Chem 481 Lecture 23: Lecture 23

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Washington University in St. Louis
University College - Chemistry
University College - Chemistry Chem 481
Patti Gary

PLP = Vitamin B6 Glycogenolysis: 3 enzymes: Processive: Glycogen Storage Site: Glycogen Active Site: Phosphorylase = Glycogen Phosphorylase Removes 6 glucose molecules (uses 6 phosphates to make 6 G1-P) Leaves 4 glucose molecules at the end (sterics block it). Stops 4 glucoses away from branch point “limit dextrin” Debranching enzyme: bifunctional Having both functions speeds up the enzyme activity. Transferase activity Moves 3 of the glucose molecules to another branch. α(1->4) bond ΔG^o’ = -15.5 kJ/mol α (1->6) bond ΔG^o’ = -7.1 kJ/mol Converting α(1->4) bonds to α(1->4) bonds requires essentially no energy, so it is thermodynamically a very favorable process. However, moving the last glucose which is an α (1->6) bond to a α (1->4) bond would be moving uphill in energy, so it is energetically more favorable to simply hydrolyze the last glucose which is thermodynamically favorable. Hydrolase activity Hydrolyzes the last glucose which is an α (1->6) bond. Thermodynamically favorable reaction. G1-P needs to be converted into G6-P for glycolysis. Phosphoglucomutase: Converts G1P G6P Makes G1P if making glycogen. Going from G1P to G6P: Phosphate starts out being attached to a serine. G1P’s C1’s -OH group attacks the phosphate. Forms Glucose-1,6-bisphosphate (G1,6P) The hydroxyl group on the Serine then attacks the phosphate attached to C6. G6P produced. Note: Phosphoglucokinase: G1P + ATP -> ADP + G-1,6-BP Used if the G1P were to fall off like the other mutase enzymes. Allows us to start 1 step further in glycolysis (stores energy) McArdle’s Disease: Defective glycogen phosphorylase. Cannot breakdown glycogen. Patient’s with this disease have a buildup of glycogen. Therefore, glycogenesis and glycogenolysis use different enzymes. Glycogenesis (making glycogen): Uses Different Enzymes: Glucose -(hexokinase)> G6P -(Phosphoglucomutase)> G1P + UTP-(UDP-glucose pyrophosphorylase)>UDP-glucose + PPi -(pyrophosphatase)> 2 Pi (irreversible) UTP is an ATP equivalent. PPi = pyrophosphate Almost immediately becomes 2 Pi Catalyzed by phosphatase (said to be omnipresent). Lactate -(gluconeogenesis)> G6P (mainly in liver) Last step: UDP-Glucose -> UDP Glucose_n -> glucose_{n+1} is glycogen being extended. UDP-Glucose Pyrophosphorylase: G1P attacks the α-phosphate of UTP which produces the PPi leaving group. Produces UDP-Glucose Reaction occurs because UMP is a good leaving group. Helps dr
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