Gluconeogenesis + Blood Gluc Control (Ben)

Question 1

What 3 glycolysis reactions must be circumvented in order for gluconeogenesis to occur?

Answer 1

1. Hexokinase (Glucose > G-6-P)
2. Phosphofructokinase 1 (F-6-P > F-1,6-BP)
3. Pyruvate Kinase (PEP > Pyruvate)

Question 2

What is the first reaction which works around the pyruvate kinase step of glycolysis to contribute to gluconeogenesis?

Reactants?

Enzyme + Coenzyme?

Product?

Notes?

Answer 2

Carboxylation of pyruvate to oxaloacetate…

Reactants: Pyruvate + ATP + CO₂

Enzyme + Coenzyme: Pyruvate Carboxylase + Biotin

Product: Oxaloacetate + ADP

Notes: endothermic and thus requires ATP

Question 3

What happens to the oxaloacetate in the mitochondria that has been formed from pyruvate via pyruvate carboxylase?

3 steps

Answer 3

1. converted to malate via malate dehydrogenase using NADH⁺ + H⁺
2. transported into the cytosol
3. oxidized back to oxaloacetate (w/ NAD)

Question 4

What is the second part of the two reaction mechanism which reverses the action of pyruvate kinase in gluconeogenesis?

Reactants?

Enzyme?

Product?

Note?

Answer 4

Decarboxylation and phosphorylation…

Reactants: Oxaloacetate (cytosolic) + GTP

Enzyme: Phosphoenolpyruvate Carboxykinase

Product: PEP + GDP + CO₂

Note: uses the GTP produced in hepatic/renal citric acid cycle

Question 5

What is the reaction for reversal of the glycolytic phosphofructokinase I reaction in gluconeogenesis?

Reactants?

Enzyme?

Products?

Answer 5

Dephosphorylation…

Reactants: F-1,6-BP + H₂0

Enzyme: Fructose 1,6-Bisphosphatase

Products: Fructose-6-Phosphate + Pi

Question 6

What is the gluconeogenic reaction for reversal of glucokinase’s reaction?

Reactants?

Enzyme?

Product?

Answer 6

Dephosporylation…

Reactants: G6P + H20

Enzyme: Glucose-6-Phosphatase

Products: Glucose + Pi

Question 7

Describe GLUT1 and GLUT3 transporters.

Where are they?

What is their function?

Affinity?

Answer 7

Where: All cells except liver and pancreatic beta cells

Function: Ensure steady glucose uptake for glucose dependent cells

Affinity: High, Km = 1 mM

Question 8

Describe GLUT2 transporters.

Where?

Function?

Affinity?

Answer 8

Where: Liver and Pancreatic B-cells

Function: Make glucose uptake proportional to [glucose]

Affinity: Low, Km = 15 mM

Question 9

Describe GLUT4 transporters.

Where?

Function?

Affinity?

Answer 9

Where: Skeletal Muscle + Adipocytes (facultative glucose-consuming cells)

Function: Insulin-dependent glucose uptake to adjust consumption to availability

Affinity: Medium, Km = 5 mM

Question 10

Describe GLUT5 transporters.

Where?

Function?

Answer 10

Where: GI + Renal Tubule Epithelium

Function: Glucose absorption/re-absorption

Question 11

Explain a mechanism of covalent modification induced by low-blood glucose__-related hormones for inhibition of glycolysis.

Start with the hormones.

(Harper’s 30th, pg 188)

Answer 11

1. Glucagon/Epinephrine released due to low blood gluc
2. cAMP increase in liver cells
3. PKA activation
4. Phosphorylation inactivates pyruvate kinase

Question 12

What are the effects of Fructose 2,6-Bisphosphate on glycolysis and gluconeogenesis?

Mention specific enzymes effected.

Answer 12

F2,6-BP allosterically activates PFK-1 and deactivates F-1,6-Bisphosphatase​ … thereby activating glycolysis

Question 13

By what enzyme is fructose 2,6-bisphosphate formed?

What else does this enzyme do?

How is this enzyme activity controlled?

Answer 13

Phosphofructokinase-2 forms F2,6-BP from F6P

• also has Fructose 2,6-Bisphosphatase activity to reverse the phosphorylation

Control:

• Fructose-6-Phosphate - allosterically activates PFK-2 function (and therefore glycolysis)
• Glucagon - increases cAMP > activating PKA which deactivates PFK-2, leading to gluconeogenesis