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Quiz 10 study guide: Glycogen Metabolism

by: Skyler Tuholski

Quiz 10 study guide: Glycogen Metabolism 3100

Marketplace > University of Georgia > BCMB > 3100 > Quiz 10 study guide Glycogen Metabolism
Skyler Tuholski
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Highlights the important lecture and book material!
Intro to Biochem and Molecular Bio
Wood & Sabatini
Study Guide
glycogen, glycogen production, pompe disease
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This 9 page Study Guide was uploaded by Skyler Tuholski on Thursday October 13, 2016. The Study Guide belongs to 3100 at University of Georgia taught by Wood & Sabatini in Fall 2016. Since its upload, it has received 59 views. For similar materials see Intro to Biochem and Molecular Bio in BCMB at University of Georgia.


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Date Created: 10/13/16
Quiz 10 Study Guide: Ch 24 + 25: Glycogen Metabolism Ch 24: Glycogen Metabolism Glyocgen: storage form of Glucose. Highly branched homopolymer of glucose present in all tissues  Largest stores of glycogen are in liver and muscle o Liver breaks down glycogen to glucose to provide energy for brain and red blood cells  Regulated by hormones glucagon, epinephrine, and insulin. Ex: Low glucose: glucagon is released, protein kinase A is stimulated, stimulate gluconeogenesis and breakdown of glycogen o Muscle glycogen provide energy for muscle contraction  Branched to make it more soluble and allows it to be synthesized and degraded more rapidly than linear molecule because more ends are exposed for enzymes to act on o Most glucose residues are linked by-1,4-glycosidic bonds, and branches are created by-1,6-glycosidic bonds. No cleavage occurs at branch points  The non-reducing ends of glycogen molecule form the surface of the glycogen granule: degradation takes places here. The core of the glycogen molecule is the protein glycogenin  The physical interaction between Glycogenin and Glycogen synthase limits size of granules Degradation by glycogen phosphorylase  Primary regulation: phosphorylation. If you affect to dimer, you affect the catalytic site. Phosphorylated: active. Dephosphorylated: less active.  Ultimate product will be glucose-6 phosphate  Glycogen phosphorylase cleaves -1,4-glycosidic bonds by the addition of Pi to yield glucose-1 phosphate: phosphorolysis reaction o Phosphoglucomutase converts glucose-1 phosphate to glucose 6-phosphate  If ATP or Glc-6-P are high, you inhibit glycogen phosphorylase. AMP stimulates.  Energetically advantageous rxn b/c the released sugar is already phosphorylated  Other cells use glycolysis directly and can take up more glucose from blood if it needs it. Why is the energy yield higher from glycogen than from free glucose? You need ATP to form glucose 6-phopshate from glucose, you don’t need ATP to make the glucose-6 phosphate from glycogen as an intermediate for glycolysis.  Liver: glucose is a negative regulator of liver phosphorylase o What if glucose conc. is low in blood? You want to stimulate the phosphorylase  Muscle: makes glc-6 phosphate to go into glycolysis for its own benefit from energy production Cleaving the branches of glycogen  About in 1 in 10 residues are branched, and -1,6-glycosidic bonds cannot be cleaved by phosphorylase  Two enzymes, transferase and -1,6-glucosidase (aka debranching enzyme) remodel the glycogen for continued degradation by phosphorylase  A transferase shifts a small oligosaccharide near the branch point to a nearby chain,  A debranching enzyme cleaves the α-1,6 bond at the branch point, releasing a free glucose Allosteric regulation of glycogen phosphorylase: differs between liver and muscle  Phosphorylase has 2 dimers: a and b. o Equilibrium for phosphorylase a favors the active R state o Equilibrium for phosphorylase b favors the less active T state  Muscle: it needs to sense the energy of your muscle. If ATP is low, it needs glucose to provide energy o Resting muscle: default form is T-state. Doesn’t need to break down glycogen, plenty of ATP (ATP is a negative regulator) o AMP shifts it to R-sate and active site is opened o ATP and glucose-6 phosphate stabilize the T-state (inhibit phosphorylase) o