be used to synthesise glycogen?
It must be phosphorylated by hexokinase to glucose-6-phosphate. This traps the glucose in the cell.
If there is an energy requirement, this can then undergo glycolysis. If there is an energy surplus, it can be used to synthesise glycogen.
What happens to glucose-6-phosphate in the synthesis of glycogen?
Glucose-6-phosphate is converted to glucose-1-phosphate by phosphoglucomutase. This enzyme simply moves the phosphate from one carbon to another, and it can catalyse forward and backward reactions.
UDP-glucose phosphorylase then converts the glucose to UDP-glucose, which can be thought of as an active form of glucose.
UDP-glucose acts as a substrate for glycogen synthase. The enzyme takes up the glucose part of UDP-glucose and adds it onto an existing glycogen. Left over UDP can be phosphorylated again to form UTP, requiring one ATP equivalent. Therefore one ATP is consumed for each glucose added to glycogen.
Why is UDP glucose highly active?
Because of the bond between the glucose and the phosphate of the UDP. The phosphate ester bond contains high energy levels which can be exploited to transfer the glucose onto another acceptor molecule.
If ATP is considered as a phosphate transfer molecule, how can UDP be considered?
As a glucose transfer molecule
What does glycogen synthase do?
What can it not do?
It synthesises glycogen from UDP-glucose, by adding one glucose to glycogen at a time. It can only extend the chains of glycogen- it cannot start a new chain or introduce new branches.
Which enzyme can introduce new branches onto glucose?
Which bonds are formed in the formation of new branches?
Transglycosylase. This forms alpha-1,6-glycosidic bonds to introduce branches at approximately every 10 glucose residues.
Which enzyme is the rate limiting enzyme in the synthesis of glycogen?
It is catalysed by glycogen phosphorylase. This frees glucose by phosphorlysis. It cleaves a glucose off the end of glycogen and adds a phosphate to the glucose to create glucose-1-phosphate. Phosphoglucomutase then transfers the phosphate from carbon 1 to carbon 6 to form glucose-6-phosphate.
What is the rate limiting step in glycogenolysis?
The step carried out by glycogen phosphorylase- which cleaves off a glucose and adds a phosphate to it.
What happens to the glucose-6-phosphate produced by glycogenonlysis in the liver?
It is de-phosphorylated and the resulting glucose is released into the bloodstream.
What happens to the glucose-6-phosphate produced by glycogenolysis in skeletal muscle?
It cannot be dephosphorylated but is used to provide energy via glycolysis and the TCA cycle.
What can glycogen phosphorylase not do?
It cannot remove the branches- this requires additional enzymes.
Which transporter moves the glucose from the liver into the bloodstream?
What does insulin do?
It stimulates glycogen synthase and inhibits glycogen phosphorylase
What does glucagon do?
It stimulates glycogen phosphorylase and inhibits glycogen synthase
Name two other hormones which stimulates glycogen phosphorylase?