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1. Many enzymes are regulated by both allosteric factors and by phosphorylation. Pleaseexplain briefly:o what pbospborylation is. why it is an effective method of controlling enzymatic activity from a structural

standpoint. why it is an effective method of contrclling enzymatic activity from a biological

perspectlve

* . key similarities and differences between the three main regulatory mechanisms thatwe have studied: allosteric effectors, selective proteolysis, and phosphorylation.

(15 points)

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2. In class, you received two handouts on the regulation of glycogen phosphorylaseactivity. One of these details the effects on the enzyme's activity of both allostericregulators and covalent regulation by phosphorylation, and the other outlines the web ofkinase and phosphatase reactions that ultimately determines the phosphorylation state ofthe enzyme. Please refer to these figures and then answer the following questions aboutglycogen phosphorylase.

a) Figure 10.20 on page 230 of your text book shows the relative activity of glycogenphosphorylase under various conditions. Please expiaino what the curves indicate (e.g., What is changing along each axis? What do the

different lines represent? How should the uninitiated interpret a high value versus aIow value?)

r how this figure demonstrates the response of the glycogen phosphorylase enzyme toboth intracellular and extracellular stimuli

r the biochemical logic that underlies these responses (i.e., why is it in the bestbiochemical interest of a cell or organism for this enzyme to respond in this manner).

Put simply, please explain what is shown by this figure and then explain why it makesgood sense from a biological perspective. In your answer, you may wish to refer to otherdiagrams or other information, and you should be sure to explain the logic of both theintracellular responsiveness and the extracellular responsiveness. ( 10 points). Jro 6*t sl^o.rs {'t. ^sye aci+,r\ (""r+nat axx) as q'fi.,.eliu- of [m,'p] (1.,-,2,*n)

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Question #2, continued...b) The complement of the glycogenolysis process catalyzed by glycogen phosphorylaseis glycogen synthesis. This process is facilitated by glycogen synthase. As shown on oneof your handouts and again on page 312 in your text, glycogen synthase is also regulatedboth by allosteric effectors and by phosphorylation. Please explain why it is logical thatboth insulin and glucose-6-phospfhate stimulare the activity of this enzyme. (5 points)

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