Glycogen is broken down to have glucose for itself, therefore is no regulation by glucose because it is not concerned with blood glucose levels  Liver: primary allosteric regulator is glucose o Default is in R-state: prepared to generate blood glucose unless signaled otherwise o Binding of glucose shifts to T-state: inactivates phosphorylase, glycogen is not degraded. o Also stimulates glycogen synthesis through activation of PP1 o when glc levels low: Glucose 6-phosphatase (unique to liver) cleaves phosphoryl group from glucose 6-phosphate to form free glucose that can be released from liver Insulin is released when glucose levels are high: stimulates glycolysis and glycogen synthesis  Activates a signal transduction pathway: phosphorylation and inactivation of glycogen synthase kinase. Prevents the phosphorylation of glycogen synthase. PP1 removes the phosphoryl groups from glycogen synthase, activating it and allowing glycogen synthesis.  Also increases number of glucose transporters (GLUT4) in plasma membrane Epinephrine (exercise condition) and Glucagon (fasting condition) trigger breakdown of glycogen  Epinephrine: released by adrenal gland in response to neural signal (fight or flight) You want your muscle and liver to start degrading glycogen to prepare for sudden energy response.  Glucagon or epinephrine bind to receptor and allows for transition to GTP. Adenylate cyclase is activated, which makes cyclic AMP- the messenger- which diffuses into the cell and activates protein kinase A, which activates phosphorylase kinase and then phosphorylation occurs to transition phosphorylase from T to R (active) state  G-protein binds GTP Recap: o Cyclic AMP activates protein kinase A, o Phosphorylates/ activates phosphorylase kinase o Phosphorylates/ activates glycogen phosphorylase o activating glycogen degradation.  Glycogen synthase- covalently modified, hormonal regulatory step. When you phosphorylate it- you inhibit it. This stimulates breakdown of glycogen so you can get more glucose since blood level is low. o Low sugar- pancreas releases glucagon, activates PKA, phosphorylates glycogen synthase which inhibits it, phosphorylates glycogen phosphorylase which activates it o PP-1 is the enzyme that reverses all of this. Activating PKA phosphorylates and deactivates everything down the line. It also inactivates PP-1 so it can’t be reversed Turning off Glycogen Breakdown Cascade:  When glucose needs are satisfied, phosphorylase kinase and glycogen phosphorylase are dephosphorylated by PP1 and inactivated  Also, glycogen synthase is dephosphorylated and activated: glycogen synthesis is activated Chapter 25: Glycogen Synthesis  Glycogen is synthesized by a pathway that uses uridine diphosphate glucose (UDP-glucose) as the activated glucose donor o UDP-glucose is synthesized by the reaction of glucose 1-phopsphate and uridine triphosphate (UTP) in a rxn catalyzed by UDP- glucose pyrophosphorylase Pyrophosphorylase : couples reactions to give you overall negative delta G. driven forward by hydrolysis of pyrophosphate Glycogen synthase catalyzes transfer of glucose from UDP-glucose to a growing chain  MAJOR regulated step o Key regulation is through phosphorylation: phosphorylating it deactivates it o Inhibited by glucagon and epinephrine  Catalyzes the transfer of glucose from UDP-glucose to a terminal hydroxyl group at C-4 of a glycogen chain to form an 1,4-glycosidic bond o Can only synthesize 1,4-linkages  Glycogen synthase can only add glucosyl residues to a chain already containing more than 4 residues  requires oligosaccharide of glucose residues as primer: glycogenin Branching enzyme forms 1,6-linkages  Glycogen synthase catalyzes only the synthesis of 1.4-linkages  Branching enzyme removes an oligosaccharide of approximately 7 resides from the nonreducing end and creates an internal 16-linkage o Non-reducing ends face outward on the glycogen granule o # of glycogenin molecules limits number of granules b/c glycogenin adds the initial glucose as primer for glycogen synthase  Branching increases solubility and rates of synthesis and degradation PP-1 is associated with phosphorylase a- it keeps it inactive. When this enzyme goes into the T- state due to high blood glucose, it releases PP1  PP1 shifts to glycogen synthesis mode. When the signal is gone (you are not making more epinephrine): o stop activating adenylate cyclase o stop making cyclic AMP o Protein phosphatase 1 reverses all the phosphorylations to end degradation and stimulate synthesis This will be a question: There is a lag because you don’t want both to be happening at the same time.  