T2 L7: Glycogen metabolism in muscle and liver Flashcards
Where is most glucose stored in the body and why?
In the muscles because there is more muscle on the body that liver
How much glycogen does the liver store?
Less than 24 hours worth
Describe the structure of stored glucose in the body
Highly branched polysaccharide consisting of (α-1,4) linked glucose molecules with a (α-1,6) branch every 8-14 glucose residues
What bonds are found between branches of a glycogen molecule?
α-1,6 bonds
How is glycogen broken down is muscles?
Gluc-6-phosphate is broken down into pyruvate and then into lactate and CO2
How is glycogen broken down in the liver?
Gluc-6-phosphate is broken down into glucose
Why can’t glucose be broken down straight into glucose in muscles?
Because muscle cells don’t have the enzyme glucose-6-phosphotase
Which enzyme breaks the α-1,4 of glycogen and what is this process called?
The enzyme Glycogen Phosphorylase breaks then by phosphorolysis . It can only break α-1,4 bonds
Describe the process by which α-1,4 bonds are broken
Glycogen Phosphorylase removes single units from non-reducing ends of glycogen to form Glucose-1-Phosphate. ATP is not involved
What is meant by phosphorolysis being analogous to hydrolysis?
The phosphate acts like water would in a hydrolysis reaction
Which enzyme breaks the α-1,6 bonds in a glycogen molecule?
α-1.6-Glucosidase activity of the debranching enzyme. by hydrolysis After the debranching enzyme has moved all the glucoses attached and only a single one remains
What is the importance of the debranching enzyme in glycogen degradation?
It’s a transferase enzyme that removes 3 residues from the branch of glycogen and transfers them to the end of a chain in a α-1,4 linkage so the phosphorylase enzyme can break them up.
Why can’t glycogen broken down in the muscles be exported in blood?
Because muscles lack the enzyme glucose-6-phosphotase so it can’t leave
What is uridine triphosphate (UTP)?
A molecule energetically equivalent to ATP
What is UDP-Glucose?
A high energy form of glucose
Describe the process that converts glucose into glycogen
Glucose is converted into glucose-6-phosphate by Hexokinase/Glucokinase (liver) using ATP. It’s then converted into glucose-1-phosphate and then UTP is added to produce UDP-Glucose. Glycogen synthase then converts it into Glycogen
What does Glycogen synthase do?
It adds glucose units by α-1,4 linkage onto glycogen but only when there are more than 4 glycosyl residues
Which protein carries out the priming function for glycogen synthesis?
Glycogenin. It remains in the centre of a glycogen molecule
Describe the priming function for glycogen synthesis
UDP-glucose donates the first glycosyl residue and attaches it onto the amino acid tyrosine in glycogenin
Which enzyme transfers a block of 7 residues from a growing glycogen chain to create a branch with a α-1,6 bond?
Branching enzyme
A new glycogen branch can form within how many residues of a pre-existing branch? Why?
4 because the enzyme can’t access the residues otherwise
Why is the sensitivity of phosphorylase and glycogen synthase beneficial?
They are very sensitive to hormones, stress, and muscle contraction so they can quickly respond to the metabolic needs of a cell
What is 1 negative of storing glycogen?
It’s hydrophilic so it associates with water therefore a lot of water is also stored
What is meant by allosteric regulation?
Changes in enzyme activity by molecules present in the cell
