Class Note for BIOC 460 at UA 3
Class Note for BIOC 460 at UA 3
Popular in Course
Popular in Department
This 8 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 15 views.
Reviews for Class Note for BIOC 460 at UA 3
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 02/06/15
Biochemistry 460 Dr Tischler GLYCOGEN HORMONAL CONTROL Related Reading Chapter 21 598604 6076llin Stryer 6 h edition OBJECTIVES l Delineate the steps by which cyclic AMPmediated hormonal regulation activates phosphorylase kinase and glycogen phosphorylase 2 Discuss events associated with the reversal of the cyclic AMPmediated cascade including the roles of inactivation of the Gs protein phosphodiesterase and protein phosphatasel 3 Describe the features in the regulation of glycogen synthase by covalent modi cation 4 Describe the regulation of the activity of protein phosphatase 1 by insulin PHYSIOLOGICAL PREMISE One metabolic problem faced by diabetics is an impaired ability to store glycogen properly This problem re ects one of the very important roles of insulin fuel storage Insulin not only promotes the storage of glucose as glycogen but also of amino acids as protein and of fatty acids as triacylglycerol Improper storage of glycogen can have important implications for the liver during food deprivation or for muscle during exercise HORMONAL CONTROL OF GLYCOGEN METABOLISM Hormonal Control Initiated by Glucogon or Epinephrine viii Cyclic AMP The preceding lecture considered the allosteric regulation of glycogen phosphorylase and glycogen synthase These enzymes also are regulated by covalent modi cation phosphorylationdephosphorylation In the absence of allosteric control glycogen phosphorylase is active when phosphorylated glycogen phosphorylasea and glycogen synthase is active when dephosphorylated glycogen synthasei A general concept to remember is that phosphorylation of enzymes is typically associated with physiological responses that require mobilization offuel eg starvation stress ght or ight response Hence phosphorylation will activate those enzymes responsible for mobilizing the fuels eg glycogen fats While concomitantly inactivating those enzymes linked to fuel synthesis and storage Conversely dephosphorylation of enzymes is associated with physiological conditions in which fuel is stored ie fed state and the activation of enzymes will follow a pattern opposite to that for fuel mobilization The regulation of glycogen phosphorylase is compleX Fig 1 Its regulation ensures that glucose remains stored as glycogen until it is mobilized from liver for maintaining blood glucose homeostasis or to supply energy to the muscle cell This enzyme is phosphorylated in response to hormone signals in a cascade The enzyme that directly catalyzes the phosphorylation of glycogen phosphorylase is phosphorylase kinase which itself can be activated either by phosphorylation or allosterically by calcium see preceding lecture Hormonal activation of glycogen phosphorylase occurs via a cascade that involves ampli cation of the initial signal In this way relatively little hormone in the blood can lead to altered activity of many molecules of enzyme Glycogen Hormonal Control l Glucagon liver Epinephrine liver muscle l Gprotein Cytoplasm ATP ADP adenylyl cyclase 1 G active Glycogen ytclfgend synthase i Syn 5939 I s I OH quot6 39 7 013 ATP cAMP 39 Protein Protein 5 kinase A 2 kinase A 9 Phosphorylase P Phosphorylase 7 kinasea kinase b OH 3 ATP ADP 9 Glycogen phosphorylase a Glycogen 7 Y 4 phosphorylase b OH 5 Glucosen1 P Glucosen GlucoselP Glycogen Figure 1 Cascade for the control of glycogenolysis and glycogenesis by elevated cyclic AMP in response to glucagon or epinephrine Active forms of enzymes are bold faced in ovals and inactive forms are italicized in rectangles The dotted lines 00000 indicate that the enzyme from which the line originates catalyzes the kinase reaction to which the dotted line points The cascade is initiated by binding of hormone glucagon or epinephrine to its speci c receptor Fig l Hormone binding causes dissociation from the Gs protein of the ocsubunit that then activates adenylyl cyclase 1 Adenylyl cyclase produces cyclic AMP that activates protein kinase A 2 Protein kinase A is activated by the binding of cyclic AMP To accelerate glycogenolysis protein kinase A catalyzes the phosphorylation of phosphorylase kinase converting it from its inactive b form to its active 21 form 3 Subsequently phosphorylase kinasea active form catalyzes the phosphorylation of the tense inactive glycogen phosphorylase b to generate the relaxed active glycogen phosphorylasea form 4 see previous lecture for terminology of tense and relaxed forms Thus glycogen degradation is triggered by this hormonal cascade 5 In parallel activated protein kinase A 6 and phosphorylase kinase 7 both directly phosphorylate the active form of glycogen synthase the i form to convert it to the inactive form glycogen synthased Thus two kinases phosphorylase kinase and protein kinase A that are activated as part of the Glycogen Hormonal Control 2 glycogenolysis cascade can inactivate glycogen synthase via phosphorylation Phosphorylation