Glucose is added. Phosphorylase activity decreases o Phosphorylase has an allosteric site for glucose, so when glucose binds it shifts it to T- state o PP-1 is released and reverses phosphorylation events, activates glycogen synthase, phosphorylates phosphorylase to deactivate it o 10:1 ratio of phosphorylate to PP1 Insulin: primary signal for glycogen synthesis  Insulin binds to receptor and stimulates IRS. Phosphorylates IRS to activate protein kinases  You phosphorylate the glycogen synthase kinase, Leads to phosphorylation o glycogen synthase: inactivation  Increases glut-4 which increases transport of glucose through membrane. Overall goal is to synthesize glycogen Inherited enzyme deficiencies  Mutations that change enzyme function or abolish enzyme activity o Most are recessive since only one functional copy of the gene is needed for activity  Lactose Intolerance o Major source of energy for nursing animals, 20% of caloric intake of infants o You don’t have the enzyme lactase, so you can’t break down lactose o Decrease in enzyme over time until you are about 6 years old you only have 10% of it left functioning o Constant drinking of milk over adulthood led to a mutation that allowed lactase to constantly be produced mutation prevents people from down-regulating the enzyme as they age o If you are lactase deficient, bacteria in your colon utilize the lactose, ferment it to lactic acid, methane, and H2 gas. o Symptoms: Leads to bloating, gas, and diarrhea, and can hinder absorption of nutrients o Treatment: avoid dietary lactose or take lactase  Galactosemia o You lack enzyme to convert galactose to glucose 6-phosphate  Galactose 1-phosphate accumulates in liver  Symptoms: high galactose in blood and urine as well decreased liver function: jaundice. High galactose in eye leads to influx of water and results in cataracts  Failure to thrive, death  Glycogen Storage disease o All defects lead to glycogen accumulation o Type I (Von Gierk’s Disease) – glucose 6-phosphatase in liver  Liver cannot release glucose into the blood (see above notes on glc-6-Pase)  No response to epinephrine or glucagon  Low blood glucose between meals  Increased [Glc-6-P]  increases glycolysis  inc lactate/pyruvate in blood (lactic acidosis)  Large amounts of glycogen in liver (Glc-6-P inhibits breakdown)  Symptoms: delay in puberty, liver enlargement, short stature  Treatment: continuous feeding of cornstarch intragastrically, drug induced inhibition of Glc uptake by liver, surgical transplant of portal vein- disconnect it from liver and extend it to the rest of your tissue so your other tissues can absorb glucose before liver gets it, low Km of hexokinase allows other body cells to uptake glucose o Type IV (Anderson’s Disease)- branching enzyme deficiency in liver  Most severe disease  Accumulate abnormal glycogen in liver, reduced solubility of glycogen, foreign body immune response, death o Type V (McArdles Disease)- glycogen phosphorylase deficiency in muscle  No breakdown of glycogen in muscle  Symptoms: Exercise quickly induces muscle cramps, otherwise normal  Effective utilization of glycogen in muscle is not essential for life  Can’t provide fuel for glycolysis to keep up demand for ATP  Muscle cramps correlate with increased ADP  Vasodilation: increased blood flow after you start to work out for a while. Muscle now has access to Glc and fatty acids in blood o Type VII (Tauri’s disease) – phosphofructokinase mutant in muscle  You do not produce ATP in your muscle  PFK deficiency directly affects glycolysis, prevents normal breakdown of glucose, slows breakdown of glycogen  Glycogen accumulates, inability to exercise  Infants: death, adult onset: debilitating  Treatment: high fat diet and no physical activity o Type II (Pompe Disease)- Glucosidase deficiency in lysosome  Defective glycogen breakdown in all organs  Glycogen buildup causes enlarged lysosome: Lysosomes lyse and release content into cells.  