Lehninger Principles of Biochemistry 5/e

Important

The 2nd Mid-Term Exam Is OnNovember17th (Monday), 2014

In Remsen 017 at 5.00 PM

1CHAPTER 15

Glycogen Metabolism&Metabolic Regulation 2

Glycogen Granules in a Hepatocyte.- Glycogen, a storage form of carbohydrate3FIGURE 15-24 Glycogen granules in a hepatocyte. Glycogen, a storage form of carbohydrate, appears as electron-dense particles, often in aggregates or rosettes. In hepatocytes glycogen is closely associated with tubules of the smooth endoplasmic reticulum. Many mitochondria are also evident in this micrograph.

Recall Glucose concentration in blood is held nearly constant to 5 mM.

2-D cross-sectional view of glycogen. A core protein of glycogenin is surroundedby branches of glucose units. The entireglobular granule may contain ~30,000 glucose unitsA view of the atomic structure of asingle branched strand of glucoseunits in a glycogen molecule.Recall in Glycogen, glucose molecules are linked together linearly by (14) glycosidic bonds from one glucose to the next. Branches are linked to the chains they are branching off from by (16) glycosidic bonds every 8-12 residues.- In vertebrates, glycogen is found primarily in the liver (10% of liver mass) and skeletal muscle (1-2% of muscles mass). 4Metabolism of Tissue Glycogen is RegulatedDigestive breakdown is Unregulated - nearly 100% of ingested food is absorbed and metabolizedBut tissue glycogen is an important energy reservoir - its breakdown is carefully controlled Glycogen consists of "granules" of high MW Glycogen phosphorylase cleaves glucose from the nonreducing ends of glycogen molecules This is a phosphorolysis, not a hydrolysis Metabolic advantage: product is a sugar-P - a potential glycolysis substrate

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Removal of a glucose residue from the nonreducing endof a glycogen chain by glycogen phosphorylase. This process is repetitive; the enzyme removes successiveglucose residues until it reaches the fourth glucose unitfrom a branch point(Pyridoxal phosphate is a cofactor) 6FIGURE 15-25 Removal of a glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase. This process is repetitive; the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point (see Figure 15-26).

Glycogen phosphorylase utilize a pyridoxal phosphate as a cofactor where its phosphate group acts as a general acid catalyst, promoting the attack by pi to glycosidic bond.

Glycogen Breakdown Near An (16) Branch PointOligo (1->6 to (1->4) Glucan Transferase7FIGURE 15-26 Glycogen breakdown near an (16) branch point. Following sequential removal of terminal glucose residues by glycogen phosphorylase (see Figure 15-25), glucose residues near a branch are removed in a two-step process that requires a bifunctional debranching enzyme. First, the transferase activity of the enzyme shifts a block of three glucose residues from the branch to a nearby nonreducing end, to which they are reattached in (14) linkage. The single glucose residue remaining at the branch point, in (16) linkage, is then released as free glucose by the debranching enzyme's (16) glucosidase activity. The glucose residues are shown in shorthand form, which omits the H, OH, and CH2OH groups from the pyranose rings.

Glucose-1-Phosphate Can Enter Glycolysis or, In Liver ReplenishBlood Glucose8FIGURE 15-27 Reaction catalyzed by phosphoglucomutase. The reaction begins with the enzyme phosphorylated on a Ser residue. In step 1, the enzyme donates its phosphoryl group (green) to glucose 1-phosphate, producing glucose 1,6-bisphosphate. In step 2, the phosphoryl group at C-1 of glucose 1,6-bisphosphate (red) is transferred back to the enzyme, re-forming the phosphoenzyme and producing glucose 6-phosphate.

