31Glycogen granules (red) associate with smooth ERIntegral membrane protein in the ERPathway for glucose export to the blood in the hepatocyteWhat is the importance of having glucose produced in the ER?After glucose-6-P is formed from glycogen hydrolysis via Phosphorylase, it can be used by the liver for Pentose phosphate pathway, glycolysis for energy, or most likely exported to the blood to maintain blood glucose concentration near 5 mM. The important function of the liver is to maintain blood glucose concentration at a constant concentration (through the help of insulin and glucagon—learn about later) because the brain depends on glucose for energy. This is the compartmentalization scheme that the liver has developed to export glucose to the blood stream. Note that this occurs on the smooth endoplasmic reticulum and glycogen particles are localized near the outside of the smooth ER In the cytosol. As Glucose-6-P is formed, it can be transported into the smooth ER via a transporter. IN the ER Lumen, Glucose-6-phosphatase (important in GNG and only present in liver and kidney—will learn about this shortly) converts Glucose-6-P to Glucose and Pi. THeglucose and Pi are then transported out of the ER via different transporters and Glucose is transported out of the liver to the blood via the Glut2 transporter.What is the importance of having glucose produced in the ER?The cell separates this reaction from the process of glycolysis and pentose P pathway. If Glucose-6-phosphatase would be in the cytosol like hexokinase/glucokinase, we could have a potential futile cycle!32How do other hexosesenter the glycolyticpathway?Fructose as F-6Por F-1-PGalactose as G-1-P Galactosemiadef. in UDPglc->gal-1-P uridylyltransferaseMannose as M6-P-->Fr-6 Ponto glycolysisepimerizationUDP functions as a coenzyme like carrier of hexose groups33Hydrolysis of Dietary CarbohydratesDextrin + nH20 n D-glucoseDextrinaseMaltose + H20 2 D-glucoseMaltaseLactose + H20 2 D-glucose + D-galactoseLactaseSucrose + H20 D-glucose + D-fructoseSucraseTrehalose + H20 2 D-glucoseTrehalase34Glycogen SynthesisDifferent pathway from BreakdownMcArdle’s Disease Glucose -1-P is activated by making UDP-Glucose(nucleotide sugars are biosynthetic precursors)Key reaction:1. Glucose-1-P + UTP UDP-Glucose + PPi∆G0’~02. PPi+ H2O2 Pi∆G0’-33.5 kJ/mol3. Glucose-1-P + UTP + H2O UDP-Glucose + 2 Pi∆G0’-33.5 kJ/molPyrophosphate hydrolysis is a common biosynthetic strategy.Enzymes: 1. UDP-Glucose pyrophosphorylase *key reaction in synthesis2. Inorganic pyrophosphatase** Cost -- 1 ATP equivalent for each glucose—1 high energy P-bondUDP glucose is the precursor (uridine diphosphate glucose)As glucose residues are added to glycogen, UDP-Glucose is the substrate and UDP is released as a reaction productNucleotide diphosphate sugars are precursors also for synthesis of other complex carbohydrates, including oligosaccharide chains of glycoproteins, etc.35Glycogen synthaseUDP-Glucose +glycogen (n residues)UDP + glycogen (n+1 residues)∆G0’ = -13.4 kJ/molPrimerWhere does the acceptor glycogen come from?glycogenin + UDP-GlucoseGlyogenin-Tyr-Glucose + UDP7 UDP-GlucoseGlycogenin (Glucose)8Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.36HOOHHOHHOHCH2OHHHCCHNHCH2OO154326HOOHHOHHOHCH2OHHHO154326P O P O UridineOO−OO−CCHNHCH2HOOtyrosine residue of Glycogenin O-linked glucose residue + UDP UDP-glucose A glycosidic bond is formed between the anomeric C1 carbon of the glucose moiety derived from UDP-glucose and the hydroxyl oxygen of the of a tyrosine side –chain of glycogenin. UDP is released as a product37Glycosylation at C4 of the O-linked disaccharide with alpha(1Æ4) glycosidiclinkage. UDP-glucose is again the glucose donor. This is repeated until a short linear glucose polymer with alpha(1Æ4) glycosidic linkages is built up on GlycogeninGlycogen synthase then catalyzes the elongation of glycogen chains initiated by GlycogeninQuestion: Where would you expect to find a Glycogenin in a cell? Answer: Some glycogenin is found associated with glycogen particles ( branched glycogen chains) in the cytoplasm, but some is freein the cytosol.Glycogen synthase catalyzes transfer of the glucose moiety of UDP-glucose at C4 of the terminal residue of a glycogen chain to form an alpha (1Æ4) glycosidiclinkage38Must be at least 11 glycosyl unitsTransfer 6 or 7 glucose residuesfrom the non-reducing endGlycogen branching enzyme =amylo-(1,4-->1,6)-transglycosylaseNew branch at least 4 units away form existing branchbranchingenzymeglycogen synthase substrate againA separate branching enzyme transfers a segment from the end of a glycogen chain to the C6 hydroxyl of a glucose residue of glycogen to yield a branch with an alpha (1Æ6) linkage.39Comparison of Glycogen synthesis to breakdown*Different reactions*Input of ATP*Recurring themeGlycogenUDP-GlucoseGlucose-1-PGlucose-6-PUTPdebranchingenzymeglycogen phosphorylasebranching enzymeglycogen synthasePiPPiUDP-glucosepyrophosphorylasephosphoglucomutaseUDP2Piinorganic pyrophosphorylaseBoth synthesis and breakdown of glycogen are spontaneousIf both pathways were active simultaneously in a cell, there would be a futile cycle, with cleavage of one ~P bond per cycle (in forming UDP-Glucose)To prevent a futile cycle, glycogen synthesis and glycogen synthase are reciprocally regulated by allosteric effectors and by phosphorylation (this will be covered later when we learn how the metabolic pathways are integrated.40pyruvate kinase∆G is -NADH “recycled”Used for 1,3 PG to Gly-3-Pmalate α kg shuttleglucose precursorsCO2addition to pyruvate: committed to GNG instead of TCAtransamination+ effectoranaplerotic OAA also to TCA cyclereduction malate DHmalate DH in bothGNG and TCA cyclepyruvate carboxylasePEP Carboxykinasemalate DH oxidBypass of pyruvate kinasepyruvate kinase catalyzes: in glycolysisPEP + ADP Æ Pyruvate + ATPFor byass of the pyruvate kinase reaction, cleavage of 2 ~P bonds in requireddeltaG for cleavage of one ~P of ATP is insufficient to drive synthesis of PEPPEP as a higher negative deltaG of phosphate hydrolysis than ATP2 steps, pyruvate carboxylase and PEP carboxykinase (See next slide)41GluconeogenesisPyruvate KinaseGNG occurs mainly in the liverSynthesis of glucose from pyruvate utilizes many of the same enzymes as GlycolysisThree glycolysis reactions have such a large