Symptoms: Muscle damage/weakness. Death by year 2 due to heart failure  Treatment: enzyme replacement therapy. Very expensive o Concept of enzyme replacement for Type II disease:  Tried in 1990  The enzyme needs to be glycosylated to get the protein to the right spot. The carbohydrates attached to the protein are important to function  Venture capitalist John Crowley started a company and hired scientists to push drug therapy testing. His kids were treated by the drug and improved.  The glucosidase needs to have certain carbs on it- important for proper trafficking to get into the lysosome and for stability once it gets there to avoid becoming degraded: mannose-6 phosphate  Sugars are typically added naturally in the Golgi apparatus  Glycolysis and Cancer: If NADH builds up, glycolysis and CAC are both inhibited because regulatory enzymes become allosterically inhibited. You still need oxygen for the CAC but not glycolysis because you need the ETC to re-oxidize NADH. Mitochondria doesn’t have an enzyme to re-oxidize NADH like glycolysis does (lactate dehydrogenase) so it relies on the ETC. o Why would a cell decide to only do glycolysis and not carry all the way through the CAC and ETC, even in the presence of O2? This is a cancer cell. They utilize glycolysis in the presence of O2 even though you get less ATP for each molecule of glucose. o Tumor grows so fast the capillaries can’t keep up, creates a low oxygen environment. ETC can’t occur. Glycolysis enzymes and glucose pumps are up- regulated o Selective advantage of using glycolysis in the presence of oxygen: tumors prefer acidic environment. Creates lactic acid through lactic acid fermentation. This facilitates tumor invasion and inhibits the immune system. Increased glucose uptake facilitates cell growth. o Cancer treatment: drug that inhibits lactate dehydrogenase so cancer cells can’t reoxidize NADH and continue to grow Breakout Quiz Review notes  Glycogenin is in the middle of glycogen- that catalyzes the first glucose addition. Glycogen synthase requires a reducing end, which is started by glycogenin. Non-reducing ends are on outside. Many more non-reducing ends than reducing ends: increased solubility and rate of synthesis/degradation  Glycogen synthesis pathway Glc 1-P and UTP to UDP-glucose: subsequent pyrophosphate hydrolysis drives rxn forward. UDP-glucose is the glucose donor. Rxn is irreversible due to the high -delta G of PPi hydrolysis.  Starting with free glucose, how many ATP eq. are utilized to extend glycogen by 1 glucose molecule? 2 ATP eq: 1 to phosphorylate glucose to glucose 6-phosphate and 1 eq. in the form of UTP to form UDP-glucose  Glycogen phosphorylase cleaves 1-4 bonds only from the non-reducing ends. Phosphorylase uses 8 inorganic phosphates to cleave  Energy yield is higher from glycogen than from glucose because you need ATP to form glucose 6-phopshate from glucose, you don’t need ATP to make the glucose-6 phosphate from glycogen. Glycogen phosphorylase uses inorganic phosphates to cleave in a phosphorolysis rxn. More efficient if your cell relies on phosphorylases rather than hydrolysis of glycogen  Liver phosphorylase and muscle phosphorylase are isozymes o Different allosteric regulation o Muscle has allosteric site for AMP/ATP. Binding of AMP shifts to R state o Liver has allosteric site for glucose. Binding of glucose shifts it to T state o Make sure you understand these regulation pathways  Key regulatory step of glycogen metabolism is phosphorylation o Know the consequences at each step in the pathway o High glucose will re-activate PP1 to reverse all the phosphorylation events that led to glycogen degradation and stimulates glycogen synthesis Glc allosterically binds to phosphorylase (which is bound to PP1), changes its shape to T state, and PP1 is released  Insulin stimulated: stimulates glc uptake (GLUT4), inactivates glycogen synthase kinase, activates glycogen synthase  Lag between glycogen synthesis and degradation happens because there’s 10:1 ratio of phosphorylase to PP1. Increases chances that all PP1 are bound to phosphorylase. You would have to shift all of them to T state to release PP1


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