Metabolism in the fed and starved states

Question 1

Metabolism in the fed state

Answer 1

LIVER
Glucose -> glycogen and TGs (secreted as VLDL) – some enters TCA cycle
Glycerol -> triacylglycerols (triglycerides)
Excess amino acids -> pyruvate -> TCA cycle OR triacylglycerols
MUSCLE
Glucose (enters via Glut 4) -> glycogen OR glycolysis & TCA cycle
Fatty acids -> oxidised to acetyl CoA to produce ATP
Amino acids -> proteins
ADIPOSE TISSUE
Glucose -> acetyl CoA -> fatty acids and triacylglycerol
Fatty acids -> triacylglycerol
Glycerol released from TGs -> liver
BRAIN
Glucose -> acetyl CoA -> ATP

Question 2

Metabolism in the early fasting states

Answer 2

LIVER – starts to export
Gluconeogenesis
Glycogen -> glucose
Protein -> amino acids, to fuel gluconeogenesis
Triacylglycerol -> fatty acids -> beta oxidation (produces citrate & acetyl CoA)
MUSCLE
No glycogenolysis
Fatty acid oxidation (inhibits glycolysis and glucose utilisation)
Proteins -> amino acids
ADIPOSE TISSUE
Glycolysis inhibited
TGs -> fatty acids
Glycerol can’t be metabolised & is recycled to liver -> gluconeogenesis
BRAIN
Takes up glucose -> acetyl CoA -> ATP

Question 3

Metabolism in the starved state

Answer 3

LIVER
TG -> fatty acids -> gluconeogenesis (excess acetyl CoA -> ketone bodies used by brain/muscle)
Proteins -> amino acids -> gluconeogenesis
Urea Synthesis stimulated (more amino acids)
MUSCLE
Little glucose entry
Proteins -> fatty acids -> acetyl CoA
Ketone bodies taken up (to reduce proteolysis)
ADIPOSE TISSUE
TGs -> Fatty acids -> acetyl CoA
Glycerol exported to liver
BRAIN
Ketone bodies -> acetyl CoA
Glucose -> acetyl CoA (glycolysis)

Question 4

cAMP dependent protein kinase

Answer 4

Activates phosphate kinase A
Inactivates glycogen synthase a
Activated by glucagon/adrenaline

Question 5

Phosphorylase kinase A

Answer 5

Phosphorylated form = active
Activates phosphorylase a (increasing glycogen breakdown)
Inactivates glycogen synthase
Activated by muscle contraction (Ca2+) and cAMP dependent protein kinase

Question 6

Phosphorylase a

Answer 6

Phosphorylated form = active
Activated by phosphate kinase a
Inactivated by protein phosphatase
Increases glycogen breakdown

Question 7

Protein phosphatase

Answer 7

Inactivates phosphorylase a

Activated by insulin

Question 8

Glycogen synthase a

Answer 8

Dephosphorylated form = active
Activated by protein phosphatase
inactivated by cAMP dependent protein kinase and phosphorylase kinase a
Decreases glycogen synthesis

Question 9

Glycogen metabolism in liver and muscle

Answer 9

Glucagon stimulates glycogenolysis: In liver - 2nd messenger is cAMP
Adrenaline stimulates glycogenolysis: in muscle & liver via b-adrenergic receptors – 2nd messenger is cAMP; in liver via a1-adrenergic receptors – second messenger is Ca2+
Insulin stimulates glycogen synthesis in both tissues

Question 10

Hormones controlling blood sugar

Answer 10

Glucagon: from pancreatic alpha cells, initiated by hypoglycaemia, causes rapid activation of glycogenolysis/gluconeogenesis
Adrenaline: from adrenal medulla, initiated by stress/hypoglycaemia, rapid activation of “ “
Cortisol: from adrenal cortex, initiated by stress, chronic activation of ‘’ ‘’
Insulin: pancreatic beta cells, initiated by hyperglycaemia, inactivation of “ “