of glycogen synthase ensures that glycogen is not synthesized under conditions where glycogenolysis is required Other kinases that can catalyze the phosphorylation of glycogen synthase include calcium calmodulindependent kinase and glycogen synthase kinase3 not shown in Figure l Reversal of the Cyclic AMPmediated Cascade W7 39 Gprotein inactive Adenylyl 1 cyclase cAMP gt AMP inactive phosphodiesterase l by insulin Protein loss 0f Protein kinase A 4 kinase A inactive active 2 Phosphorylase Phosphorylase 39Pi kinase a kinase 17 OH I O Glycogen 4 synthase d In protein activated via insulin pi phosphatase1 signal transduction 5 3 1 Glycogen synthase i 4 O GlyCOgen Glycogen phosphorylase a phosphorylase b OP I OH Figure 2 Reversal of phosphorylation events initiated by protein kinase A Cyclic AMP is degraded by phosphodiesterase causing inactivation of protein kinase A Insulin activates protein phosphatase which removes phosphate from the enzymes to which the dotted lines 00000 point ie phosphorylase kinase glycogen phosphorylase glycogen synthase Active forms of enzymes are bold faced in ovals and inactive forms are italicized in rectangles Glycogen Hormonal Control 3 Dissociation of the hormone from its receptor leads to and inactivation of the Gprotein leads to inactivation of adenylyl cyclase Fig 2 1 Hydrolysis of cyclic AMP by phosphodiesterase stimulated by insulin leads to inactivation of protein kinase 2 Despite these events the effects of glucagon and epinephrine would be sustained as long as the enzymes in the cascade remain phosphorylated The primary antagonist to the cascade therefore is insulin through its ability to activate protein phosphatasel 3 Once activated protein phosphatase removes phosphate from phosphorylase kinase and glycogen phosphorylase leading to their inactivation 4 and from glycogen synthase to restore its activity 5 Regulation of the Activity ofPratein Phosphatase1 Extracellular space Cytoplasm active receptor P tyrosine kinase P PLC tyr OH ATP 1 IRTK t 1 d ADP ca a yze dephosphorylation v of enzvmes 4 quot activation GPI contained in 2 gt 3 1nner lea et of GPI IPG plasma membrane Figure 3 Mechanism for insulin to activate protein phosphatase Abbreviations used GPI glycosylphosphatidyl inositol IPG inositol phosphate glycan IRTK insulin receptor tyrosine kinase PLC phospholipase C PP protein phosphatase Glycogen Hormonal Control 4 Insulin counterregulates the effects of glucagon or epinephrine that activate protein kinase A by increasing the activity of phosphodiesterase that hydrolyzes cyclic AMP and thereby leads to deactivation of protein kinase A Similarly there are mechanisms to control the activation and deactivation of protein phosphatasel in a coordinated fashion Insulin regulates pathways in carbohydrate and lipid metabolism through activation of protein phosphatase Earlier in this lecture we discussed the phosphorylation of proteins in glycogen metabolism by protein kinase A and other kinases We will discuss a role for phosphorylation of enzymes in conjunction with lipid regulation in a later lecture Phosphate is removed from these phosphorylated enzymes by protein phosphatasel in a coordinated fashion Fig 2 These events are essential for promoting glucose storage as glycogen and increasing the synthesis of fatty acids and cholesterol while at the same time preventing the mobilization of stored fuels ie glycogen triacylglycerol The signaling pathway shown below is initiated by activation of a speci c phospholipase C PLC through insulin receptor tyrosine kinasecatalyzed phosphorylation Fig 3 1 Phospholipase C catalyzes the hydrolysis of a membrane phospholipid glycosylphosphatidylinositol GPI to produce inositol phosphate glycan IPG as a product 2 IPG seems to be a second messenger molecule cf cyclic AMP that ultimately signals the activation of protein phosphatase PP 3 leading to enzyme dephosphorylation 4 Let s rst examine the mechanism that ensures protein phosphatasel is active in the presence of insulin and thus able to carry out the dephosphorylation events depicted in gure 2 The protein phosphatasel catalytic subunit is active only when associated with glycogen through the RG1 subunit g 4 The RG1 subunit contains two different sites for phosphorylation on hydroxyl groups of amino acid side chains The site that potentially can be phosphorylated as a consequence of insulin signaling is designated OHins in gure 4 In the fed state when insulin is plentiful its binding to the cell membrane activates the receptor tyrosine kinase that ultimately activates an insulinsensitive protein kinase g 4 1 The insulinsensitive protein kinase then phosphorylates the OHins on the RG1 subunit 2 This converts the protein phosphatasel from its low activity form to a fully activated compleX These events ensure that protein phosphatasel can rapidly dephosphorylate enzymes see Fig 2 fully activated complex ADP S1gna1 v1a receptor tyrosine kinase ctl iated Insulin 1nsu11n binds to sensitive 2 receptor 1 protein kinase ATP low activity form Figure 4 Activation of protein phosphatasel In the fed state full activation of protein phosphatasel by insulin