03: Carbohydrate Metabolism: Glycogenesis and Glycogenolysis

Glycogenesis

Structure of Glycogen

• Storage form of glucose; branched polymer

• Multiple α–D-glucose units linked by α-(1,4) glycosidic bonds

  • Linear chain of α–D-glucose

• Every 12 or 14 glucose residues, a α-(1,6) bond is created

  • α-(1,6) bond creates a branch point on the linear chain

• Advantages of branching

  • More glucose units can be “packed” into glycogen’s structure

  • Prevents crystallization of glucose

• 

General Structure of a Glycogen Particle

Glycogen

• Glycogen is the storage form of carbohydrate in animals

• Glycogenesis: Synthesis of glycogen from α-D-glucose

• Anabolic process (anabolic, catabolic)

• Main stores of glycogen: Liver and skeletal muscle

• Liver glycogen:

  • Function: maintain blood glucose concentration

  • Increase in the well-fed state

  • Decrease in the starvation state

• Muscle glycogen:

  • Function: fuel reserve for ATP synthesis during exercise

  • Decreased levels due to strenuous muscle activity

  • Synthesized when glycogen stores are depleted

Flow Diagram of Glycogen Metabolism

Glycogenesis: Full Process

• 

• step #	action type	enzyme
  1	conversion of glucose 6-phosphate to glucose 1-phosphate	phosphoglucomutase
  2	synthesis of UDP-glucose	UDP- glucose pyrophoshorylase
  3	synthesis of an initiating primer	glycogenin
  4	chain elongation	glycogen synthase
  5	formation of branches (creation of 1.6 branches and extension of 1,4 branched chain)	4:6 transferase (branching enzyme)

Step 1: conversion of glucose 6-phosphate to glucose 1-phosphate

• Glucose 1-phosphate and glucose 6-phosphate: isomers

  • Interconversion catalyzed by phosphoglucomutase

• 

Step 2: synthesis of UDP-glucose

• UDP-glucose is synthesized from glucose 1-phosphate (step 1) and UTP

  • UDP-glucose pyrophosphorylase hydrolyzes UTP by UDP-pyrophosphorylase

• 

  • This α-D-glucose molecule is added to the growing glycogen chain in step 4

Step 3: synthesis of an initiating primer

• Glycogenin is a priming glucosyltransferase

• Glycogenin serves as an acceptor of glucose residues

• A glucose unit is attached to glycogenin via Tyr –OH group

• This forms the initiating primer

• Incoming α-D-glucose units from UDP-glucose are transferred to the primer

• Chain begins to grow

• 

• 

Step 4: Chain Elongation by Glycogen Synthase

• The transfer of glucose units from UDP-glucose by glycogen synthase leads to chain elongation of glycogen

  • The transfer occurs at the non-reducing end

    • The reducing end is the glycogenin-linked region

• The glucose units are linked serially as α-(1,4) glycosidic bonds

• 

• 

Step 5: branching

• 4:6 transferase (glycogen branching enzyme)

  • Amylo-α-(1,4),α-(1,6) transglucosidase

• Transfers 5-8 residues from the non-reducing end [which has α-(1,4) linkage] to another glucose 1-phosphate residue on the chain

  • This creates a branch point with [α-(1,6) linkage] point

  • Branching allows more non-reducing ends to be created

  • Helps packs in more α-D-glucose units on the glycogen particle

• 

Glycogenolysis

Glycogenolysis

• Definition: The breakdown of glycogen to release glucose 1-phosphate and α-D-glucose

• Glycogenolysis is a catabolic pathway (anabolic, catabolic)

  • A different set of enzymes than in glycogenesis

  • Pathway is not a direct reversal of the reactions of glycogenesis

• Glycogenolysis involves the following steps, catalyzed by specific enzymes

• step #	action type	enzyme
  1	shortening of glycosidic chains	glycogen phosphorylase
  2	de-branching (move 3 resides)	4:4 transferase (debranching enzyme #1)
  3	de-branching (move 1 glucose)	1:6 glucosidase (debranching enzyme #2)
  4	conversion of glucose 1-phosphate to glucose 6-phosphate	phosphoglucomutase

Step 1: shortening of glycosidic chains

• Glycogen phosphorylase

  • Phosphorolysis

  • Pyridoxal phosphate (prosthetic group)

• α-(1,4)-linkage of glycogen cleaved from non-reducing end

  • Glucose 1-phosphate is released

• 

• Enzyme continues until four glucosyl units remain from the branch point

  • This four unit “stump” is called limit dextrin

• 

Step 2: de-branching (remove 3 resides)

• Limit dextrin initiates the action of a debranching enzyme

• 4:4 transferase enzyme (debranching enzyme #1)

  • Oligo-α-(1,4), α-(1,4)-glucantransferase

• 

• 4:4 transferase enzyme removes 3 residues from limit dextrin

  • These 3 residues are transferred to the non-reducing end of another chain

  • 1 residue is left attached (removed in step 3)

• One α-(1,4) bond is broken and a new α-(1,4) bond is made

• 

Step 3: de-branching (remove 1 glucose)

