Gluconeogenesis

Question 1

Gluconeogenesis

Answer 1

• production of new glucose from non-carbohydrate sources

Question 2

The need for gluconeogenesis

Answer 2

• Some tissues require a constant supply of glucose as a metabolic fuel
• Humans must be able to synthesise glucose from other precursors and maintain blood glucose with narrow limits
• Liver and kidney cortex are the primary gluconeogenic tissues

Question 3

Tissues that require constant glucose supply as fuel

Answer 3

• Brain & CNS
• Erythrocytes
• Kidney medulla
• Lens and cornea
• Testes
• Exercising muscle

Question 4

Synthesis and use of glucose

Answer 4

• Liver glycogen an essential post-prandial source of glucose can meet the needs for ~24 hrs in the absence of dietary intake
• however in a prolonged fast when glycogen reserves are depleted, glucose must be made from non-carbohydrates sources

Question 5

Precursors required for gluconeogenesis

Answer 5

• Pyruvate 3C (from glycolysis)
• Glycerol 3C (from hydrolysis of TAGs)
• Lactate 3C (from anaerobic glycolysis)
• α-keto acids 3, 4, 5C (obtained from the metabolism of AAs)

Question 6

Glycolytic pathway

Answer 6

• Reactions 1, 3 & 10 of Glycolysis are so strongly exergonic (high ΔG0’) as to be essentially irreversible
• In gluconeogenesis different enzymes are used at each of these steps

Question 7

Bypass 1 of gluconeogenesis

Answer 7

• CO2 from bicarbonate is activated and transferred by pyruvate carboxylase to its biotin prosthetic group
• enzyme transfers CO2 to pyruvate, form oxaloacetate
• oxaloacetate can’t cross mitochondrial membrane and reduced to malate
• malate reoxidised to oxaloacetate, which is oxidatively decarboxylated to phosphophenolpyruvate by PEP carboxykinase

Question 8

Bypass 2 of gluconeogenesis

Answer 8

• Conversion of Fructose 1,6-bisphosphate to fructose 6-Phosphate
• catalysed by Fructose 1,6-bisphosphatase
• hydrolysis reaction
• phosphate removed

Question 9

Bypass 3 of gluconeogenesis

Answer 9

• Conversion of Glucose 6-phosphate to Glucose
• catalysed by Glucose 6-phosphatase
• hydrolysis reaction
• phosphate removed
• Glucose 6-phosphatase is expressed predominantly in the liver and kidney and is embedded in the ER membrane

Question 10

Amount and location of gluconeogenesis

Answer 10

• During an overnight fast ~90% of Gluconeogenesis occurs in the liver and ~10% in the kidney.
• During a prolonged fast ~40 % of Gluconeogenesis will take place in the kidneys.
• A small amount can take place in the small intestine too

Question 11

Cori cycle - glucose from lactate

Answer 11

• pyruvate to lactate
• catalysed by lactate dehydrogenase
• Lactate from anaerobic glycolysis is released into the blood – from cells that lack mitochondria or excising Muscle.
• Lactate taken up by liver and oxidised to pyruvate which can then be converted to glucose
• cycle is especially in muscular exercise

Question 12

Glucose-alanine cycle in muscle

Answer 12

• Pyruvate is converted to alanine (transamination)

- Prevents toxic levels of ammonia in muscle & in blood

Question 13

Glucose-alanine cycle in liver

Answer 13

• Alanine donates NH3 to a ketoglutarate to form glutamate (transamination) and Pyruvate is reformed which can be used for gluconeogenesis.
• Glutamate is deaminated releasing NH3
• cycle is especially active in starvation

Question 14

Regulation of hepatic gluconeogenesis

Answer 14

• Regulation of CHO metabolism
• High insulin/glucagon ratio (fed state): reduced glycogenolysis and gluconeogenesis and instead favours anabolic reactions e.g. muscle synthesis and storage.
• Low insulin/glucagon ratio (fast state): favours glycogenolysis and gluconeogenesis
• Epinephrine also promotes gluconeogenesis

Question 15

Glycolytic specific enzymes

Answer 15

• Phosphofructokinase-1 (PFK-1)

- Pyruvate kinase

Question 16

Gluconeogenic specific enzymes

Answer 16

• Glucose 6-phosphatase
• Fructose bisphosphatase -1 (FBP-1)
• Phosphoenolpyruvate carboxykinase
• Pyruvate carboxylase

Question 17

Effects of glucagon in fasting state

Answer 17

• High levels produced during fasting state
• Activates adenylate cyclase which synthesise cAMP
• cAMP activates Protein Kinase A (PKA)
• PKA targets & modulates several enzymes involved in gluconeogenesis + glycolysis
• PKA phosphorylates Pyruvate Kinase, inactivating it
• Prevents Phosphoenolpyruvate being converted to pyruvate and therefore favours gluconeogenesis

Question 18

Increase in Fructose 2, 6 bisphosphate

Answer 18

• ↑ PFK1 activity and so activates glycolysis

- ↓ FBP1 activity and so inhibits gluconeogenesis

Question 19

Decrease in Fructose 2, 6 bisphosphate

Answer 19

• ↓PFK1 activity and so inhibits glycolysis

- ↑FBP1 activity and so activates gluconeogenesis

Question 20

What controls PFK-2/FBPase-2 activities?

Answer 20

- Phosphorylation:
↓ kinase activity of PFK-2
↑phosphatase activity of FBPase-2
- Dephosphorylation:
↑ kinase activity of PFK-2
↓ phosphatase activity of FBPase-2

Question 21

Phosphorylation of PFK-2/FBPase-2

Answer 21

• PKA is activated by high glucagon.
• PKA phosphorylates PFK-2/FBPase-2: P decreases activity of PFK-2 and increases FBPase-2 – this impedes the synthesis of fructose 2,6-bisP
• So fructose 2,6-bisP ↓: gluconeogenesis ↑, Glycolysis ↓
• Increases synthesis of glucose

Question 22

Dephosphorylation of PFK-2/FBPase

Answer 22

• PKA is not activated in high insulin
• favours dephosphorylation of PFK-2/FBPase-2: increases activity of PFK-2 and decreases FBPase-2 – this increases the synthesis of fructose 2,6-bisP
• So fructose 2,6-bisP ↑: gluconeogenesis ↓, glycolysis ↑
• Prevents synthesis of glucose