<p>1. REGULATION OF GLUCONEOGENESIS</p><p>Regulation of fructose-1,6-bisphosphatase - PFK-1 and fructose-1,6-Bpase form futile cycle; both reactions are irreversible and when unregulated they consume net amounts of ATP with production of heat</p><p>Inhibition by AMP - AMP is a signal of low energy in cytoplasm - fructose-1,6-BPase requires energy whether it is used for gluconeogenesis in liver or to permit conversion of lactate  glycogen in muscle</p><p>Activation by citrate - in liver  citrate activates fructose-1,6-BPase and feedback inhibits glycolysis by inactivating PFK-1; if accumulation of citrate in cytoplasm becomes excessive  gluconeogenesis activated to send carbons back to glucose</p><p>Inhibition by fructose-2,6-BP - most important allosteric regulator of gluconeogenesis in liver is fructose-2,6-BP - blood glucose increases after meal  amount of fructose-2,6-BP increases - in fed state gluconeogenesis is unnecessary  fructose-2,6-BP inhibits fructose-1,6-BPase - food deprivation  [fructose-2,6-BP] declines to relieve inhibition of fructose-1,6-BPase and gluconeogenesis</p><p>2. REGULATION OF GLYCOGEN METABOLISM - regulation of glycogen synthase and glycogen phosphorylase oppose each other - synthesis of glycogen requires energy; glycogenolysis does not directly produce energy - both enzymes regulated by hormonal control through covalent modification (phosphorylation/dephosphorylation) - phosphorylation activates enzymes responsible for mobilizing fuels (glycogen, fats), while inactivating enzymes linked to fuel synthesis/storage - dephosphorylation associated with fuel storage and activation of enzymes follow pattern opposite to that for fuel mobilization</p><p>Glycogen phosphorylase - regulation of glycogen phosphorylase ensure that glucose remains stored as glycogen until it is mobilized from liver for maintaining blood glucose homeostasis or to supply energy to muscle cell - enzyme is phosphorylated in response to hormone signals in a cascade - enzyme that directly catalyzes the phosphorylation of glycogen phosphorylase is phosphorylase kinase (can be activated either by phosphorylation or allosterically by calcium) - muscle  calcium released from sarcoplasmic reticulum; liver  calcium released from ER in response to hormonal signaling; rise in cytoplasmic calcium activates phosphorylase kinase  phosphorylates glycogen phosphorylase</p><p>Activation of glycogenolysis by a kinase cascade initiated by glucagon or epinephrine via cAMP - glycogen phosphorylase is active when phosphorylated (glycogen phosphorylase-a form) - glycogen synthase active when dephosphorylated (glycogen synthase-i form) - occurs via a cascade that involves amplification of initial signal allowing little hormone to alter activity of many molecules of enzyme</p><p>- glucagon or epinephrine binds to its receptor  AC  cAMP  PKA  activation of glycogenolysis by PKA phosphorylation of phosphorylase kinase (b to a form)  phosphorylation of glycogen phosphorylase (ba)  removal of glucose units from glycogen</p><p>Relaxed and tense form of glycogen phosphorylase - active forms of glycogen phosphorylase are relaxed; inactive forms are tense - phosphorylation causes enzyme to revert to relaxed state</p><p>Reversal of the cAMP-mediated cascade by insulin - dissociation of hormone from receptor  inactivation of AC - insulin activates phosphodiesterase (PDE) : cAMP  AMP  inactivation of PKA  shuts of cascade - insulin also activates protein phosphatase  hydrolyzes phosphate from phosphorylase kinase and glycogen phosphorylase  inactivation - glycogenolysis activity is diminished in fed state</p><p>3. Allosteric regulation of glycogen phosphorylase - phosphorylase kinase (besides being activated by phosphorylation) is allosterically activated by calcium during muscle contraction - when glucsose-6-P is in excess  it inhibits phosphorylase kinase to prevent cell from generating more glucose-6-P by glycogen breakdown - when glycogen phosphorylase in tense b state is phosphorylated  reverts to relaxed active form a form</p><p>- AMP binding to its allosteric site on the tense form of phosphorylase-b promotes conversion to active relaxed site - enzyme can be rapidly activated in absence of hormone signal - in muscle  this effect of AMP coordinates with ability of AMP to activate PFK-1 in glycolysis - high levels of ATP  maintain phosphorylase-b in tense state</p><p>- increase [glucose-6-P] keeps phosphorylase-b in tense conformation in two ways 1. glucose-6-P allosterically inhibits phosphorylase kinase to prevent phosphorylation of glycogen phosphorylase 2. glucose-6-P binds to the relaxed phosphorylase-b form causing enzyme to convert to tense conformation - inhibitory effect of glucose-6-P reduces the trapping of phosphate as phosphorylated sugar, under conditions where utilization of glucose-6-P has diminished (trapped phosphate is unavailable for synthesis of ATP) - one way to diminish glucose-6-P is through inhibition of glycogenolysis - excess amounts of glucose in cell promote conversion of glycogen phosphorylase-a from relaxed to its tense form making it available for dephosphorylation by protein phosphatase - fed state  glucose  insulin  activates protein phosphatase-1</p><p>4. GLYCOGEN SYNTHASE Hormonal inactivation of glycogen synthase - PKA directly phosphorylates the active (nonphosphorylated) form of glycogen synthase to convert it to its inactive form - inactivation of glycogen synthase prevents resynthesis of glycogen from glucose-1-P (product of glycogenolysis) - other kinases exist that catalyze the phosphorylation of glycogen synthase  inactivating enzyme - one is phosphorylase kinase-a - two kinases which are activated as part of the cascade can inactivate glycogen synthase via phosphorylation to ensure that glycogen is not synthesized under conditions where glycogenolysis is required</p><p>The ‘d’ and ‘i’ forms of glycogen synthase - glycogen synthase is responsible for fuel synthesis/storage and is active when dephosphorylated and inactive when phosphorylated - protein phosphatases (after activation by insulin) catalyzes the dephosphorylation of glycogen synthase - phosphorylated form  glycogen synthase-d (inactive); ‘d’ indicates that for phosphorylated form to be active, it depends on allosteric binding of glucose-6-P; this allosteric activation provides an additional mechanism to ensure that glucose units are stored when glucose-6-P is abundant (such as glucose influx into lover or uptake by muscle in response to insulin) - the allosteric effect of glucose-6-P can override the hormonal effects of glucagon or epinephrine to protect cell - activity of dephosphorylated form of glycogen synthase is independent (glycogen synthase-i, active) of glucose-6-P for its activity since dephosphorylation places the enzyme in a favorable conformation to carry out its catalytic role</p><p>-hormonal activation of glycogen synthase occurs in presence of insulin (fed state) and is mediated by the removal of the covalently attached phosphate catalyzed by protein phosphatase - inhibitory effects mediated by PKA, phosphorylase kinase-a and glycogen synthase kinase-3 are reversed - inactivation of these kinases  activation of PDE removes cAMP  PKA inactive  phosphorylase kinase can be dephosphorylated by that action of protein phosphatase and be inactivated</p>