• The lone glucose residue attached by α-(1,6)-glycosidic bond is removed by hydrolysis

  • Amylo-α-(1,6) glucosidase activity (1:6 glucosidase)

  • Debranching enzyme #2

  • Glycogen phosphorylase is used again for degradation if needed

• 

Step 4: Conversion of glucose 1-phosphate to glucose 6-phosphate

• Glucose 1-phosphate (from Step 1) is converted to glucose 6-phosphate by phosphoglucomutase

• In the Liver

  • Glucose 6-phosphate is transported to the ER by glucose 6-phosphate

    translocase

  • In the ER, glucose 6-phosphate is converted to glucose by glucose 6- phosphatase

  • Glucose is released into the blood stream

  • 

• In the Muscle

  • Glucose 6-phosphate levels build up

    • Muscle lacks glucose 6-phosphatase (see gluconeogenesis)

    • What should the muscle cells do now? They will not make glucose … ☹

      • Cell needs energy. What should it do? Start GLYCOLYSIS ... from step 2 (Glucose 6-phosphate to Fructose 6-phosphate)

Glycogen Breakdown: Summary

• Glycogen phosphorylase works on non-reducing ends until it reaches four residues from an (α1 → 6) branch point.

• Debranching enzyme #1 transfers a block of three residues to the non-reducing end of the chain

• Debranching enzyme #2 cleaves the single remaining (α1 → 6)-linked glucose, which becomes a free glucose unit (i.e., NOT glucose-1- phosphate).

• 

Regulation of Glycogen Metabolism

• Synthesis and breakdown of glycogen are tightly regulated

  • Importance in maintaining blood glucose levels

• In the liver

  • Well fed state: glycogenesis accelerates

  • Fasting state: glycogenolysis accelerates

• In the muscle

  • Active exercise: glycogenolysis accelerates

  • At rest: glycogenesis accelerates

• Regulation occurs at two levels

  • Receptor level

    • Allosteric control of glycogenesis and glycogenolysis

  • Hormonal level

    • Glycogen phosphorylase and glycogen synthase are hormonally regulated by insulin, glucagon and epinephrine (aka adrenaline)

      • Insulin generally opposes the effects of glucagon and epinephrine

      • Insulin favors glycogen synthesis

      • Glucagon and epinephrine favor glycogen breakdown

Hormonal Stimulation of Glycogen Synthesis

• 

  • Glycogen synthase a : active form, dephosphorylated

  • Glycogen synthase b : inactive form, phosphorylated

  • Glycogen phosphorylase a : active form, phosphorylated

  • Glycogen phosphorylase b : inactive form, dephosphorylated

• Insulin binds to a receptor and activates a tyrosine kinase

• The receptor tyrosine kinase phosphorylates an insulin-sensitive kinase

• Insulin-sensitive kinase phosphorylates Phosphatase-1

  • “active phosphate group” is created

• Activated Protein Phosphatase-1 (PP-1) activates glycogen synthase

  • Leads to glycogen synthesis

• Activated Protein Phosphatase-1 (PP-1) inhibits phosphorylase kinase

  • Phosphorylase kinase activates glycogen phosphorylase

    • Inhibition of phosphorylase kinase thus prevents the breakdown of glycogen

    • Indirect inhibition of glycogen phosphorylase

Influence of insulin on glucose transport, glycolysis, and glycogenesis

Hormonal Stimulation of Glycogen Breakdown

• Glucagon and epinephrine promotes glycogen breakdown and stops glycogen synthesis

• Glycogen synthase exists in two forms:

  • Glycogen synthase a : active form, dephosphorylated

  • Glycogen synthase b : inactive form, phosphorylated

• Glycogen phosphorylase exists in two forms:

  • Glycogen phosphorylase a : active form, phosphorylated

  • Glycogen phosphorylase b : inactive form, dephosphorylated

• Hormonal Regulation of Glycogen Breakdown

  • 

    • Glycogen synthase a : active form, dephosphorylated

    • Glycogen synthase b : inactive form, phosphorylated

    • Glycogen phosphorylase a : active form, phosphorylated

    • Glycogen phosphorylase b : inactive form, dephosphorylated

• Epinephrine or glucagon activates adenylyl cyclase

• Adenylyl cyclase catalyzes the generation of cyclic AMP (cAMP)

• cAMP activates a cAMP-dependent protein kinase

  • Higher the levels of cAMP, greater the number of activated protein kinase molecules

• cAMP-dependent protein kinase phosphorylates both glycogen synthase and glycogen phosphorylase

  • Glycogen synthase becomes inactive; glycogen synthesis stops

  • Glycogen phosphorylase becomes active; glycogen breakdown is favored

Influence of glucagon and epinephrine on glycolysis, gluconeogenesis and glycogenesis

Coordination of glycogen breakdown with glycogen synthesis

• Glycogen phosphorylase

• Glycogen synthase

• Synthesis and breakdown of glycogen is coordinated

• When one process is active, the other is shut off

• The hormone signaling system is common to both pathways

• The effects of phosphorylation on the two enzymes have opposing effects

• Reciprocal regulation!

Flow Diagram of Glycogen Metabolism