Gluconeogenesis

Question 1

What are the non-carbohydrate precursors in glucose synthesis?

Answer 1

• pyruvate
• lactate
• alanine
• glycerol
• propionate

Question 2

What components of the body require a constant supply of glucose?

Answer 2

brain and erythrocytes

Question 3

Where does gluconeogenesis take place?

Answer 3

liver (90%) and kidney (10%)

Question 5

What are the 3 irreversible steps in glycolysis?

Answer 5

• glucose –> glucose-6- phosphate
  
  • hexokinase
• fructose-6-phosphate –> fructose-1,6-bisphosphate
  
  • phosphofructokinase
• phosphoenolpyruvate –> pyruvate
  
  • pyruvate kinase

Question 6

What is the first enzymatic step involved gluconeogenesis from pyruvate?

Answer 6

pyruvate + CO₂ –> oxaloacetate + ADP + P

pyruvate carboxylase

Question 7

Where is pyruvate kinase found? It requires what prosthetic group?

How is it regulated?

Answer 7

mitochondrial enzyme

requires biotin as prosthetic group

regulated by acetyl-CoA

Question 8

How is Biotin involved in the transfer of CO₂ to pyruvate to convert it to oxaloacetate?

Answer 8

1. CO₂ is covalently bound to biotin-enzyme
   
   • ATP hydrolysis to ADP + Pi is required for this step
2. Biotion serves as a carrier of activated CO₂
3. activated CO₂ is transferred to pyruvate to form oxaloacetate

Question 9

How does gluconeogenesis finish in the cytoplasm if it starts in the mitochondria & oxaloacetate is unable to cross the mitochondrial membrane?

Answer 9

Malate / Aspartate Shuttle

oxaloacetate is not transported across the mitochondrial inner membrane

malate dehydrogenase converts oxaloacetate to malate – malate can be transported to the cytoplasm

OR

aspartate transaminase converts oxaloacetate to aspartate – aspartate can be transported to the cytoplasm

once in the cytoplasm malate / aspartate are conveted back to oxaloacetate

Question 10

What is the second enzymatic step in gluconeogenesis from pyruvate?

Answer 10

Oxaloacetate –> phosphopheonolpyruvate

simultaneous decarboxylation & phosphorylation: phosphoenolpyruvate carboxykinase

GTP required for reaction

Question 11

What gluconeogenesis steps circumvent the pyruvate kinase step of glycolysis?

Answer 11

pyruvate carboxylase and phosphoenolpyruvate carboxykinase

Question 12

How is PEP converted to Fru-1,6-bisphosphate during gluconeogenesis?

Where in the cell does this reaction take place?

Answer 12

using reversible glycolytic enzymes

cytosol

Question 13

What is the third regulated step in gluconeogenesis from pyruvate?

Answer 13

Fru-1,6-bisphosphate –> Fru-6-phosphate

fructose-1,6-bisphosphatase

irreversible reaction

major control point

Question 14

What is the function of glucose-6-phosphatase?

What is a unique characteristic about this enzyme?

Answer 14

glucose-6-phosphate –> glucose

irreversible reaction

expressed only in liver and kidney

ER enzyme

Question 15

What is the overall energy cost of gluconeogenesis?

Answer 15

• pyruvate –> oxaloacetate: 2 ATP (per glucose)
• oxaloacetate –> phosphoenolpyruvate: 2 GTP
• 3-phosphoglycerate –> 1,3-bisphosphoglycerate: 2 ATP

SUM: 6 high energy phosphate bonds used per glucose produced

Question 16

Glycololysis / gluconeogenesis have what two types of regulation?

Answer 16

allosteric (local) and hormonal (systemic)

Question 17

What are the two major steps of regulation in glycolysis / gluconeogenesis?

Answer 17

1. fru-6-phosphate / fru-1,6-bisphosphate
2. PEP / pyruvate

Question 18

How is hexokinase allosterically regulated in the liver?

Answer 18

• hexokinase
  
  • inhibited by G-6-P

Question 19

How does AMP serve as an allosteric regulator in the liver?

Answer 19

stimulates PFK1 activity

inhibits F-1,6BPase activity

Question 20

How does ATP serve as an allosteric regulator in the liver?

Answer 20

ihibits PFK1

inhibits PK activity

Question 21

How does citrate serve as an allosteric regulator in the liver?

Answer 21

inhibits PFK1

Question 22

How does alanine serve as an allosteric regulator in the liver?

Answer 22

inhibits PK activity

Question 23

How does AcetylCoA serve as an allosteric regulator in the liver?

Answer 23

stimulates pyruvate carboxylase activity

Question 24

How does fructose-1,6-BP act as an allosteric regulator in the liver?

Answer 24

stimulates PK activity

Question 25

What are the hormonal regulators for gluconeogenesis in the liver?

Answer 25

**insulin / glucagon ratio** regulates glycolysis / gluconeogenesis in the liver

Question 26

How is hormonal regulation of gluconeogenesis in the liver accomplished?

Answer 26

varying the concentrationof fructose-2,6-bisphosphate * Fru-2,6-BP stimulates PFK1 * Fru-2,6-BP inhibits F-1,6-BPase pyruvate kinase * glucagon decreases PK activity * epinephrin increases [cAMP] -- effects similar to glucagon

Question 27

What is the name of the cycle of lactate to the liver & glucose to the erythrocytes and muscle?

Answer 27

Cori cycle

Question 28

What is the final product of glycolysis in erythrocytes?

Answer 28

lactate b/c they do not contain mitochondria

Question 29

What is the name for the transfer of alanine for glucose? Where does this occur?

Answer 29

The alanine cycle 1. Muscle * pyruvate transaminate dto alanine & exported to blood 2. Liver * alanine & lactate converted to pyrucate and subsequently glucose

Question 30

What amino acids are substrates for gluconeogenesis?

Answer 30

All except Leu and Lys

Question 31

Glycerol is released from what process? It enters gluconeogenesis at what step?

Answer 31

released from hydrolysis of triacylglycerols by adipose tissue enters at level of DHAP

Question 32

How is propinoyl-CoA able to enter the TCA cycle? How is this relevant to gluconeogenesis?

Answer 32

converted to succinyl-CoA only this part of the rare fatty acids is gluconeogenic