MCB 25

Question 1

Describe how glucose regulates glycogen metabolism in the liver

Answer 1

Glucose binds to glycogen phosphorylase A (R state) and inhibits T state

T state glycogen dissociates from PP1, activating it

PP1 is free, dephosphorylates glycogen phosphorylase A and glycogen synthase b, inhibiting glycogen degradation and stimulating glycogen synthesis

Question 2

State two organs in the body where glycogen is found

Answer 2

Liver
Muscle

Question 3

State the purpose of glycogen degradation in the liver and muscle

Answer 3

Liver: glycogen broken down to release glucose to provide energy for red blood cells and brain

Muscle: Glycogen broken down to release glucose to provide energy for muscle contractions

Question 4

State the two types of bonds / linkages between glucose molecules in a glycogen polysaccharide

Answer 4

Alpha 1,4 glycosidic bonds

Alpha 1,6 glycosidic bonds

Question 5

Glycogen degradation Step 1

Answer 5

Glycogen phosphorylase cleaves glycogen from non-reducing ends

Catalyses phosphorolysis of glycogen into glucose-1-phosphate

Question 6

State the issue with glycogen phosphorylase

Answer 6

Only cleaves a-1,4-glycosidic bonds
Cannot cleave branched areas of glycogen chain (alpha-1,6-glycosidic bonds)

Question 7

State the issue with glycogen phosphorylase

Answer 7

Only cleaves a-1,4-glycosidic bonds
Cannot cleave branched areas of glycogen chain (alpha-1,6-glycosidic bonds)

Glycogen phosphorylase stops 4 residues (molecules near a branch point)

Question 8

Glycogen degradation step 2

Answer 8

Glycogen remodelling

Transferas transfers small oligosachharide from branching point to near chain, exposes glucose to glycogen phosphorylase

Then, debranching enzyme, a-1,6-glucosidase cleaves alpha-1,6-glycosidic bonds at branch points, releasing free glucose

Question 9

Glycogen degradation step 3

Answer 9

Phosphoglucomutase catalyses glucose-1-phosphate to glucose-6-phosphate, this includes a glucose,1-6, bisphosphate intermediate

This involves a transfer of phosphoryl group from the enzyme to the substrate and then the transfer of a different phosphate group back to the enzyme to restore it to its initial state

Then, glucose-6-phophate is converted to glucose via glucose-6-phosphatase in the liver. Free glucose released. Enters blood

Question 10

State the key regulatory enzyme for glycogen degradation

Answer 10

Glycogen phosphorylase

Question 11

Describe the structure of glycogen phosphorylase

Answer 11

Dimer, exists in 2 forms

a form (R state) - more actvie
b form (T state) - less active

Question 12

State the structural difference between glycogen phosphorylase in the R and T state

Answer 12

R state - phosphorylated serine residue (phosphoserine)

T state - no phosphorylated serine residue (serine)