Chapter 21

Question 1

The efficient breakdown of glycogen requires requires 4 enzyme activities. Name them

Answer 1

1. to degrade glycogen
2. to remodel glycogen (2)
3. convert the product of glycogen into a form suitable for further metabolism

Question 2

What does glycogen phosphorylase do?

Answer 2

cleaves the substrate by the addition of Pi to yield G1P

Question 3

What do phosphorylases do?

Answer 3

catalyze the sequential removal of glucosyl residues from the nonreducing ends of the glycogen molecule (the ends with a free OH group on carbon 4)

Question 4

Why is a debranching enzyme necessary in the breakdown of glycogen?

Answer 4

the alpha1,6 bonds at branch points are not susceptible to cleavage by phosphorylase

Question 5

How do the debranching enzymes work?

Answer 5

the transferase shifts a block of three glucosyl residues from one outer branch to the other
a1,6-glucosidase hydrolyzes the a1,6 glycosidic linkage exposed by the transferase
a free glucose molecule is released and then phosphorylated by hexokinase

Question 6

T/F

Phosphoglucomutase has kinase activity.

Answer 6

True

Question 7

How is G1P converted to G6P?

Answer 7

Phosphoglucomutase transfers a phosphoryl group to G1P forming G16BP
the enzyme then from the phosphoryl group from C1 creating G6P

Question 8

What are the two forms of phosphorylase?

Answer 8

a: mostly in R state (has Pi)
b: mostly in T state (no Pi)

Question 9

What form of phosphorylase resides in the muscle?

Answer 9

Since the muscle stores G, the b form of phosphorylase is in the muscle

Question 10

What are the activators of phosphporylase in the muscle?

Answer 10

AMP - put b in R state
ATP - put b in T state
G6P - put b in T state

Question 11

What form of phosphorylase resides in the liver?

Answer 11

since the liver uses G, the a form of phosphorylase resides in the liver

Question 12

What is the only activator of phosphorylase in the liver?

Answer 12

glucose (G) - puts a in the T state

Question 13

What enzyme phosphorylates phosphorylase?

Answer 13

glycogen phosphorylase kinase
ATP + GP”b” —————-> ADP + GP”a”-Pi
GPK

Question 14

Who takes Pi off phosphorylase?

Answer 14

protein phosphatase 1 (PP1)

Question 15

What does glycogen synthase need to build a glycogen granule?

Answer 15

It needs 4 glucose molecules to add more glucose molecules. However, a primer is needed to put 4 glucose molecules together to “start” glycogen granule

Question 16

How is a glycogen granule “started?”

Answer 16

glycogenin attaches glucose to Tyr in its active site and acts as a primer to build 4 glucose molecules

Question 17

What type of linkage does glycogen synthase form?

Answer 17

a-1,4

Question 18

Why does glucose need a-1,6 branches?

Answer 18

to increase solubility/directly store glucose

Question 19

How does the branching enzyme work?

Answer 19

has glucosidase activity; cut a-1,4 (after 7 G)

transfers 7 G molecules down to make new a-1,6 linkage

Question 20

What does it mean when the enzymes are reciprocally regulated?

Answer 20

1 works while the other doesn’t

synthase works ——–phosphorylase doesn’t

Question 21

What are the two forms of the synthase?

Answer 21

a —- active (R state)

b —- inactive (T state, w/Pi)

Question 22

What is one of the major functions of the liver?

Answer 22

to maintain a nearly constant level of glucose in the blood

Question 23

How does glucose leave the liver?

Answer 23

the liver contains a hydrolytic enzyme, glucose-6-phosphotase that enables glucose to leave

Question 24

What hormones signal the need for glycogen breakdown?

Answer 24

epinephrine and glucagon

Question 25

How do hormones trigger the breakdown of glycogen?

Answer 25

they initiate a cyclic AMP signal-transduction cascade

Question 26

What is the first step of cAMP?

Answer 26

the signal molecules epinephrine and glucagon bind to specific transmembrane receptors in the plasma membrane of target cells
these binding events activate Gs protein
a specific external signal has been transmitted into the cell through structural changes, first in the receptor and then in the G protein

Question 27

What is the second step of cAMP?

Answer 27

The GTP bound subunit of Gs activates the transmembrane protein adenylate cyclase. This enzyme catalyzes the formation of the second messenger cyclic AMP from ATP.

Question 28

What is the third step of cAMP?

Answer 28

the elevated cytoplasmic level of cyclic AMP activates protein kinase A. The binding of cyclic AMP to inhibitory regulatory subunits triggers their dissociation from the catalytic subunits. The free catalytic subunits are now active

Question 29

What is fourth step of cAMP?

Answer 29

protein kinase A phosphorylates phosphorylase kinase, which subsequently activates glycogen phosphorylase