glycogen metabolism

Question 1

what enzyme removes a G-1-P from glycogen?
in what state does it do this (active/inactive)

Answer 1

glycogen phosphorylase a
active

Question 2

what converts glycogen phosphorylase a to glycogen phosphorylase b, inactivating it

Answer 2

protein phosphatase 1G
(and glucose-6-phosphate)
dephosphorylation reaction

Question 3

what converts glycogen phosphorylase b to glycogen phosphorylase a, activating it

Answer 3

phosphorylase kinase a

Question 4

what converts phosphorylase kinase b to a, activating it?

Answer 4

PKA

Question 5

what increases intracellular PKA levels?

Answer 5

glucagon and adrenaline via the cAMP cascade

Question 6

what enzyme catalyses the addition of G-1-P to glycogen?

Answer 6

glycogen synthase a
(active)

Question 7

what converts glycogen synthase a to glycogen synthase b, inactivating it?

Answer 7

GSK3 (glycogen synthase kinase 3)
phosphorylation reaction

Question 8

what converts GSK3-P to its active form GSK3?

Answer 8

PKB

Question 9

what converts glycogen synthase b to a, activating it?

Answer 9

protein phosphatase 1G

Question 10

what inhibits protein phosphatase 1G?

Answer 10

protein phosphatase inhibitor-1
PPI1 activated by PKA

Question 11

how is protein phosphatase inhibitor-1 inactivated?

Answer 11

phosphorylation by protein phosphatase-2A

Question 12

what are the biochemical effects of insulin on the balance of glycogenesis to glycogenolysis

Answer 12

1. PI3 cascade, activates PKB, inactivates GSK3, stops conversion of glycogen synthase a to b (activates)
2. activates protein phosphatase 1G, promotes conversion of glycogen synthase b to a (activates)
3. activates protein phosphatase 1G, promotes conversion of glycogen phosphorylase a to b (inactivates)
   overall promotes conversion of glucose to glycogen

Question 13

what are the biochemical effects of cAMP (causes by glucagon/adrenaline secretion) on the balance of glycogenesis to glycogenolysis

Answer 13

1. activates PKA, activates phosphorylase kinase, activates glycogen phosphorylase a
2. activates PKA to convert PP1G into a low activity form, therefore glycogen synthase b is not converted to a
   overall the breakdown of glycogen is promoted
3. activates PKA to phosphorylate sites 1 and 2 of regulatory subunit 1G to inactivate PP1 as above during the adrenaline-induced emergency, but immediately dephosphorylates site 2 after adrenaline stops so that PP1 can activate glycogen synthase to replenish stores

Question 14

which hormones stimulate glycogenolysis in the liver

Answer 14

glucagon and adrenaline

Question 15

which hormone stimulates glycogenolysis in the muscles

Answer 15

just adrenaline
(do not have receptors for glucagon)

Question 16

how much adrenaline is needed to mobilise enough glucose to use as respiratory fuel?

Answer 16

nanomolar amounts

Question 17

what happens when adrenaline docks to its receptor in muscle?

Answer 17

signal transduced through G protein, adenylyl cyclase, ATP -> cAMP, activates PKA

Question 18

what are the metabolic opponents of PKA

Answer 18

protein phosphatases

Question 19

how does cAMP activate PKA?

Answer 19

when inactive, PKA is a tetramer of 2 catalytic and 2 regulatory subunits
cAMP binds the 2 regulatory units and liberates the active subunits

Question 20

how is protein phosphatase-1 inactivated?

Answer 20

PKA phosphorylates site 2 of regulatory subunit of protein phosphatase-1G complex
protein phosphatase 1 dissociates from the complex
is now vulnerable to attack from protein phosphatase inhibitor 1

PPI1 has to be activated by phosphorylation by PKA
it is deactivated at rest by protein phosphatase 2A

Question 21

where is glycogen phosphorylase b phosphorylated to form a?

Answer 21

serine at N-14 by phosphorylase kinase

Question 22

what state is phosphorylase b in in resting muscle?

Answer 22

inactive, T form

Question 23

what is the effect of AMP on phosphorylase b?

Answer 23

stimulates it by forming R form which has higher affinity for phosphate

Question 24

what molecules counter the effect of AMP on phosphorylase b?

Answer 24

ATP and G-6-P
phosphorylase b returns to T form

Question 25

when is glycogenesis most active and why?

Answer 25

in the fed state as insulin is needed to transport glucose into muscle cells

Question 26

what is the action of insulin that counteracts the production of cAMP by glucagon/adrenaline?

Answer 26

an insulin-stimulates series of reactions leading to actiavtion of cAMP phosphodiesterase 3B and conversion of cAMP to AMP

Question 27

what chemicals lead to the occurrence of glycogen synthesis?

Answer 27

protein phosphatase 1 and 2A

Question 28

what 6 things determine whether glucose is stored or used in glycolysis?

Answer 28

1. cell energy needs
2. glucose availability
3. hormonal regulation
4. enzyme regulation
5. cell signalling
6. tissue specificity