glycogen metabolism

Glucose homeostasis 20 g 190 g glucose in body fluids, mainly blood Glycogen - liver ~ 24 hrs starvation Glycogenolysis ~ Gluconeogenesis after Carbohydrate/glucose reserve „Buffer role” in the maintenence of blood glucose level

Structure of glycogen

Glycogen synthesis

G-6-P - G-1-P conversion DIPF: diisopropylfuorophosphate - inhibitor

Activated glucose

Reaction is pulled in the forward direction by the hydrolysis of PPi

UDP-glucose pyrophosphorylase

Primer is required

glycogenin Autocatalytic activity for glycosylation Human glycogenin gene- 1 muscle, -2 liver 5 exons 0.3% of glycogen is protein Glycogenin content determines the cellular glycogen content

Glycogen branching enzyme: glycosyl (4,6) transferase, -more soluble glycogen -more non reducing terminal residues increased rate of metabolism

Glycogenesis

Energy balance of glycogenesis for one glycosyl unit G-6-P + ATP + glycogen (n) + H2O Glycogen (n+1) + ADP + 2Pi

Glycogen degradation

Phosphorolysis = cleavage of a bond by Pi Energetically advantageous – released sugar is phosphorylated Glycogen phosphorylase

Debranching enzyme Single polypeptide chain

Glycogenosis = glycogen storage disease Targets: liver (blood glucose homeostasis – hypoglycaemia, hepatomegaly) muscle (ATP production, muscle contraction convulsions, weakness, unable for muscle work)

Glucose-6 phosphatase enzyme system in the ER membrane

ADP increases during exercise in McArdle disease measured byNMR

Glycogen phosphorylase Muscle dimer or tetramer, Ser 14 phosphorylation/monomer AMP binding site Liver Glucose sensor function Regulated by allosteric interactions and Reversible phosphorylation

Glycogen phosphorylase Pi binding site PLP: pyridoxal phosphate – each catalytic site contains PLP group

PLP - Schiff base linkage at active site of phosphorylase

active usually inactive not phosphorylated phosphorylated

Equilibrium favors Equilibrium favors

Allosteric binding site for nucleotides Transition is controlled by the energy charge of the muscle cell

Glycogen phosphorylase Phosphorylase a is fully active regardless of the levels of ATP/AMP, G-6-P Phosphorylase b is usually inactive under physiological circumstances because of the inhibitory effect of ATP and G-6-P

Allosteric binding site for glucose – glucose sensor function – only in liver inactive Under physiological conditions there is no AMP dependent regulation

Activation of phosphorylase kinase e.g. epinephrine δ subunit: calmodulin – calcium sensor

Glycogen synthase 9 sites for phosphorylation PKA and other protein kinases can phosphorylate the enzyme Phosphorylation converts the active a form of the enzyme to inactive b form

Reciprocal regulation in glycogen metabolism

PP1: protein phosphatase 1 PP1 inactivates phosphorylase kinase and phosphorylase a PP1 decreases glycogen breakdown PP1 converts glycogen synthase b to much more active a form PP1 accelerates glycogen synthesis

PP1: protein phosphatase 1 Rgl: glycogen binding subunit PP1 is active, when associated with glycogen Rgl can be phosphorylated by PKA - causes dissociation from PP1 - inactive

Rgl can be phosphorylated by PKA - causes dissociation from PP1 - inactive Rgl can be phosphorylated by insulin sensitive protein kinase - causes association to PP1 - active

Blood glucose regulates liver glycogen metabolism

Only in liver Muscle phosphorylase is unaffected by glucose

Signal amplification

Regulation of blood glucose level. Hyperglycaemia -1 Liver increased glucose uptake – GLUT2 Glucokinase – „extra glucose” Increased glycogenesis – insulin; PP1 – glycogen synthase Decreased glycogenolysis – glucose sensor function – glycogen phosphorylase PDH active – increased fatty acid synthesis

Regulation of blood glucose level. Hyperglycaemia -2 Peripheral tissues pancreas increased glucose uptake – GLUT2 Glucokinase – insulin secretion muscle, adipocytes GLUT4 increased number in membranes Increased glycogenesis Decreased glycogenolysis increased glycolysis – PFK1

Regulation of blood glucose level. Hyperglycaemia -3 Long term effects Decreased amount of PEPCK – decrease in gluconeogenesis

Regulation of blood glucose level. Hypoglycaemia liver Increased gluconeogenesis Increased glycolysis

Regulation of blood glucose level. Hypoglycaemia newborns Limited ketone body synthesis Brain/body rate – Increased glucose demand PEPCK is not induced, gluconeogenesis is not enough Glycogen storage is limited Glucokinase, G-6-P-ase are not induced