Glycogen breakdown

Question 1

what is glycogen?

Answer 1

it is a branched polymer and the storage form of glucose

Question 2

how is glycogen turned into glucose in the liver?

Answer 2

glycogen to glucose-1-phosphate to glucose-6-phosphate

Question 3

what are the links of glycogen?

Answer 3

and alpha (1-4) linked D-glucose with alpha (1-6) every 8-14 residues

Question 4

where are glucose unites removed from glycogen?

Answer 4

from the non reducing ends. Note that there is only one reducing end (linked to glycogenin)

Question 5

how is glycogen found in the body?

Answer 5

it is stored in glycogen granules, of 100-400 A° diameter and it stores up to 120,000 glucose

Question 6

muscle and liver contain how much glycogen by weight?

Answer 6

muscle=1-2%, liver=10%

Question 7

what are the enzymes of glycogenolysis

Answer 7

glycogen phosphorylase, glycogen debranching enzyme, phosphoglucomutase

Question 8

function of glycogen phosphorylase?

Answer 8

catalyzes glycogen phosphorolysis to make G1P. it only acts on glucose units at least 5 units away from a branch point

Question 9

function of phosphoglucomutase?

Answer 9

convert G1P into G6P

Question 10

what is phosphorolysis

Answer 10

bond cleavage occurs by substitution of a phosphate group

Question 11

what is PLP (pyridoxal 5’ phosphate)

Answer 11

it is the prosthetic group of phosphorylase, it is covalently linked to the enzyme by a Schiff base (between its aldehyde group and the E-amino group of Lys-680)

Question 12

what is the mechanism of glycogen phosphorylase?

Answer 12

1- we have glycogen + a phosphate group + the glycogen phosphorylase enzyme.
2- all three together form a ternary complex
3- the non-reducing end of glycogen breaks its bond with the chain by donating an electron (from 1-4) to the phosphate group which is passed onto the enzyme.
4- this forms a shielded oxonium ion intermediate which has a half chair formation.
5- all the electrons travel back and form and alpha-D-glucose-1-phosphate

Question 13

what are the 2 catalytic properties of the glycogen debranching enzyme?

Answer 13

alpha (1-4) transglycosylase and alpha(1-6) glucosidase (this yields glucose)

Question 14

which is faster the glycogen debranching enzyme or the glycogen phosphorylase?

Answer 14

the glycogen phosphorylase

Question 15

explain the phosphoglucomutase mechanism

Answer 15

1- we have G1P + the enzyme (which has a phosphate group)
2- the enzyme phosphorylate G1P to make it glucose -1,6-biphosphate
3- the enzyme gets phosphorylate from the substrate at C1, this leaving us with a G6P

Question 16

G6P is turned into glucose by the action of which enzyme in the ER membrane? in the cytosol?

Answer 16

G6Pase, G6P translocase

Question 17

how does glucose leave the liver?

Answer 17

in GLUT2

Question 18

The inactive T form of phosphorylase b into the active R form of phosphorylase b is controlled By? The reverse?

Answer 18

AMP. The reverse is controlled by ATP/G6P. It is kept mostly in the T form

Question 19

Phosphorylase A is usually in the ? form. It turns into the T form if there is a high level of ?

Answer 19

Active form. Glucose

Question 20

Which enzyme catalyses phosphorylase b inactive into phosphorylase a inactive? Which does the reverse?

Answer 20

Phosphorylase kinase. Phosphoprotein phosphatase

Question 21

Explain the conformational change from the T state to the R state

Answer 21

The side chain of Arg 569 reorients in the transition so as to interact with the substrate phosphate.
The T state enzyme has a buried active site hence a low affinity for its substrate, while the R state has an accessible catalytic site and a high affinity phosphate binding site.
AMP bind to the R state in its allosteric effector site, and causes the phosphorylases tower helixes to tilt and pull apart so it has a more efficient packing.
ATP bonds to the T state enzyme and it inhibits the shift onto the R state

Question 22

What are glycogen storage diseases? Which enzymes deficiency leads to glycogen build-up? What are a few symptoms of this disease?

Answer 22

They are genetic enzyme deficiencies associated with excessive glycogen accumulation. G6Pase and phosphofructokinase.
When it effects the liver it cause hypoglycemia. When it effects muscle it causes weakness and cramps upon exertion

Question 23

Go to notes and read lysosomal storage disease

Answer 23

It’s only a paragraph