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Practice%20Problems%20No_2 - by protein phosphatase the...

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by protein phosphatase, the resulting de-phosphorylation further converts the T state of glycogen phosphorylase a to the T state of glycogen phosphorylase b, thus achieving the inhibition of the activity of glycogen phosphorylase. 8. There is a statement in the textbook, saying “under usual physiological conditions, the enzymatic activity of glycogen phosphorylase is essentially determined by its rates of modification and demodification”. What does this mean? (note: the modification and de- modification here refer to phosphorylation and de-phosphorylation of glycogen phosphorylase.) (Please also refer to Fig. 15-13.) Key: Under usual physiological conditions, i.e. under resting state, cellular [ATP] and/or [G6P] are high, whereas cellular [AMP] and [glucose] are low. Therefore, it is clear from the above keys to question 6 and 7 that, under resting state, the T conformational state of glycogen phosphorylase b (i.e. the un-phosphorylated enzyme species) represents the predominant inactive enzyme species, and the R conformational state of glycogen phosphorylase a (i.e. the phosphorylated enzyme species) represents the predominant active enzyme species. (note: G6P is also an allosteric inhibitor for glycogen phosphorylase.) 9. Based on what we have learned so far, please explain how an increased cellular [cAMP] promotes glycogen degradation. (Please also refer to Fig. 15-20 and 15-21.) Key : cAMP is able to bind to the two regulatory subunits of the tetrameric cAMP-dependent protein kinase (also known as protein kinase A or PKA).
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