3: The Role Of Gluconeogenesis In Glucose Homeostasis Flashcards
Why is gluconeogenesis so important in glucose homeostasis?
- glucose is sole nutrient for nervous tissue and erythrocytes and for foetus.
- if there is insufficient carbs from diet, glucose demands Is met via gluconeogenesis
- it is the major source of blood glucose during periods of reduced glucose intake like starvation, high protein diets.
- gluconeogenesis is important for clearing lactate produced by muscles and cells, needed to maintain acid-base balance
Definitition of gluconeogenesis
Gluconeogenesis is the metabolic process whereby glucose is synthesised in the liver (and kidney) from non-carbohydrate precursors such as lactic acid, glycerol, amino acids and propionate
Why don’t ruminants absorb much glucose under normal dietary conditions?
The carbs in their diet are fermented to volatile fatty acids (VFA) in the rumen by action of microbes.
Therefore ruminants don’t absorb much glucose under normal conditions.
Gluconeogenesis is essential all the time in ruminants for glucose homeostasis.
-in well fed ruminants glucose comes from the conversion of propionate (C3 fatty acid) to glucose via gluconeogenesis
What are the four main compounds used for the synthesis of glucose via gluconeogenesis?
- Lactate (lactic acid) from muscle
- glycerol from white adipose tissue
- amino acids from protein degradation (mainly from muscle)
- proprionate from rumen fermentation
During starvation amino acids are the major source of carbon
Where does gluconeogenesis take place?
-90% liver
-t capable of kidney gluconeogenesis
Glucose needs an intact hepatocyte, all parts are needed if the starting substrate is lactate or alanine
Describe the pathway of gluconeogenesis
There are thermodynamic barriers to just reversing glycolysis.
There are 3 barriers with 3 irreversible kinase enzymes of glycolysis: hexokinase, phosphofructokinase (PFK 1), and pyruvate kinase. There are 4 enzymes required to reverse these 3 steps. They are titled the obligatory quartet
Name each enzyme apart of the obligatory quartet and state its function in glycolysis.
Only the liver and kidney are tissues that express all 4 so only them are capable of gluconeogenesis.
1) pyruvate carboxylase (a mitochondrial enzyme)
2) phosphoenolpyruvate carboxykinase (PEPCK, and inducible enzyme mainly present in cytosol
3) fructose-1,6 bisphosphate (F-1,6bisPase) present in the cytosol and which is the major regulatory enzyme determining the overall rate of gluconeogenesis
4) glucose-6- phosphatase: present in the smooth endoplasmic reticulum which releases the phosphate from the G-6-P thereby allowing the glucose to pass across the plasma membrane to the blood.
Refer to lecture notes book for how each non carb enters cycle
Do it
How is gluconeogenesis regulated?
Give me 1st way.
- Induction/ repression of enzymes:
- GLUCAGON indices the synthesis of PEPCK (phosphoenolpyruvate carboxykinase) in newborn animals, starving animals or animals with low carb intake.
- INSULIN induces the synthesis of the key enzymes of glycolysis
How is gluconeogenesis regulated?
Give me 2nd way.
- Allosteric regulation
A) -Pyruvate carboxylase is an allosteric enzyme with AcetyleCoA as its positive allosteric affecter.
-AcetyleCoA is also an allosteric inhibitor of pyruvate dehydrogenase which turns pyruvate to acetyl coA
-the action of acetylCoA links both enzymes using pyruvate. It ensures the supply of oxaloacetate ➡so it can condense with acetyleCoA to form citrate
-therefore the supply of OAA and acetylCoA is ensured for the TCA cycle.
-in starving animals fat oxidation and gluconeogenesis are linked via action of acetylCoA in stimulating pyruvate carboxylase and inhibiting pyruvate dehydrogenase
B) the major point of regulation is at the enzymatic steps between fructose 1,6 bis phosphate.
-the 2 enzymes involved are phosphofructokinase (PFK1) and fructose 1,6 bisphosphatase (1,6BisPase)
-both enzymes are allosteric enzymes.
-both have the same allosteric effector which is fructose 2,6 bisphosphate. However has opposite effects.
-it stimulated PFK1 and inhibits F-1,6BisPase.
-therefore in the presence of F-2,6bisP glycolysis is functioning and gluconeogenesis is inhibited.
Look at slide 8 for image on lecture 4
How is gluconeogenesis regulated?
Give me 3rd way.
- Covalent modulation➡ something comes in and phosphorylates enzyme which speeds it up or slows it down.
-concentration of fructose 2,6 bisphosphate determines direction of glycolysis/gluconeogenesis.
-its produced by phosphofructokinase 2 which takes fructose6P from glycolysis through to fructose-2,6bisP = glycolysis.
-has another part to this enzyme which acts as fructose2,6bisphosphatASE. Which makes this enzyme a bi functional enzyme. Ie one protein, 2 functions.
Both parts of enzyme have opposite effects.
-depends on wether enzyme is phosphorylated or not as to what part of enzyme is active.
If not phosphorylated➡ acts as PFK2➡ which converts F6P to F1,6bisP = glycolysis
If it is phosphorylated then it acts as F2,6BisPase➡ destroys F2,6bisP ➡ gluconeogenesis is no longer inhibited. :)
-phosphorylation state depends on presence of glucagon.
-glucagon binds to plasma membrane of liver cells
-binding stimulates production of secondary messenger 3’,5’ cyclicAMP or cAMP inside the cell.
-cAMP allosterically increases activity of cAMP-dependant protein kinase
-protein kinase phosphorylates the bi functional enzyme So that PFK2 is inhibited and F2,6BisPase activity is stimulated.
-overall result is that glucagon ⬇ concentration of F2,6bisPase therefore increases activity of F2,6BisPase so gluconeogenesis is stimulated.
Summary of control of PFK2/F2,6BisPase Hormonal control (covalent modulation) Substrate control (allosteric)
Hormonal control (covalent modulation) -glucagon leads to phosphorylation ➡ phosphorylated bifunctional enzyme: F2,6BisPase active/ PFK2 inactive so gluconeogenesis
Substrate control (allosteric) -if fructose 6 P is abundant (ie from glucose after a meal), then F6-P allosterically stimulates kinase and inhibits phosphatase
