Ch. 25 Glycogen Synthesis

What is used to add glucose to glycogen?

UDP-glucose

How is UDP-glucose made?

glucose 1-P + UTP

What catalyzes the transfer of glucose from UDP-glucose?

glycogen synthase

What type of glycosidic bond does glycogen synthase create?

a-1,4

How many molecules of ATP are required to incorporate glucose into glycogen?

2

step 1: glucose 1-P —>

glucose 6-P

step 2: glucose 1-P + UTP —→

UDP-glucose + Ppi

Step 3: ppi + H2O —→

2 Pi

Step 4: UDP-glucose + glycogen_n —→

glycogen_n+1 + UDP

Step 5: UDP + ATP —→

UTP + ADP

What steps require ATP to make glucose into glycogen?

glucose —> glucose 6-P and UDP —> UTP

What does glycogen synthase need to start adding glucose to a chain?

polysaccharide of glucose residues as a primer

glycogenin

generates glucose chain of 10-14 units

branching enzyme

cleaves a-1,4 linkage, moves 7 glucoses, synthesizes a-1,6 bond

glycogen branching mechanism

add glucose molecules onto glycogen, break a-1,4 bond, make a-1,6 bond

glycogen synthase a form

active, dephosphorylated

glycogen synthase b form

inactive, phosphorylated

glycogen phosphorylase a form

active, phosphorylated

glycogen phosphorylase b form

inactive, phosphorylated

What hormones inhibit glycogen synthesis?

glucagon and epinephrine

Why does PKA phosphorylate glycogen synthase a?

to form glycogen synthase 6 and inhibit glycogen synthesis

What other enzyme phosphorylates and inhibits glycogen synthase?

glycogen synthase kinase

When would glycogen synthase be turned off and phosphorylase a be turned on?

during exercise or fasting to generate glucose

protein phosphatase 1 (PP1)

shifts glycogen from degradation to synthesis mode

What does PP1 do to glycogen synthase?

removes phosphate to convert to active form; promotes synthesis

What does PP1 do to glycogen phosphorylase and phosphorylase kinase?

removes a phosphate to inactivate it; stops glycogen breakdown

What does PP1 do to glycogen synthase kinase?

adds a phosphate to inactivate it

How does insulin facilitate glycogen storage?

increases the number of GLUT4 in plasma membrane; increased uptake of glucose

Phosphorylation turns glycogen

break down on

Dephosphorylation turns glycogen

storage on

What does PP1 promote?

storing glucose to build glycogen

What do PKA and phosphorylase kinase promote?

glycogen breakdown