Regulation of Fructose-1,6-Bisphosphatase

1. REGULATION OF GLUCONEOGENESIS

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

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

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

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

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

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

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

- 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

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

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

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

- 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

- 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

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

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

-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