Glycogen Phosphorylase

Question 1

What does glycogen phosphorylase catalyse?

Answer 1

The phosphorolytic cleavage of glycogen

Question 2

What is the role of glycogen in the muscle?

Answer 2

• ATP needed for muscle contraction
• undergoes very large changes in ATP demand
• no gluconeogenesis

Question 3

What regulates glycogen phosphorylase in the muscle?

Answer 3

It is regulated by AMP/ATP

Question 4

What is the role of glycogen in the liver?

Answer 4

• maintains Glucose homeostasis in the blood for the whole body

Question 5

What regulates glycogen phosphorylase in the liver?

Answer 5

glucose sensors

Question 6

What is the active form of GP in the muscle?

Answer 6

GPa

• phosphorylated form of GP by phosphorylase kinase at Ser14
• equilibrium lies heavily on the R state side
• less sensitive to allosteric regulators

Question 7

What does Liver GPa allosterically responsive to?

Answer 7

[Glucose] in the blood

Question 8

What is the inactive form of GP in the muscle?

Answer 8

GPb

• dephosphorylated form by phosphorylase phosphatase
• equilibrium lies heavily on the T state side
• dependent on allosteric controls (inhibited by resting cellular ATP, G6P bind to T preventing unnecessary glycogenolysis)

Question 9

Is Liver GPb activated by AMP like Muscle GPb is?

Answer 9

no

Question 10

What is the structure of GP?

Answer 10

• large dimer
• multiple distinct site locations
• glycogen binding
• active site
• phosphorylation site
• allosteric site

Question 11

Where does AMP interact with the GP dimer?

Answer 11

AMP interacts with 3 parts of 1 subunit, and binds to the other subunit via its adenine, ribose and Pi moieties, linking the active site, interface and N-terminal regions

Question 12

What happens when AMP binds to the GP dimer?

Answer 12

AMP promotes T to R state transition by binding to R

• inducing conformational change at interface leading to long-range changes
• both active sites switch to become active
• tower helices tilt and pull apart, triggering T to R state transition
• tower motion also displaces a loop masking the active site
• induces rotation of an Arginine towards active site
• increased GP binding affinity for substrate phosphate

Question 13

What are the differences between the active sites in the T and R state?

Answer 13

T state:
- active site is malformed
- 280s loop (asparagine) masks substrate access to its binding site
R state:
- loop no longer masks access
- reorientated arginine increases binding of substrate orthophosphate

Question 14

What occurs during the hormonal stimulation of GPb to GPa?

Answer 14

• phosphorylation near the GP dimer interface greatly changes conformation of the first 20 a.a. near the N terminus (basic a.a)
• major tertiary and quaternary changes
• addition of phosphate to Ser14 moves 34A and disrupts electrostatic charge between the basic and acidic a.a
• subunit rotation 10 degrees
• active site is freed up
• changes from T to R state

Question 15

Which hormone triggers the hormonal stimulation of GPb to GPa?

Answer 15

adrenaline

Question 16

Which form of GP is allosterically sensitive in the Liver?

Answer 16

GPa to glucose

Question 17

What does the binding of glucose (when above 5mM in blood) do to GPa?

Answer 17

• shifts equilibrium to the T state
• glucose binds to an allosteric site per subunit on liver GPa inducing a conformational change
• exposes its phosphorylated serines to protein phosphatase
• cleaves phosphates
• inactivating the enzyme as converts GPa to GPb
• glycogen synthesis in liver increases

Question 18

What does insulin do when high blood [glucose] is detected?

Answer 18

• activates protein phosphatase (PP1) which catalyses the de-phosphorylation of GP