Glycogen Synthesis

Question 1

What evidence convinced researchers that glycogen synthesis and glycogen breakdown occur via separate pathways?

Answer 1

McArdle’s Disease: Pts lack glycogen phosphorylase and cannot break down glycogen (so have muscle pain/cramping on exertion); however, their muscles contain high amounts of normal glycogen. So glycogen synthesis does not require glycogen phosphorylase.

Question 2

Which step in glycogen synthesis pulls the entire reaction to completion?

Answer 2

UDP-Glucose Pyrophosphorylase (the first step)

Question 3

What reaction is coupled with UDP-Glucose Phosphorylase’s conversion of G1P to UDPG and drives forward this otherwise unfavorable reaction.

Answer 3

The hydrolysis of PPi (pyrophosphate).

Question 4

What does PPi stand for?

Answer 4

Pyrophosphate; it is an unstable molecule that reacts easily with H20 to release a lot of free energy.

Question 5

What prevents the reverse reaction of UDPG to G1P in the first step of glycogen synthesis?

Answer 5

PPi is totally eliminated, preventing the reaction from moving in reverse

Question 6

What is the energetic cost of making glycogen?

Answer 6

1 molecule of UTP per molecule of glycogen synthesized

Question 7

How do the energies of UTP, ATP, GTP, and CTP compare?

Answer 7

They are all about the same.

Question 8

Which enzyme in glycogen synthesis transfers a glucose from UDP-glucose to glycogen?

Answer 8

Glycogen synthase

Question 9

Which enzyme in glycogen synthesis creates alpha (1-6) branch linkages?

Answer 9

Glycogen branching enzyme

Question 10

Which enzyme in glycogen synthesis forms alpha(1-4) linkages on non-reducing ends of glycogen?

Answer 10

Glycogen synthase

Question 11

True or false: UTP is equivalent to ATP.

Answer 11

True

Question 12

Which form of glycogen synthase is active?

Answer 12

Form a (dephosphorylated form)

Question 13

Which form of glycogen synthase is inactive?

Answer 13

Form b (phosphorylated form)

Question 14

How is the dephosphorylated form (form a) of glucose synthase activated?

Answer 14

By G6P

Question 15

How do forms a and b of Glycogen synthase compare to forms a and by of Glycogen phophorylase?

Answer 15

In glycogen synthase, form a is dephosphorylated and the active form; in glycogen phosphorylase, phosphorylase a is phosphorylated.
In glycogen synthase, form b is phosphorylated; in glycogen phosphorylase, phosporylase b is dephosphorylated.

Question 16

What controls glycogen synthase activity?

Answer 16

[G6P] and the amount in dephosphorylated form a.

Question 17

True or false: glycogen synthesis can be accomplished by simply joining two glucose molecules together.

Answer 17

False; can only extend a pre-existing chain through alpha (1-4 linkages)

Question 18

What acts as a base structure for the synthesis of glycogen when there is no glycogen present?

Answer 18

Glycogenin

Question 19

What is glycogenin?

Answer 19

The base of every glucose molecule.

Question 20

How many molecules of glycogenin and glycogen synthase does each glucose molecule contain?

Answer 20

one of each

Question 21

How many residues away from another branched point must a branch of glucose be?

Answer 21

At least 4 residues away.

Question 22

How is a branch created by glycogen branching enzyme?

Answer 22

A 7-residue segment from an 11 reside chain to a C6-OH on the same or a different chain.