Gluconeogenesis

Question 1

What is gluconeogenesis?

Answer 1

Gluconeogenesis is the reverse of glycolysis except at rate-limiting steps

-need to “bypass” the 3 rate-limiting steps in glycolytic pathway in order to go in opposite direction

Question 2

What are the glycogen in solutions?

Answer 2

• primary source is amino acids
• lactate
• glycerol

Question 3

Are there reactions that are unique to gluconeogenesis ?

Answer 3

7 f the reactions in glycolysis are reversible- used in synthesis from lactate or pyruvate

-3 glycolytic reactions are irreversible and are substituted by 4 reactions unique to gluconeogenesis

Question 4

Explain the first step of gluconeogenesis

Answer 4

Pyruvate —> oxaloacetate (OAA) -carboxylation by Pyruvate carboxylase

Biotin is a coenzyme

Biotin is bound to apoenzyme of Pyruvate carboxylase through an E-amino group of a lysyl residue to form biocytin

Cleavage of ATP drives formation of apoenzyme-biotin- CO2 intermediate

-intermediates carboxylase Pyruvate to form OAA

Question 5

Where is Pyruvate carboxylase found?

Answer 5

Pyruvate carboxylase round in mitochondria of liver and kidney cells but not muscle

Question 6

Explain the second step for gluconeogenesis

Answer 6

B: transport of oxaloacetate to the cytosol

• OAA is produced in mitochondria but subsequent gluconeogenesis reactions occur in the cytosol
• OAA is converted to a late for transport to cytosol
• Recall this is one of the shunts for transport of reducing equivalents between mitochondria and cytosol

Question 7

What is the third step of gluconeogenesis ?

Answer 7

C: Decarboxylation of cytosolic oxaloacetate

OAA—> PEP by decarboxylation and phosphorylation by PEP-carboxykinase (PEPCK) using hydrolysis of GTP.

This represents a thermodynamically favorable path from Pyruvate to PEP

PEP is then converted through reverse glycolysis reactions up to fructose 1,6-bisphosphate

Question 8

What is the 4th step of gluconeogenesis?

Answer 8

D: Dephosphorylation of fructose 1,6-bisphosphate

• Conversion of fructose 1,6-bisphosphate —> fructose 6-phosphate cannot use phosphofructokinase 1(irreversible in opposite direction - glycolysis)
• The enzyme fructose 1,6-bisphosphate is used instead
• Important regulatory site for gluconeogenesis

Question 9

What is the 5th step of gluconeogenesis ?

Answer 9

• Hexokinase/Glucokinase reaction in glycolysis reaction irreversible
• Glucose 6-phosphatase converts glucose 6-phosphate —> glucose
• Found in liver and kidney but not muscle(like pyruvate carboxylase)
• In liver, if the enzyme glucose 6-P phosphatase is defective, it results in a glycogen storage disorder (glycogen storage disease type 1: Von Gierke’s disease)

Question 10

Define and differentiate the glucose 6-phosphate deficiencies

Answer 10

Type 1a: Von Gierke’s disease (glucose 6-phosphatase deficiency )

Type 1b: Glucose 6-Phosphate translocase deficiency

Question 11

What are the glucose 6-phosphatase deficiencies, what can they cause?

Answer 11

Type 1a: Von Gierke’s disease (glucose 6-phosphatase deficiency )

Type 1b: Glucose 6-Phosphate translocase deficiency

• Affects liver and kidney
• Fasting hypoglycemia- severe
• Fatty liver, hepato- and renomegaly
• Progressive renal disease
• Growth retardation and delayed puberty
• Hyperlacticacidemia, hyperlipidemia, and hyperuricemia
• Normal glycogen structure, increased glycogen stores

Question 12

Describe treatment of glucose 6-phosphate deficiency

Answer 12

Type 1a: von Guerke’s disease( glucose-6 phosphatase deficiency )

Type 1b: glucose 6-phosphate translocase deficiency

Treatment: Nocturnal gastric infusions of glucose or regular administration of uncooked cornstarch

Question 13

How does glucagon regulate gluconeogenesis ?

Answer 13

Stimulates gluconeogenesis by 3 mechanisms

1. changes in allosteric effectors: lowers levels of fructose 2,6-bisphosphate
2. Covalent modification of enzyme activity
3. Induction of enzyme synthesis: it increases transcription of the gene for PEPCK

Question 14

How does glucagon use covalent modification to regulate enzyme activity ?

Answer 14

• elevates cAMP levels
• cAMP-dependent protein kinase converts Pyruvate kinase to a phosphorylated inactive form
• This decreases conversion of PEP to pyruvate switching off glycolysis and switching on gluconeogenesis

Question 15

Outline the step by step process of glucagon regulating gluconeogenesis

Answer 15

1. High glucagon/insulin ratio causes elevated cAMP and increased levels of active protein kinase A
2. Increased protein kinase A activity favors the phosphorylated form of the PFK-2/FBP-2 complex
3. Phosphorylated PFK-2 is insctive, whereas FBP-2 is active; this impedes formation of fructose 2,6-bisphosphate
4. Decreased levels of fructose 2,6-bisphosphate decreases the inhibition of FBP-1, which leads to an increased rate of gluconeogenesis

Question 16

Aside from glucagon, what may affect the rate of gluconeogenesis

Answer 16

• Gluconeogenesis rate affected by a availability of precursors especially glucogenic amino acids
• Decreased insulin levels mobilize amino acids from muscles (especially alanine) to provide carbon skeletons for gluconeogenesis

Question 17

Explain the allosteric activation of gluconeogenesis by glucagon

Answer 17

Allosteric activation by Acetyl CoA

• Acetyl CoA allosterically activates hepatic Pyruvate carboxylase during starvation
• During starvation, lypolysis in adipose tissue flood blood stream with fatty acids and glycerol
• Rate of formation of Acetyl CoA by B-oxidation of fatty acids >rate of conversion of Acetyl CoA —> CO2 + H2O
• Accumulating Acetyl CoA stimulates pyruvate carboxylase

Question 18

How do energy levels within the cell regulate glucagon?

Answer 18

• Fructose 1,6-bisphosohatase inhibited by high levels of AMP (signal of energy poor state of cell)
• Stimulated by Hugh levels of ATP and low levels of AMP

Question 19

What are the three substrates in glucogenesis?

Answer 19

1. Glycerol
2. Lactate
3. Alanine

Question 20

What is the importance of substrates for gluconeogenesis?

Answer 20

The various gluconeogenesis precursors are intermediates in protein (deamination of glucogenic amino acids) and lipid metabolism

Question 21

Explain glycerol as a gluconeogenesis precursor

Answer 21

• Produced by hydrolysis of triacylglycerols (triglyceride fats) in adipose tissue
• Delivered to liver by blood
• Glycerol is phosphorylated by glycerol kinase —> glycerol phosphate —> dihydroxyacetone-phosphate —> glyceraldehyde 3-P - inetermediwte in glycolysis

Question 22

Explain the importance of lactate in the Cori cycle

Answer 22

Lactate:

Released into blood by cells performing anaerobic glycolysis (either cells lack mitochondria or exercising muscle)

Cori cycle:

• Muscle converts blood glucose —> lactate
• lactate converted to glucose by gluconeogenesis in liver
• glucose re-enters circulation and travels back to muscle as fuel

Question 23

Explain the importance of alanine as a gluconeogenic precursor

Answer 23

• Alanine from the degradation of muscle proteins is released in the blood stream and transported to the liver
• Liver converts alanine to pyruvate, which in turn can be cinvertdd to glucose