2/22/151Biochemistry401Lecture 23 Beginning Glycogen Metabolism Glycogen DegradationMobilization of glucose Glycogen synthesisFormation of the glucose storage unitGlycogen is a highly branched polymer of glucose. 1) Straight chains: α-1,4 linkage 2) Branch points: α-1,6 linkage ~ every 10 units. 3) Initiator primer molecule: glycogenin Non-reducing ends G Primary storage sites are skeletal muscle and liver. Skeletal muscle cells contain glycogen for their own needs (2% by wt.). Liver cells contain glycogen primarily for the needs of the body as a whole (10% by wt.). Glycogen is stored as dense granules in the cytoplasm. Why glycogen? Why not Fatty Acids? Glucose is sole RBC and primary Brain fuel Utilization of glucose can proceed under anaerobic conditions. (Glycolysis) Glycogen has many branch-points for rapid, regulated mobilization.2/22/152• Linear: α-1,4 linkage • Branch-points: α-1,6 linkage Straight Chain Phosphorolysis Glycogen free ends are broken down releasing Glucose1-P. Enzyme: Glycogen phosphorylase Glucose 1-P is transformed to glycolytically usable glucose 6-P. Enzyme: Phosphoglucomutase Three fates of Glucose 6-P 1) Glycolysis 2) Free glucose (Liver, kidneys) 3) Pentose phosphate pathway to make NADPH Fate depends on: cell type current metabolic needs.2/22/153Glycogen Phosphorylase Cleaves glucose through addition of inorganic phosphate. Can catalyze phosphorolysis of the straight chain to within 4 glucosyl units of a branch point. Yields: α-Glucose 1-P monomers Energetically favorable: Released glucose is phosphorylated. Also favorable because this intermediate cannot diffuse out of cell. Glycogen Phosphorylase overview: Dimeric enzyme that cleaves α - 1,4 glycosidic bond between C1 and the glycosidic oxygen, through phosphate addition. Cofactor: Pyridoxal Phosphate It is important to make sure that this is a phosphorylytic not a hydrolytic reaction. A loss of the phosphate must not occur. Must exclude water from this reaction.2/22/154Glycogen Phosphorylase: Allosterically regulated homodimer. One subunit is shown in yellow, and the other is in white. Pyridoxal Phosphate is shown with Pi and glycogen in the active site. Excludes water. The enzyme is processive and can cleave many residues without leaving the substrate. Pyridoxal Phosphate, a B vitamin derivative, is a cofactor that is covalently linked to a lysine in the enzyme through Schiff-base linkage. This cofactor acts as an acid-base catalyst. Acid (donor) Base (acceptor) Glycogen Phosphorylase mechanism Acid (Proton Donor) Base (Proton Acceptor) Two step reaction keeps the alpha configuration. Inorganic phosphate donates a proton to the C4 oxygen, while accepting a proton from PLP. Pi attacks the carbocation and donates a proton back to PLP. Attack of Pi on the carbocation Donation of a proton to C4 Oxygen2/22/155Phosphoglucomutase catalyzes the transformation of glucose 1-P to glucose 6-P for use in glycolysis. In muscle cells: Phosphate keeps glucose in the cell for the muscle’s own energy needs. In Liver and Kidney cells: The phosphate is removed from C6 by hydrolysis in a reaction catalyzed by Glucose 6-phosphatase. This enzyme also functions during gluconeogenesis and is found in the ER . Free glucose is released to meet the metabolic needs of other tissues. Glycogen phosphorylase cannot catalyze the breakdown of α-1,6 linked glucose at the branch points or any of the four α-1,4 glucose units terminal to the α-1,6 glycosidic bond. Two additional enzyme functions, transferase and α-1,6 glucosidase, are supplied by the Debranching enzyme, to release these monomers. Hexokinase in muscles phosphorylates free glucose. hexokinse P The Problem of Branches hexokinse P Debranching Enzyme2/22/156α-1, 6-Glucosidase hydrolyses the α-1, 6 glycosidic bond to release free glucose. Glycogen Synthesis Glycogen + UDP - Glucose > Glycogenn+1 + UDP Activated glucose: UDP-G UDP-Glucose Pyrophosphorylase Activating Glucose This reaction is driven forward by the hydrolysis of PPi.2/22/157Glycogen Synthesis Glycogen + UDP - Glucose > Glycogenn+1 + UDP Activated glucose: UDP-G Glycogen synthase Glycosyl unit of UDP-glucose is transferred to C4 of the growing chain - must have at least 4 residues already there. A primer is needed to start a new molecule . Adding Activated Glucose Glycogenin, a dimeric protein with α-1,4 linked oligosaccharides, is used as a primer to initiate synthesis of a new glycogen molecule2/22/158Branching Enzyme Forms α-1,6 linkages. 1) Breaking an α-1,4 linkage 2) Formation of an α-1,6 linkage A block of 7 residues (usually) is transferred to a more interior location. The block is transferred to a branch containing 3 or more residues. Why branching? Increases free ends. Increases solubility. This is the end of Lecture 23. !Have a great
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