BIOC 460 - DR. TISCHLER LECTURE 32 GLYCOGEN METABOLISM: HORMONAL REGULATION

OBJECTIVES 1. Steps by which cyclic AMP-mediated hormonal regulation activates phosphorylase kinase and glycogen phosphorylase. 2. Events associated with reversal of the cyclic AMP-mediated cascade - roles of GTPase activity of the s-subunit, phosphodiesterase, protein phosphatase-1. 3. Features in regulation of glycogen synthase by covalent modification 4. Regulation of the activity of protein phosphatase-1. 5. Mechanisms by which insulin facilitates the activation of glycogen synthase to promote glycogen storage.

Glucagon or epinephrine inactive enzyme G-protein adenylyl cyclase - inactive adenylyl cyclase -active ATP ADP OH Glycogen synthase-i OP Glycogen synthase-d ATP cAMP Protein kinase-A Protein kinase-A OP Phosphorylase kinase-a OH Phosphorylase kinase-b ATP ADP OP Glycogen phosphorylase-a HO Glycogen phosphorylase-b ATP ADP Pi + Glucosen Glucosen-1 + (Glycogen) Glucose-1-P Figure 1. Cascade for the control of glycogenolysis and glycogenesis

phosphodiesterase (+insulin) G-protein Adenylyl cyclase Adenylyl cyclase cAMP  AMP phosphodiesterase (+insulin) active enzyme cAMP loss Protein kinase-A Protein kinase-A inactive enzyme OP Phosphorylase kinase-a HO Phosphorylase kinase-b -Pi Glycogen synthase-d OP protein phosphatase activated via insulin signal transduction -Pi OH Glycogen synthase-i -Pi OP Glycogen phosphorylase-a OH Glycogen phosphorylase-b Figure 2. Reversal of phosphorylation events initiated by protein kinase A.

Figure 3. Regulation of protein phosphatase-1. Fed state OPins PP-1 OHpka fully-activated complex RGI Signal via receptor tyrosine kinase [1] ATP ADP [2] activated insulin-sensitive protein kinase Insulin binds to receptor o OHins PP-1 OHpka low activity form RGI glycogen Figure 3. Regulation of protein phosphatase-1. Fed state

Figure 3. Regulation of protein phosphatase-1 – starvation/stress OHins PP-1 OHpka low activity form RGI ADP ATP [4] Signal via G-protein [3] activated protein kinase A glucagon or epinephrine binds to receptor OHins OPpka RGI o RGI PP-1 PP-1 minimally active form PP-1 PP-1 activated PKA PP-1 OP I I OH OP I OP I PP-1 OP I OP I ADP ATP [5] inactive form [6] inactive form active form Figure 3. Regulation of protein phosphatase-1 – starvation/stress

Figure 4. Insulin inactivates glycogen synthase kinase-3. Insulin signal mediated by IRS, p85, PI-3 kinase, PIP3, PKB ATP ADP OH Glycogen synthase kinase-3 (active) OP Glycogen synthase kinase-3 (inactive) Inactive glycogen synthase kinase-3 leads to active glycogen synthase Figure 4. Insulin inactivates glycogen synthase kinase-3.

Mechanisms by Which Insulin Facilitates the Activation of Glycogen Synthase to Promote Glycogen Storage PP-1 removes the phosphate from phosphorylase kinase so that it cannot phosphorylate glycogen synthase Phosphodiesterase degrades cyclic AMP so PKA becomes inactive and unable to phosphorylate glycogen synthase Insulin promotes dephosphorylation (activation) of glycogen synthase through activation of protein phosphatase-1 4) Insulin prevents phosphorylation of glycogen synthase inactivating glycogen synthase kinase-3