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28 - Monday November 1 2010 Lecture 28 Announcements 1 Quiz...

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Monday, November 1, 2010 Lecture 28 Announcements 1. Quiz 7 this Wednesday 11/3 (no PyMol). Quiz 8 will be Wednesday, 11/10. Quiz 9 will be on 11/17. The last quiz, #10, will be on 12/1. 2. This week's assignments as in the LG, but also one problem from last week, Ch.15 #4. Also, do PyMOL: #8, phosphofructokinase 3. No lecture on Nov. 24, the day of the start of the Thanksgiving Recess. Friday's lecture: .   control of glycolysis at the steps of large negative G .   rxn mechanism of GAPDH .   glycogen phosphorylase and hormone amplification principles Today's lecture: How is GP activity controlled? P. 199 GP exists in two IV structures, R and T. .   The T state is inactive, largely because substrate binding is blocked .   The R state is active, with substrate access unblocked. The MAIN switch between R and T states is the phosphorylation state of serine- 14 (we'll see why soon). Without a phosphate on Ser-14, the enzyme is called GP-b; with a phosphate, GP-a (top of page 199). - But very high [glucose] can switch GP-a into the T state - And very high [AMP] together with low [G-6-P] can switch GP-b into the R state. p. 200 How does GP work? GP is a dimer of identical 841 AA, 98 kD subunits. Each subunit has the following sites: 1. Glycogen granule fits the "glycogen storage" binding site, i.e. the GP enzyme fits the surface of a glycogen granule; 2. Substrates: The glycogen polymer α - D -Glc-(1 4)- α - D -Glc, together with a P i fit into the active site; 3. Ser-14 is the site of covalent modification (hence binding by a kinase); 4. Glucose can also fit into the substrate site; 5. AMP, ATP, and G-6-P each can fit into the allosteric binding site (but only one at a time!). The structure of the T-state has the “tunnel to the active site” blocked.
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In the R-state the two subunits of the dimer are linked in a different IV structure from the T-state. In the R-state, either (i) Ser-14 bridges the subunits with an ion pair to Arg-69 from one subunit, Arg-43' of the other subunit; or else (ii) AMP in its site bridges the subunits with several tight interactions. (In the LG, the prime on a residue indicates the other subunit).
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