In Liver and Kidneys ER Exists Glucose-6-Phosphatase..Skeletal muscle and adipose tissues lack this G-6-Phosphatase system9FIGURE 15-28 Hydrolysis of glucose 6-phosphate by glucose 6-phosphatase of the ER. The catalytic site of glucose 6-phosphatase faces the lumen of the ER. A glucose 6-phosphate (G6P) transporter (T1) carries the substrate from the cytosol to the lumen, and the products glucose and Pi pass to the cytosol on specific transporters (T2 and T3). Glucose leaves the cell via the GLUT2 transporter in the plasma membrane.In liver and kidneys ER only exists Glucose-6-phosphatase system (but NOT in other tissues, such as skeletal muscle and adipose tissues so glycolysis can continue in these cells). So the supply of glucose to blood is done effectively. How Is Glycogen Synthesized?Glucose units are activated for transfer by formation of sugar nucleotides Luis Leloir showed in the 1950s that glycogen synthesis depends on Sugar Nucleotides UDP-Glucose Pyrophosphorylase catalyzes a phosphoanhydride exchange driven by pyrophosphate hydrolysisGlycogen synthesis takes place in virtually all animal tissues to some extent, but is especially PROMINENT in the liver and skeletal muscles. 10Glycogen synthesis takes place in virtually all animal tissues but is especially prominent in the liver and skeletal muscles. How Is Glycogen Synthesized?

UDP-glucose is one of the sugar nucleotides. Discovered by Luis Leloir in the 1950s, they are activated forms of sugar.11

The mechanism of the UDP-Glucose Pyrophosphorylase reaction. Attack by a phosphate oxygen of glucose-1-P on the -phosphorus of UTP is followed by departure of the pyrophosphate anion.12Glycogen Synthase Catalyzes Formation of (14) Glycosidic Bonds in GlycogenForms -(1 4) glycosidic bonds in glycogen The very large glycogen particle is built around a single protein, glycogenin, at the coreThe first (and 7 more) glucose is linked to a tyrosine -OH on the protein (by Glycogenins Glucosyl-transferase activity)Sugar units are then added by the action of glycogen synthaseGlycogen synthase transfers glucosyl units from UDP-glucose to C-4 hydroxyl at a nonreducing end of a glycogen strand.Note the oxonium ion intermediate13

Figure. The Glycogen Synthase Reaction.14Glycogen synthase can NOT make alpha 1->6 linkages. Another enzyme is needed for that (amylo (1->4) to (1->6) transglycosylase). Advanced Glycation End Products A Serious Complication of DiabetesSugars can react nonenzymatically with proteinsThe C-1 carbonyl groups of glucose form Schiff bases linkages with lysine side chains of proteinsThese Schiff base adducts undergo Amadori rearrangements and subsequent oxidations to form irreversible glycation end products (AGEs)AGEs are implicated in circulation, joint, and vision problems in diabeticsMeasurement of glycated hemoglobin is a better diagnostic yardstick for type-2 diabetes than serum glucose levels15

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Glycogen Synthesis Requires PrimedInitial Sugar Made By Glycogenin17FIGURE 15-33a Glycogenin and the structure of the glycogen particle. (a) Glycogenin catalyzes two distinct reactions. Initial attack by the hydroxyl group of Tyr194 on C-1 of the glucosyl moiety of UDP-glucose results in a glucosylated Tyr residue. The C-1 of another UDP-glucose molecule is now attacked by the C-4 hydroxyl group of the terminal glucose, and this sequence repeats to form a nascent glycogen molecule of eight glucose residues attached by (14) glycosidic linkages.- Glycogen synthase can NOT make alpha 1->6 linkages. Another enzyme is needed for that (amylo (1->4) to (1->6) transglycosylase).

18FIGURE 15-33b Glycogenin and the structure of the glycogen particle. (b) Structure of the glycogen particle. Starting at a central glycogenin molecule, glycogen chains (12 to 14 residues) extend in tiers. Inner chains have two (16) branches each. Chains in the outer tier are unbranched. There are 12 tiers in a mature glycogen particle (only 5 are shown here), consisting of about 55,000 glucose residues in a molecule of about 21 nm diameter and Mr ~1 107.Note that highly branched glycogen is more soluble than unbranched glycogen. In addition, both glycogen synthase and glycogen phosphorylase act at the nonreducing ends of glycogen chains. Branched glycogen has far more ends for these enzymes to work on than would the equivalent amount of linear glycogen chains. Having more ends effectively increases the concentration of substrate for the enzymes, thereby increasing the rate of glycogen synthesis and breakdown.

