Glycogen and the Pentose Phosphate Pathway

Question 1

Glycogen

Answer 1

a highly branched polymer of glucose monomers
found primarily in liver and muscle
can be depleted in 12-24 hours (fasting)

Question 2

Key regulated enzyme in glycogen synthesis

Answer 2

• glycogen synthase
• catalyzes transfer of glucose from UDP-glucose to growing chain (G6P–>G1P–>UDP glucose)
• forms alpha 1,4 glycosidic linkage (adds to C4 terminus of glycogen)
• glycogen synthase can only add glucose if polysacc chain has been initiated and already contains more than 4 glucose residues
• Glycogenin forms initiating site for glycogen synthesis.

Question 3

Branching enzyme for glycogen synthesis

Answer 3

• branching enzyme: transfers 6-7 glucose residues to form alpha 1,6 linkages that produce a branch point (at least 4 units away from existing branch point)
• branching provides terminal residues: glycogen breakdown via glycogen phosphorylase
• branching increases rate of glycogen synthesis and degradation

Question 4

steps of glycogen degradation

Answer 4

1. release of G1P from glycogen
2. remodeling of the remaining glycogen to permit further degradation
3. conversion of G1P to G6P

enzymes:
glycogen phosphorylase
debranching enzyme
phosphoglucomutase

Question 5

Glycogen phosphorylase

Answer 5

• cleavage of glycogen to G1P
• glycogen released can be converted to G6P by phosphoglucomutase (interconverts between two)
• glycogen phosphorylase STOPS when 4 residues away from alpha 1,6 glycosidic bond branch point.

Question 6

debranching enzyme

Answer 6

shifts block of 3 glycosyl residues from one outer branch to the other and glycogen phosphorylase continues. This exposes a single glucose residue joined by alpha1,6 glycosidic linkage.
-Glucosidase hydrolyzes the last alpha 1,6 glycosidic bond to yield free glucose molecule.

Question 7

G6P

Answer 7

in muscle: can enter glycolysis
in liver: can be converted to free glucose and exported to maintain blood glucose levels during fasting state (glucose-6-phosphatase)

Question 8

Glucagon results in ___

Insulin results in ___

Answer 8

Glucagon (exercise/fasting) results in phosphorylation

Insulin (fed state) results in dephosphorylation

Question 9

Glycogen synthase activation in liver/ muscle

Answer 9

G6P (+)

Question 10

Glycogen phosphorylase inhibition/activation in liver/muscle

Answer 10

Liver
G6P (-)
ATP (-)
Glucose (-)

Muscle
inhibition:
G6P
ATP
activation:
AMP (allosteric/direct)
Ca

Question 11

Activation of glycogen degradation by cAMP producing pathways

Answer 11

-glucagon or epi (counter reg hormones) binds to recep to cell surface–> signals need for glycogen degradation
-2 enzymes are affected: phosphorylase kinase and glycogen phosphorylase
-Binding of epi/gluc activates PKA, which phosphorylates phosphorylase kinase.
-Phosphorylase kinase b (inactive) is converted to phosphorylase kinase a (active) by phosphorylation thru PKA. (Insulin can initiate a cascade that inactivates phosphorylase kinase a using protein phosphatase 1)
-Glycogen phosphorylase also exists in two forms:
dephosphorylated inactive b form
phosphorylated active a form
-Active phosphorylase kinase phosphorylates glycogen phosphorylase b to its active a form, which begins glucose breakdown
-Phosphoprotein phosphatase 1 (PP1) converts glycogen phosphorylase a to b by dephosphorylation
-Phosphorylase kinase b is activated by phosphorylation and phosphorylates glycogen phosphorylase.

Question 12

Inhibition of glycogen synthesis by cAMP-directed pathway

Answer 12

• Glycogen synthase a and b
• a is the active form, but is DE-phosphorylated
• b is inactive and phosphorylated
• Glucose 6 phosphate binds to an allosteric site on glycogen synthase b: making it a better substrate for dephosphorylation by PP1
• Epi and glucagon lead to activation of cAMP protein kinase (PKA)
• PKA phosphorylates and inactivates glycogen synthase
• PP1 can remove phos groups from: phosphorylase kinase, glycogen phosphorylase, and glycogen synthase
• Insulin stimulates glycogen synthesis by activating PP1 and by inactivating GSK3

Question 13

What can PP1 dephosphorylate?

Answer 13

phophorylase kinase
glycogen phosphorylase
glycogen synthase

Question 14

Regulation of glycogen phosphorylase in liver

Answer 14

• low blood glucose, glucagon activates glycogen phosphorylase kinase, which converts b to a form of glycogen phosphorylase–> glucose into blood
• glucose normal: glucose enters hepatocytes and binds to allosteric site on glycogen phosphorylase a–> conformational change–> phosphatase removes phos, converts it to inactivate glycogen phosphorylase b, turns off glycogenolysis and glucose release

Question 15

What inhibits glycogen phosphorylase?

Answer 15

ATP
G6P
Glucose (liver)
AMP (muscle)

Question 16

What does the Pentose phosphate pathway produce?

Answer 16

• NADPH (for synth of fatty acids and steroids)
• ribose-5-phosphate (for synthesis of nucleotides– purines and pyrimidies (like ATP, GTP, UTP)
• glycolytic intermediates

NADPH is part of oxidative phase
ribose 5 phosphate and glycolytic int part of non-oxidative phase

Enzymes of pathway located in cytosol.

Question 17

Where is the PPP prominent?

Answer 17

mammary gland
adrenal cortex
liver
adipose tissues
(where fatty acid and steroid synthesis are common)

Question 18

First rxn in PPP

Answer 18

• first rxn is catalyzed by glucose-6-phosphate dehydrogenase (G6PD) (key committed and rate related step)
• generates first NADPH
• a subsequent step of dehydrogenation and decarboxylation produce another NADPH and 5 C sugar ribulose-5-phosphate

Question 19

Oxidative vs non-oxidative phase of PPP

Answer 19

oxidative: generates NADPH

Non-oxidative: interconverts sugars back to glucose-6-phosphate if more NADPH or pentose phosphates are needed or to glycolytic intermediates if more energy is needed.
-rearrangements/transfers convert ribulose-5-phosphate to ribose-5-phosphate and to glycolytic intermediates. Accomplished by transfer of 2C and 3C units. (3 5C sugars are converted to 2 6C sugars and 1 3C sugar)

Question 20

G6PD deficiency

Answer 20

• NADPH provides the reducing equivalent for redox rxns, esp those involving glutathione (GSH).
• NADPH maintains glutathion in a reduced state
• G6PD deficiency? unable to regenerate GSH to guard agains ROS, and sulfhydryl groups in hemoglobin become oxidized–> cross links and aggregates in RBCs called “Heinz bodies”
• rigid RBC membranes leading to destruction and hemolytic anemia