What 3 glycolysis reactions must be circumvented in order for gluconeogenesis to occur?
Hexokinase (Glucose > G-6-P)
Phosphofructokinase 1 (F-6-P > F-1,6-BP)
Pyruvate Kinase (PEP > Pyruvate)
What is the first reaction which works around the pyruvate kinase step of glycolysis to contribute to gluconeogenesis?
Enzyme + Coenzyme?
Carboxylation of pyruvate to oxaloacetate...
Reactants: Pyruvate + ATP + CO2
Enzyme + Coenzyme: Pyruvate Carboxylase + Biotin
Product: Oxaloacetate + ADP
Notes: endothermic and thus requires ATP
What happens to the oxaloacetate in the mitochondria that has been formed from pyruvate via pyruvate carboxylase?
- converted to malate via malate dehydrogenase using NADH+ + H+
- transported into the cytosol
- oxidized back to oxaloacetate (w/ NAD)
What is the second part of the two reaction mechanism which reverses the action of pyruvate kinase in gluconeogenesis?
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
What is the reaction for reversal of the glycolytic phosphofructokinase I reaction in gluconeogenesis?
Reactants: F-1,6-BP + H20
Enzyme: Fructose 1,6-Bisphosphatase
Products: Fructose-6-Phosphate + Pi
What is the gluconeogenic reaction for reversal of glucokinase's reaction?
Reactants: G6P + H20
Products: Glucose + Pi
Describe GLUT1 and GLUT3 transporters.
Where are they?
What is their function?
Where: All cells except liver and pancreatic beta cells
Function: Ensure steady glucose uptake for glucose dependent cells
Affinity: High, Km = 1 mM
Describe GLUT2 transporters.
Where: Liver and Pancreatic B-cells
Function: Make glucose uptake proportional to [glucose]
Affinity: Low, Km = 15 mM
Describe GLUT4 transporters.
Where: Skeletal Muscle + Adipocytes (facultative glucose-consuming cells)
Function: Insulin-dependent glucose uptake to adjust consumption to availability
Affinity: Medium, Km = 5 mM
Describe GLUT5 transporters.
Where: GI + Renal Tubule Epithelium
Function: Glucose absorption/re-absorption
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)
Glucagon/Epinephrine released due to low blood gluc
cAMP increase in liver cells
- PKA activation
- Phosphorylation inactivates pyruvate kinase
What are the effects of Fructose 2,6-Bisphosphate on glycolysis and gluconeogenesis?
Mention specific enzymes effected.
F2,6-BP allosterically activates PFK-1 and deactivates F-1,6-Bisphosphatase ... thereby activating glycolysis
By what enzyme is fructose 2,6-bisphosphate formed?
What else does this enzyme do?
How is this enzyme activity controlled?
Phosphofructokinase-2 forms F2,6-BP from F6P
- also has Fructose 2,6-Bisphosphatase activity to reverse the phosphorylation
- Fructose-6-Phosphate - allosterically activates PFK-2 function (and therefore glycolysis)
- Glucagon - increases cAMP > activating PKA which deactivates PFK-2, leading to gluconeogenesis