Glycogen Metabolism - Glycogen Synthesis

Question 1

Why does glycogen biosynthesis require an additional exergonic step?

Answer 1

Since the direct conversion of G1P to glycogen and Pi is thermodynamically unfavourable (positive delta G) under all physiological Pi concentrations

Question 2

What additional exergonic step does glycogen biosynthesis require?

Answer 2

The combination of G1P with uridine triphosphate (UTP) to form uridine diphosphate glucose (UDP-glucose or UDPG): UDGS’s “high-energy” status permits it to spontaneously donate glycosyl units to the growing glycogen chain

Question 3

What enzymes are involved in catalyzing the three steps involved in the glycogen synthesis pathway?

Answer 3

1) UDP-glucose pyrophosphorylase
2) Glycogen synthase
3) Glycogen branching enzyme

Question 4

What reaction does UDP-glucose pyrophosphorylase catalyze?

Answer 4

The reaction of UTP and G1P (phosphoanhydride exchange)

Question 5

Glycogen synthase cannot simply link together two glucose residues, but it can do what?

Answer 5

Extend an already existing alpha(1–>4) -linked glucan chain and yields alpha-amylose

Question 6

How is branching to form glycogen accomplished?

Answer 6

By a 700 residue monomeric enzyme, amylo-(1,4–>1,6)-transglycosylase (glycogen branching enzyme) that transfers a terminal chain segment of 7 glycosyl residues to the C6-OH groups of glucose residues o n the same or another glycogen chain

Question 7

What is the difference between debranching and branching?

Answer 7

• Debranching: Breaks and reforms alpha(1–>4)-glycosidic bonds and hydrolyzes alpha(1–>6)-glycosidic bonds
• Branching: Breaks alpha(1–>4)-glycosidic bonds and reforms alph(1–>6)-glycosidic linkages

Question 8

What is the biological function of glycogen?

Answer 8

To maximize the density of stored glucose units consistent with the need to rapidly mobilize it under conditions of high metabolic demand