- Glycogen synthase can NOT make alpha 1->6 linkages. Another enzyme is needed for that (amylo (1->4) to (1->6) transglycosylase). Further note that branching in glycogen creates more opportunity to provide non-reducing end to glycogen phosphorylase. How Is Glycogen Metabolism Controlled? A highly regulated process, involving Reciprocal Control of Glycogen Phosphorylase (GP) and Glycogen Synthase (GS)GP allosterically activated by AMP, Glucogon (Liver), Epinephrine (Muscle), Ca++, and inhibited by ATP, glucose-6-P and caffeine GS is stimulated by glucose-6-P Both enzymes are regulated by covalent modifications - phosphorylation19

Regulation of Muscle Glycogen Phosphorylase(GP) by Covalent ModificationPP1 Inactivates Glycogen Phosphorylase

Phosphoprotein phosphatase 1 (PP1)20FIGURE 15-34 Regulation of muscle glycogen phosphorylase by covalent modification. In the more active form of the enzyme, phosphorylase a, Ser14 residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active form, phosphorylase b, by enzymatic loss of these phosphoryl groups, catalyzed by phosphorylase a phosphatase (also known as phosphoprotein phosphatase 1, PP1). Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase b kinase. (See also Figure 6-36 on glycogen phosphorylase regulation.)Glycogen Synthase is Regulated by Covalent ModificationGlycogen Synthase exists in two distinct formsActive, Dephosphorylated GS-ILESS active, Phosphorylated GS-DThe phosphorylated form is allosterically activated by glucose-6-PAt least 9 serine residues are phosphorylated4 different protein kinases are involvedDephosphorylation is carried out by phosphoprotein phosphatase-1 (PP1)PP1 Activates Glycogen Synthase21Insulin Modulates the Action of Glycogen Synthase in Several WaysBinding of insulin to plasma membrane receptors in the liver and muscles triggers protein kinase cascades that stimulate glycogen synthesisInsulins effect include stimulation of lipid synthesis, glycogen synthesis, protein synthesis, glycolysis, and active transport, AND Inhibition of gluconeogenesis and lipid breakdownGlucose uptake provides substrate for Glycogen Synthesis and Glucose-6-P, which allosterically activates the otherwise inactive form of glycogen synthase

22Insulin Modulates the Action of Glycogen Synthase in Several Ways

Figure. Insulin triggers protein kinases that stimulate glycogen synthesis23

Again The Path from Insulin to GSK3 and Glycogen Synthase24FIGURE 15-39 The path from insulin to GSK3 and glycogen synthase. Insulin binding to its receptor activates a tyrosine protein kinase in the receptor, which phosphorylates insulin receptor substrate-1 (IRS-1). The phosphotyrosine in this protein is then bound by phosphatidylinositol 3-kinase (PI-3K), which converts phosphatidylinositol 4,5-bisphosphate (PIP2) in the membrane to phosphatidylinositol 3,4,5-trisphosphate (PIP3). A protein kinase (PDK-1) that is activated when bound to PIP3 activates a second protein kinase (PKB), which phosphorylates glycogen synthase kinase 3 (GSK3) in its pseudosubstrate region, inactivating it by the mechanisms shown in Figure 15-38b. The inactivation of GSK3 allows phosphoprotein phosphatase 1 (PP1) to dephosphorylate and thus activate glycogen synthase. In this way, insulin stimulates glycogen synthesis. (See Figure 12-16 for more details on insulin action.)The Actions of Insulin on Metabolism

Figure. The metabolic effects of insulin are mediated through protein phosphorylation and second messenger modulation.25So Hormones Regulate Glycogen Synthesis and DegradationStorage and utilization of tissue glycogen and other aspects of metabolism are regulated by hormones, including glucagon, epinephrine, and the glucocorticoids (steroid hormone)Insulin is a response to increased blood glucoseInsulin triggers Glycogen Synthesis when blood glucose risesBetween meals, blood glucose is 70-90 mg/dLGlucose rises to 150 mg/dL after a meal and then returns to normal within 2-3 hoursGlucagon and Epinephrine stimulate Glycogen Breakdown

