Lecture 7 - Glycogen Metabolism

Question 1

a

Question 2

where is glycogen stored?

Answer 2

glycogen is stored predominantly in the liver and muscle as energy store

Question 3

what are the differences in uses for the glycogen found in the liver and glycogen found in the muscles?

Answer 3

liver glycogen is utilised to maintain plasma glucose levels between meals whereas muscle glycogen is required to maintain muscle contractions

Question 4

how will glycogen exist chemically at equilibrium?

Answer 4

at equilibrium, the mixture consists of about 36% a-D-glucose, 64% b-d-glucose [both cyclic] & less than 0.02% of the open chain aldehyde form

Question 5

what two linkages are used in the formation of glycogen?

Answer 5

1,6 linkages & 1,4 linkages

Question 6

how are the 1,4 linkages in glycogen broken?

Answer 6

by phosphorolysis catalysed by the enzyme (glycogen) phosphorylase to give 1 molecule of glucose 1-phosphate from each end

Question 7

mobilisation:

Answer 7

process in which metabolic need cells can switch on the breakdown of stored glycogen very rapidly

Question 8

what is pyridoxal phosphate (PLP)?

Answer 8

a derivative of vitamin B6 it serves as a prosthetic group for glycogen phosphorylase

Question 9

what reaction does phosphoglucomutase facilitate?

Answer 9

Glucose-1-phosphate → Glucose-6-phosphate

Question 10

what does phosphoglucomutase give from glycogen when it reacts?

Answer 10

it will give one molecule of glucose from each end

Question 11

what is PHOSPHOROLYSIS is analogous to?

Answer 11

PHOSPHOROLYSIS is analogous to hydrolysis (with phosphate acting like water in hydrolysis reactions).
NOTE that ATP is NOT involved in this reaction

Question 12

what activates cAMP cascades in the liver and the muscle?

Answer 12

Glucagon activates cAMP cascades in the liver and Epinephrine (Adrenalin) does so in muscle cells

Both hormones, are produced in response to low blood glucose, thus releasing glucose from glycogen when needed

Question 13

when does adrenaline increase greatly in the muscles?

Answer 13

Epinephrine (Adrenalin) levels rise greatly during exercise (such as medium to long distance running etc.) when the metabolic demands of muscle are high

Question 14

what do the rises of cAMP (as a result of cAMP cascades) do in the liver and muscles?

Answer 14

In both cases, the rise in cAMP levels result in activation of glycogen phosphorylase

Question 15

kinases and phosphorylase’s:

Answer 15

Kinases and Phosphorylase, both refer to enzymes involved in phosphorylation of a substrate in cellular processes.
-Kinases transfer phosphate groups from ATP to proteins.
- Phosphorylases catalyse the transfer of a phosphate group from a donor to an acceptor (or a substrate) molecule.

Question 16

in response to epinephrine:

Answer 16

The cAMP cascade results in phosphorylation of a serine hydroxyl of muscle glycogen phosphorylase, which promotes transition to the active (relaxed) state

Question 17

what happens during glycogen breakdown when you hit a 1-6 linkage?

Answer 17

α1-6 linkages at the branch points need to be digested by the α1-6 glucosidases enzyme by hydrolysis.

Question 18

what is the difference between the digestion of 1,4 & 1,6 linkages?

Answer 18

1,4 linkages are broken by phosphorylase

1,6 linkages are digested by a1-6-glucosidases

Question 19

phosphorylase can only break 1:4-links up to: [enzymes as a team]

Answer 19

Phosphorylase can only break -1:4-links up to within 4 residues (glucose units) of a branch point.

At this point a transferase activity takes three residues and transfers them to the end of another chain.

The single glucose unit left from the branch is removed by the action of -1:6-glucosidase. esidues (glucose units) of a branch point

Question 20

what gives gives UDP-glucose from Glucose 1- P and UTP

Answer 20

UDP-glucose pyrophosphorylase

Question 21

how is glycogen formed from UDP-glucose?

Answer 21

glycogen synthase

Question 22

How does a glycogen synthesis start?

Answer 22

The synthesis involves ADDITION of new UDP-glucose units onto the end of an EXISTING glycogen chain – sometimes called a PRIMER.

Question 23

Glycogen synthase can add glucose units only to a:

Answer 23

pre-existing chain of more than four glucosyl residues.

Question 24

the priming function of glycogen synthesis is carried out by:

Answer 24

glycogenin

Question 25

where is glycogenin found?

Answer 25

Glycogenin is found free in the cell but also remains at the core of the glycogen granule it has initiated

Question 26

Glycogenin:

Answer 26

is a protein dimer composed of two identical subunits of Mr 37,000. An oligosaccharide of four glucosyl units is built up on each molecule

Question 27

what’s elongates the chains in glycogen synthesis and adds branches?

Answer 27

Glycogen synthase can then elongate the chains. A separate branching enzyme introduces the branches

Question 28

UDP-GLUCOSE forms a covalent link with a conserved tyrosine residue of Glycogenin

Answer 28

UDP glucose donates the first glucosyl residue to glycogenin

Carbon 1 of the glucose forms a covalent link with the hydroxyl group of a specific tyrosine residue in the glycogenin protein sequence

Question 29

how does glycogen branch?

Answer 29

Glycogen synthase can add glucose units only to a pre-existing chain of more than four glucosyl residues.
Another enzyme is required to generate the α-1:6 linkages needed to form a branch.

The branching enzyme transfers a block of 7 residues (including the non-reducing end) to a more interior site, creating a new α-1:6 -glycosidic linkage.

Question 30

why is glycogen a good energy store?

Answer 30

Glycogen is a GOOD energy store because it can be mobilised very rapidly (branched structure)

Question 31

Glycogen is a BAD energy store because:

Answer 31

Glycogen is a BAD energy store because glucose is hydrophilic. Therefore, water associates with glycogen granules, increasing the overall weight and bulk of

Question 32

The enzymes phosphorylase and glycogen synthase are very sensitive to regulation by:

Answer 32

hormones, stress and muscle contraction