MCB 25 Flashcards
Describe how glucose regulates glycogen metabolism in the liver
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
State two organs in the body where glycogen is found
Liver
Muscle
State the purpose of glycogen degradation in the liver and muscle
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
State the two types of bonds / linkages between glucose molecules in a glycogen polysaccharide
Alpha 1,4 glycosidic bonds
Alpha 1,6 glycosidic bonds
Glycogen degradation Step 1
Glycogen phosphorylase cleaves glycogen from non-reducing ends
Catalyses phosphorolysis of glycogen into glucose-1-phosphate
State the issue with glycogen phosphorylase
Only cleaves a-1,4-glycosidic bonds
Cannot cleave branched areas of glycogen chain (alpha-1,6-glycosidic bonds)
State the issue with glycogen phosphorylase
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)
Glycogen degradation step 2
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
Glycogen degradation step 3
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
State the key regulatory enzyme for glycogen degradation
Glycogen phosphorylase
Describe the structure of glycogen phosphorylase
Dimer, exists in 2 forms
a form (R state) - more actvie
b form (T state) - less active
State the structural difference between glycogen phosphorylase in the R and T state
R state - phosphorylated serine residue (phosphoserine)
T state - no phosphorylated serine residue (serine)
