Glycogen Phosphorylase Flashcards
What does glycogen phosphorylase catalyse?
The phosphorolytic cleavage of glycogen
What is the role of glycogen in the muscle?
- ATP needed for muscle contraction
- undergoes very large changes in ATP demand
- no gluconeogenesis
What regulates glycogen phosphorylase in the muscle?
It is regulated by AMP/ATP
What is the role of glycogen in the liver?
- maintains Glucose homeostasis in the blood for the whole body
What regulates glycogen phosphorylase in the liver?
glucose sensors
What is the active form of GP in the muscle?
GPa
- phosphorylated form of GP by phosphorylase kinase at Ser14
- equilibrium lies heavily on the R state side
- less sensitive to allosteric regulators
What does Liver GPa allosterically responsive to?
[Glucose] in the blood
What is the inactive form of GP in the muscle?
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)
Is Liver GPb activated by AMP like Muscle GPb is?
no
What is the structure of GP?
- large dimer
- multiple distinct site locations
- glycogen binding
- active site
- phosphorylation site
- allosteric site
Where does AMP interact with the GP dimer?
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
What happens when AMP binds to the GP dimer?
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
What are the differences between the active sites in the T and R state?
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
What occurs during the hormonal stimulation of GPb to GPa?
- 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
Which hormone triggers the hormonal stimulation of GPb to GPa?
adrenaline
Which form of GP is allosterically sensitive in the Liver?
GPa to glucose
What does the binding of glucose (when above 5mM in blood) do to GPa?
- 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
What does insulin do when high blood [glucose] is detected?
- activates protein phosphatase (PP1) which catalyses the de-phosphorylation of GP
