T2 L6 Glycogen metabolism in muscle & liver Flashcards
Describe glycogen
Polysaccharide storage form of glucose in the body
Stored in granules in liver & muscle as an energy reserve
Formed from dietary glucose by glycogenesis
How is glycogen degraded?
By glycogenolysis pathway to produce glucose-1-phosphate
Glucose-1-phosphate is converted to free glucose & exported into the bloodstream to maintain plasma glucose levels.
What is the structure of glycogen?
Forms granules within cells
Highly branched polysaccharide of glucose
Consists of alpha-1,4 linked glucose molecules with an alpha-1,6 branch every 8-14 residues. This provides a large number of ends for phosphorylase & glycogen synthase to act on to ensure rapid breakdown & resynthesis.
How much does glycogen storage weigh?
In the fed state glycogen makes up 10% of the weight of the liver & 2% of the weight of muscle
40% of human body weight is muscle
2.5% of human body weight is liver
Liver contains less glycogen than is required to sustain glucose metabolism for 24 hours, therefore de novo synthesis is required by gluconeogenesis
What is the equation for glycogen breakdown?
Glucose –> glucose-1-phosphate glucose-6-phosphate
Phosphate group is either on carbon 6 or carbon 1. G-1-P is used to build up glycogen molecules
What does phosphoglucomutase do?
It catalyses the reversible conversion of glucose-1-phosphate to glucose-6-phosphate
What happens to glycogen stores between meals?
They fall as glycogen is released from liver glycogen to stabilise the concentration of glucose in the blood
Glycogen stores are mobilised overnight to help maintain blood glucose concentration
How are the alpha-1,4 linkages broken?
By phosphorolysis
Catalysed by glycogen phosphorylase enzyme
Removes single units from non-reducing ends of glycogen to form glucose-1-phosphate
ATP is not involved
What is the major enzyme for controlling glycogen breakdown?
Glycogen phosphorylase
Breaks bond between alpha-1,4 linked glucose residues by adding a phosphate to produce glucose-1-phosphate in a phosphorolysis reaction
What is glycogen formed by?
UDP glucose - high energy form of glucose
Consumption of UTP is energetically equivalent to ATP consumption
What does hexokinase catalyse?
The conversion of glucose to glucose-6-phosphate
What does phosphoglucomutase catalyse?
The conversion of glucose-6-phosphate to glucose-1-phosphate
What does glycogen synthase do?
Adds glucose units in an alpha-1,4 linkage onto the glycogen chain using UDP-glucose
What is the priming function carried out by?
The glycogenin protein as glycogenin can accept the first glucose residue & doesn’t dissociate
How are branches formed?
A branching enzyme transfers a block of 7 residues from a growing chain to create a new branch with an alpha-1,6 linkage.
The new branch can’t be within 4 residues of a pre-existing branch due to conformation
