Phosphorylase b is maintain in an inactivated state by Insulin

Question 29

Phosphorylase b is maintain in an inactivated state by

a.       ATP

b.      cAMP

c.       Calcium

d.      Insulin

Answer :

d. Insulin

Reference:

Harper 27^th Edition Page 162, Page 163 Figure 19.6 and Lippincott 3^rd Edition Page 131. Figure 11.11

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Discussion

·        Phosphorylase is an enzyme give catalyses the following reaction

Glygogen(n+1) + Pi à Glycogen(n) + Glucose-1-Phosphate

·        Phosphorylase b is the inactive form and Phosphorylase a is the active form of the enzyme) The conversion of inactive phosphorylase to active phosphorylase (the reaction given below) is catalysed by phosphorylase kinase

Phosphorylase b + ATP à Phosphorylase a + ADP

·        Increasing the concentration of cAMP activates cAMP-dependant protein kinase which catalyses the phosphorylation by ATP of inactive phosphorylase kinase to active phosphorylase kinase

Phosphorylase kinase b + ATP à Phosphorylase kinase a + ADP

Explanation

·        We can see that the following factors active Phosphorylase b into phosphorylase a

·        ATP – Needed for phosphorylation of Phosphorylase b as well as phosphorylase kinase b

·        cAMP- Activates inactive cAMP depended protein kinase to active cAMP depended protein kinase so that phosphorylase kinase b is activated to phosphorylase kinase a

·        The conversion of Phosphorylase b to Phosphorylase (ie activation) in inhibited by

1.      G6P

2.      Insulin by virtue of it increasing the concentration of G-6-P

Comments:

If you are still not convinced, please read the table below

Action	Enzyme	Activated form	Inactivated form	Enzyme Activated from inactive form to active form by	Other factors needed
	cAMP-dependant protein kinase	inactive cAMP depended protein kinase	active cAMP depended protein kinase	cAMP
	Phosphorylase kinase	Phosphorylase kinase b	Phosphorylase kinase b	activates cAMP-dependant protein kinase	ATP Calcium Calmodulin component of phosphorylase kinase
Glycogenolysis	Phosphorylase a	Phosphorylase a	Phosphorylase b	Phosphorylase kinase a	ATP

Tips

What will you do if you have AMP instead of cAMP as one of the choices in another exam. Well there is nothing to worry as “in muscle under extreme conditions of anoxia and depletion of ATP, AMP activated glycogen phosphorylase b without it being phosphorylated