26Epinephrine (ep-uh-nef-rin, -reen) is also known as adrenaline. It is a hormone that is secreted by the adrenal glands. Epinephrine is involved in the fight or flight response in humans. The fight or flight response occurs when a person is subject to a threat. This causes a signalling process to occur, which causes the body to react to the potential danger.Specifically, once a threat is perceived, a signal is sent to the brain. The brain then sends nerve impulses to the adrenal gland in the kidneys. When the nerve signal reaches the adrenal gland, chromaffin cells, in the medulla of the adrenal gland, release epinephrine.

Glucagon has a major role in maintaining normal concentrations of glucose in blood, and is often described as having the opposite effect of insulin. That is, glucagon has the effect of increasing blood glucose levels. Glucagon is a linear peptide of 29 amino acids. Glucagon is synthesized as proglucagon and proteolytically processed to yield glucagon within alpha cells of the pancreatic islets.Glucagon stimulates breakdown of glycogen stored in the liver AND Glucagon activates hepatic gluconeogenesis.Hormones Regulate Glycogen Synthesis and Degradation

Figure. The portal vein system carries pancreatic secretions such as INSULIN and GLUCAGON to the liver.27Insulin 2-Ways Action: Stimulating Glycogen Synthesis and Inhibiting Glycogen Breakdown

Glucagon and Epinephrine Stimulate Glycogen Breakdown and Inhibit Glycogen SynthesisGlucagon and epinephrine stimulate glycogen breakdown the opposite effect of insulinGlucagon (a 29-residue peptide), is secreted by pancreas alpha cellsGlucagon acts in LIVER and ADIPOSE tissue onlyEpinephrine (adrenaline) is released from adrenal glands Epinephrine acts on LIVER and MUSCLESWhen either hormone binds to its receptor on the outside surface of the cell membrane, a phosphorylase cascade amplifies the signal28

Glucagon and Epinephrine Activate a cascade of reactions that Stimulate Glycogen Breakdown and Inhibit Glycogen Synthesis in liver and muscles, respectively.29Adenylyl cyclase will catalyze the conversion of ATP to cAMP. Epinephrine and GlucagonThe difference:Both are Glycogenolytic in Liver, but for different reasonsEpinephrine is the fight or flight hormoneIt rapidly mobilizes large amounts of energy Glucagon is for long-term maintenance of steady-state levels of glucose in the bloodIt activates glycogen breakdownIt activates liver gluconeogenesis 30

Regulation of CarbohydrateMetabolism inthe Liver: 31FIGURE 15-41 Regulation of carbohydrate metabolism in the liver. Arrows indicate causal relationships between the changes they connect. For example, an arrow from A to B means that a decrease in A causes an increase in B. Pink arrows connect events that result from high blood glucose; blue arrows connect events that result from low blood glucose.

Recall PFK-1 catalyzes the important "committed" step of glycolysis, the conversion of fructose 6-phosphate and ATP to fructose 1,6-bisphosphate and ADP.- Recal that GLUT2 is the name of the transporter of ER of Liver/kidney involved in releasing Glucose (when sunthesized by Gluconeogenesis)

Difference in the Regulation of CarbohydrateMetabolism in Liver and Muscle32FIGURE 15-42 Difference in the regulation of carbohydrate metabolism in liver and muscle. In liver, either glucagon (indicating low blood glucose) or epinephrine (signaling the need to fight or flee) has the effect of maximizing the output of glucose into the bloodstream. In muscle, epinephrine increases glycogen breakdown and glycolysis, which together provide fuel to produce the ATP needed for muscle contractionSuggested Problems3, 4, 5, 6, 7, 8, 10, 12, 13Hormones Regulate Glycogen Synthesis and DegradationInsulin is secreted from the pancreas (to liver) in response to an increase in blood glucose Note that the portal vein is the only vein in the body that feeds an organInsulin acts to lower blood glucose rapidly in several ways, stimulating Glycogen Synthesis and Inhibiting Glycogen Breakdown 34