Lecture 3: Gluconeogenesis Flashcards
How long do glycogen stores last in a fasting state?
About a day
Where does gluconeogenesis occur?
In the liver
Discuss the energetics of gluconeogenesis.
Gluconeogenesis is a very energetically costly process: it uses up a lot of ATP (6 ATP equivalents for every glucose molecule synthesised: 4 ATP and 2 GTP)
When and why is gluconeogenesis essential?
Gluconeogenesis is essential when food intake is low, for producing glucose for the brain and red blood cells.
Give the overall equation for gluconeogenesis, indicating the number of each molecule.
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2H+ + 4H2O
—>
Glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi
Where does the pyruvate used in gluconeogenesis come from?
- Lactate
- Some amino acids (during starvation - from diet or breakdown of muscle)
- Glycerol (released form fats (triglycerides))
What is gluconeogenesis the opposite of substrate-wise? