Lecture 6 GLYCOGEN METABOLISM

Introduction Stores of available glucose to supply the tissues with an oxidizable energy source are found principally in the liver, as glycogen. A second major source of stored glucose is the glycogen of skeletal muscle.

muscle glycogen is not generally available to other tissues, because muscle lacks the enzyme glucose-6-phosphatase. The major site of daily glucose consumption (75%) is the brain via aerobic pathways. Most of the remainder of is utilized by erythrocytes, skeletal muscle, and heart muscle.

Glycogenolysis Degradation of stored glycogen (glycogenolysis) occurs through the action of glycogen phosphorylase. The action of phosphorylase is to phosphorolytically remove single glucose residues from a-(1,4)-linkages within the glycogen molecules.

The product of this reaction is glucose-1-phosphate. The glucose-1-phosphate produced by the action of phosphorylase is converted to glucose-6-phosphate by phosphoglucomutase: The advantages of the phosphorolytic step are:

The glucose is removed from glycogen is an activated state, i. e The glucose is removed from glycogen is an activated state, i.e. phosphorylated and this occurs without ATP hydrolysis. The concentration of Pi in the cell is high enough to drive the equilibrium of the reaction the favorable direction since the free energy change of the standard state reaction is positive. Ensures that the glucose residues do not freely diffuse from the cell.

The conversion of G6P to glucose, which occurs in the liver, kidney and intestine, by the action of glucose-6-phosphatase does not occur in skeletal muscle as these cells lack this enzyme. So, any glucose released from glycogen stores of muscle will be oxidized in the glycolytic pathway. In the liver the action of glucose-6-phosphatase allows glycogenolysis to generate free glucose for maintaining blood glucose levels.

Glycogen phosphorylase cannot remove glucose residues from the branch points (a-1,6 linkages) in glycogen. The removal of the these branch point glucose residues requires the action of debranching enzyme (also called glucan transferase) which contains 2 activities: glucotransferase and glucosidase.

Regulation of Glycogenolysis Glycogen phosphorylase exists in two distinct conformational states: b ( less active) and a ( more active) state. Phosphorylase is capable of binding to glycogen when the enzyme is in the R state. This conformation is enhanced by binding of AMP and inhibited by binding ATP or glucose-6-phosphate. The enzyme is also subject to covalent modification by phosphorylation as a means of regulating its activity.

Pathways involved in the regulation of glycogen phosphorylase Pathways involved in the regulation of glycogen phosphorylase. See the text for details of the regulatory mechanisms. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Whether a factor has positive (+ve) or negative (-ve) effects on any enzyme is indicated. Briefly, phosphorylase b is phosphorylated, and rendered highly active, by phosphorylase kinase. Phosphorylase kinase is itself phosphorylated, leading to increased activity, by PKA (itself activated through receptor-mediated mechanisms). PKA also phosphorylates PPI-1 leading to an inhibition of phosphate removal allowing the activated enzymes to remain so longer. Calcium ions can activate phosphorylase kinase even in the absence of the enzyme being phosphorylated. This allows neuromuscular stimulation by acetylcholine to lead to increased glycogenolysis in the absence of receptor stimulation.

In response to lowered blood glucose the a cells of the pancreas secrete glucagon which binds to cell surface receptors on liver and several other cells. The response of cells to glucagon is the activation of the enzyme adenylate cyclase which is associated with the glucagon receptor. leads to a large increase in the formation of cAMP. cAMP binds to an enzyme called cAMP-dependent protein kinase, PKA. this binding phosphorylate a number of proteins inside the cell of significance phosphorylase kinase which enhances activity of glycogen breakdown of phosphorylase enzyme.

Representative pathway for the activation of cAMP-dependent protein kinase (PKA). In this example glucagon binds to its' cell-surface receptor, thereby activating the receptor. Activation of the receptor is coupled to the activation of a receptor-coupled G-protein (GTP-binding and hydrolyzing protein) composed of 3 subunits. Upon activation the alpha subunit dissociates and binds to and activates adenylate cyclase. Adenylate cylcase then converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. Once released the catalytic subunits of PKA phosphorylate numerous substrate using ATP as the phosphate donor.

This identical cascade of events occurs in skeletal muscle cells as well. But muscle cells lack glucagon receptors. so Epinephrine is released from the adrenal glands in response to neural signals indicating an immediate need for enhanced glucose utilization in muscle, the so called fight or flight response.

Glycogen Synthesis Synthesis of glycogen from glucose is carried out the enzyme glycogen synthase. This enzyme utilizes UDP-glucose as one substrate and the non-reducing end of glycogen as another. The activation of glucose to be used for glycogen synthesis is carried out by the enzyme UDP-glucose pyrophosphorylase.

This enzyme exchanges the phosphate on C-1 of G1P for UDP. glycogen synthase catalyze the incorporation of glucose into glycogen. protein known as glycogenin located at the core of glycogen molecules. Glycogenin attaching C-1 of a UDP-glucose to a tyrosine residue on the enzyme. The attached glucose is believed to serve as the primer required by glycogen synthase.

The a-1,6 branches in glucose are produced by amylo-(1,4 - 1,6)-transglycosylase, also termed the branching enzyme. This enzyme transfers a terminal fragment of 6-7 glucose residues (from a polymer at least 11 glucose residues long) to an internal glucose residue at the C-6 hydroxyl position.

Regulation of Glycogen Synthesis The activity of glycogen synthase is regulated by phosphorylation of serine residues in the subunit proteins. Phosphorylation of glycogen synthase reduces its activity towards UDP-glucose. When in the non-phosphorylated state, glycogen synthase does not require glucose-6-phosphate as an activator---when phosphorylated it does.

Pathways involved in the regulation of glycogen synthase. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor.

Glycogen synthase activity can be affected by epinephrine binding to a-adrenergic receptors through a pathway like that described above for regulation of glycogen phosphorylase. When a-adrenergic receptors are stimulated there is an increase in the activity of PLC with a resultant increase in PIP2 hydrolysis. which phosphorylates and inactivates glycogen synthase.

. Pathways involved in the regulation of glycogen synthase by epinephrine activation of a-adrenergic receptors. See the text for details of the regulatory mechanisms. PKC is protein kinase C. PLC-g is phospholipase C-g. The substrate for PLC-g is phosphatidylinositol-4,5-bisphosphate (PIP2) and the products are IP3, inositol trisphosphate and DAG, diacylglycerol.

The effects of these phosphorylations leads to: Decreased affinity of synthase for UDP-glucose. Decreased affinity of synthase for glucose-6-phosphate. Increased affinity of synthase for ATP and Pi. Reconversion of synthase-b to synthase-a requires dephosphorylation. This is carried out predominately by protein phosphatase-1 (PP-1) The activity of PP-1 is also affected by insulin. since the role of insulin is to increase the uptake of glucose from the blood. .

Glycogen Storage Diseases Since glycogen molecules can become large, an inability to degrade glycogen can cause cells to become abnormal; it can also lead to the functional loss of glycogen as a source of cell energy and as a blood glucose buffer. Although glycogen storage diseases are quite rare, their effects can be most dramatic.

Table of Glycogen Storage Diseases Type: Name Enzyme Affected Primary Organ Manifestations Type 0 glycogen synthase liver hypoglycemia, early death, hyperketonia Type Ia: von Gierke's glucose-6-phosphatase hepatomegaly, kidney failure, thrombocyte dysfunction Type Ib microsomal glucose-6-phosphate translocase like Ia, also neutropenia, bacterial infections Type Ic microsomal Pi transporter like Ia Type II: Pompe's lysosomal a-1,4-glucosidase, lysosomal acid a-glucosidase acid maltase skeletal and cardiac muscle infantile form = death by 2; juvenile form = myopathy; adult form = muscular dystrophy-like

Type: Name Enzyme Affected Primary Organ Manifestations Type IIIa: Cori's or Forbe's liver and muscle debranching enzyme liver, skeletal and cardiac muscle infant hepatomegaly, myopathy Type IIIb liver debranching enzyme normal muscle enzyme liver symptoms same as type IIIa Type IV: Anderson's branching enzyme liver, muscle hepatosplenomegaly, cirrhosis Type V: McArdle's muscle phosphorylase skeletal muscle excercise-induced cramps and pain, myoglobinuria

Type: Name Enzyme Affected Manifestations Type VI: Her's liver phosphorylase liver hepatomegaly, mild hypoglycemia, hyperlipidemia and ketosis, improvement with age Type VII: Tarui's muscle PFK-1 muscle, RBC's like V, also hemolytic anemia Type VIa, VIII or Type IX phosphorylase kinas liver, leukocytes, muscle like VI Type XI: Fanconi-Bickel glucose transporter-2 (GLUT-2) failure to thrive, hepatomegaly, rickets, proximal renal tubular dysfunction

Explain Von gierke disease can cause: Lactic acidosis hyperuricemia Hypoglycemia Renal stones Brain damage Hyperlipidemia