is mediated by an insulinsensitive protein kinase that phosphorylates OHins on the RG1 subunit Glycogen Hormonal Control 5 In contrast in starvation glucagon or stress epinephrine it is essential that phosphorylated enzymes eg glycogen phosphorylase phosphorylase kinase glycogen synthase remain covalently modi ed as occurs in the cascade depicted in gure 1 Consequently the protein phosphatasel catalytic subunit must be inactivated in the presence of glucagon or epinephrine This mechanism involves rst the activation of protein kinase A g 5 1 Since protein kinase A catalyzes the various phosphorylation events this is an ef cient scenario whereby protein kinase A concomitantly causes inactivation of protein phosphatase 1 This is accomplished through two phosphorylation events catalyzed by protein kinase A low activity form ATP glucagon or Signal via epinephrine Gprotein activated binds to protein 2 receptor 1 klnase A ADP minimally active form PPl Inhibitory protein activated protein kinase A lnacthe form 1 7T 4 3 ATP ADP OP OH Inactive Active form form Figure 5 Inhibition of protein phosphatasel In starvation or stress protein kinase A after activation by glucagon or epinephrine phosphorylates OHpka on the R51 subunit causing protein phosphatasel to dissociate The dissociated form is completely inactivated by binding an inhibitory protein that also requires phosphorylation catalyzed by protein kinase A Phosphorylation of the RG1 subunit on a different hydroxyl group designated as OHpka makes interaction with the protein phosphatasel catalytic subunit unfavorable so that the regulatory RG1 and catalytic PPl subunits dissociate generating a minimally active form of protein phosphatasel g 5 2 Additionally protein kinase A catalyzes the phosphorylation of an inhibitory protein I g 5 3 that binds to protein phosphatasel to completely inactivate it g 5 4 Thus when epinephrine or glucagon activate protein kinase A each concurrently shuts off protein phosphatasel providing coordinated regulation of the phosphorylationdephosphorylation events Glycogen Hormonal Control 6 Cascade for preventing inactivation of glycogen synthase will not be tested in exam Extracellular DP active receptor tyrosine kinase Cymplasm IRS P0 1 IRTK catalyzed active IRS2 PIP PIP3 2 3 activated PI3K catalyzes PIP3 formation that activates PDK PDK activates PKB IRS 2 activated by docking 4 PKB phosphorylates GSK 3 to make it inactive Glycogen LL synthase 5 Inactive glycogen synthase kinase3 kinase3 leads to active glycogen Inactive synthaSe OP Glycogen synthase kinase 3 active OH Figure 6 Insulin inactivates glycogen synthase kinase3 A signal from insulin is mediated through a variety of intermediates that include insulin receptor substrate IRS phosphatidylinositol kinase PI3 kinase phosphatidylinositol tiisphosphate PIP3 phospholipiddependent kinase PDK and protein kinase B PKB to ultimately inactivate glycogen synthase kinase3 by phosphorylation Glycogen Hormonal Control 7 Besides protein kinase A and phosphorylase kinase glycogen synthase kinase 3 plays an important role in catalyzing the phosphorylation and therefore the inactivation of glycogen synthase Glycogen synthase kinase3 eXists in both an active dephosphorylated and an inactive phosphorylated form The pathway for activating glycogen synthase kinase3 is unknown but requires a protein phosphatase which is not regulated by insulin More is known however about the process by which glycogen synthase kinase3 is inactivated One pathway of insulin signal transduction includes phosphorylation of insulin receptor substrate IRS by the insulin receptor tyrosine kinase IRTK to an active form Fig 6 1 The activated IRS can then dock with phosphatidylinositol kinase PI3 kinase via p85 Fig 6 2 PI3 kinase catalyzes the addition of phosphate to phosphatidylinositol bisphosphate PIPZ to form phosphatidylinositol trisphosphate PIP3 Fig 6 3 PIP3 then serves as a second messenger that activates phospholipiddependent kinase PDK PDK directly activates protein kinaseB PKB that then catalyzes the phosphorylation of glycogen synthase kinase3 Fig 6 4 Phosphorylation of glycogen synthase kinase3 causes it to be inactivated Once inactivated glycogen synthase activity can be restored by its return to its non phosphorylated state Summary of Mechanism by Which Insulin Facilitates the Activation of Glycogen Synthase to Promote Glycogen Storage Insulin has several mechanisms for increasing the activity of glycogen synthase In this way insulin ensures that glucose is stored as glycogen 1 Protein phosphatase1 removes the phosphate from phosphorylase kinase so that it cannot phosphorylate glycogen synthase Fig 2 2 Phosphodiesterase degrades cyclic AMP so protein kinase A becomes inactive and unable to phosphorylate glycogen synthase Fig 2 3 Insulin promotes the dephosphorylation activation of glycogen synthase through activation of protein phosphatase1 Fig 4 4 Insulin prevents phosphorylation of glycogen synthase by causing the phosphorylation of glycogen synthase kinase3 thereby inactivating it Fig 6 Glycogen Hormonal Control 8
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'