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Monday, December 15, 2008

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 27th Edition Page 162, Page 163 Figure 19.6 and Lippincott 3rd 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

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