Action of insulin & Glycogen revision

Question 1

What does Glucagon act via? and what is the target organs?

Answer 1

• Glucagon acts via cAMP and PKA to phosphorylate enzymes involved in carbohydrate metabolism: net effect is to raise blood sugar
• Target organs = liver and adipose tissue

Question 2

What does Insulin act via? and what is the target organs?

Answer 2

• Insulin acts via PP1 to dephosphorylate enzymes involved in carbohydrate metabolism: net effect
  is to lower blood sugar
• Target organs = liver, adipose tissue, skeletal muscle

Question 3

Explain the Insulin receptor ?

Answer 3

• The insulin receptor is an RTK consisting of a cross-linked tetrameric α2β2 complex
• The extracellular α subunit contains the insulin binding sites
• The β subunit anchors the α subunit to the plasma membrane through a transmembrane region (TM) and contains the intracellular tyrosine kinase domain (TK)
• Three tyrosine residues in the TK domain must be autophosphorylated by the ligand-bound receptor before the kinase can phosphorylate other substrates

Question 4

Explain how receptor activation by autophosphorylation happens?

Answer 4

• Insulin binding to the extracellular domains of the receptor activates the catalytic domain inside the cell
• The catalytic domain in one receptor phosphorylates Tyr residues in another receptor
• Receptor autophosphorylation allows binding and phosphorylation of protein IRS-1 (insulin receptor-substrate-1)
• This is a family of related proteins: there are tissue-specific differences in IRS expression and different downstream pathway
  recruitment in order to regulate different metabolic responses in different tissues

Question 5

What are the insulin receptor has two key downstream pathways ?

Answer 5

There is a primarily mitogenic arm via Ras/MAPK, and a primarily metabolic arm via PI3K/Akt

Question 6

What does MAPk switch on ?

Answer 6

Insulin-sensitive genes e.g. glycolytic enzymes, lipogenic enzymes, and activates PP1, which reverses effects of PKA and other kinases

Question 7

What does PP1 dephosphorylate ?

Answer 7

The glycogen enzymes: glycogen synthase ON, phosphorylase and phosphorylase kinase OFF

Question 8

What does Akt switch off ?

Answer 8

GSK3, which allows glycogen

synthase to be activated

Question 9

What does activated AKT phosphorylate ?

Answer 9

A number of substrates at Ser/Thr residues. These
effector proteins mediate the effects of insulin on glucose production, utilisation and
uptake, as well as the synthesis of glycogen, protein and lipid

Question 10

When does Glycogen breakdown (glycogenolysis) occur ?

Answer 10

In response to low glucose in the plasma (liver) or muscle contraction

Question 11

Glycogen phosphorylase works on ?

Answer 11

Nonreducing ends breaking α1,4 bonds to release glucose-1-phosphate until it reaches four residues from an (α1,6) branch point

Question 12

What does Transferase do ?

Answer 12

Transfers a block of three residues to the nonreducing end of the chain in an α1,4 linkage

Question 13

What do the Debranching enzymes cleave ?

Answer 13

The single remaining (α1,6)-

linked glucose, which becomes a free glucose unit (i.e., NOT glucose-1-phosphate)

Question 14

What is the glycogen phosphorylase conformations?

Answer 14

Has tense (inactive) and relaxed (active) conformation

Question 15

What is the R state stabilised by ?

Answer 15

AMP, Pi and phosphorylation

Question 16

What is the T state stabilised by ?

Answer 16

TP, G6P and glucose (liver)

Question 17

Explain Glucagon signalling to phosphorylase ?

Answer 17

• The primary message is the hormone, which binds
  to the glucagon receptor
• This signal is then transduced by the G-protein and adenylate cyclase
• cAMP made by adenylate cyclase is the second
  messenger
• PKA, activated by cAMP, is the effector molecule
• Phosphorylase kinase and glycogen phosphorylase
  are the target molecules

Question 18

What is Phosphorylase kinase regulated by ?

Answer 18

Calcium and phosphorylation: it switches on glycogen

phosphorylase by phosphorylating it

Question 19

What happens to animals deficient in phosphorylase kinase ?

Answer 19

Unable to break down liver glycogen

Question 20

What is Phosphorylase kinase activated by ?

Answer 20

Activated by:

• Increased Ca2+: e.g. from muscle contraction, α1-adrenergic agents (liver), Angiotensin (liver), Vasopressin (liver)
• Phosphorylation: by PKA downstream of glucagon (liver only) and adrenaline (muscle and liver). Decreases requirement for calcium

Question 21

What is Phosphorylase kinase inactivated by ?

Answer 21

Dephosphorylation by PP1 (protein phosphatase-1), downstream of insulin signalling

Question 22

What is the default state of muscle phosphorylase ?

Answer 22

Tense, b form: don’t make glucose unless signalled to

Question 23

What is the default state of liver phosphorylase ?

Answer 23

Active, a form: make glucose unless signalled not to

Question 24

Explain Glycogenesis ?

Answer 24

• Glycogen synthase can only synthesize α-1,4-linkages
• A branching enzyme generates branches by cleaving an α-1,4-linkage, moving a block of approximately seven glucoses and synthesising an α-1,6-linkage
• Glycogen synthase can then extend the branched
  polymer: regulated allosterically and by
  phosphorylation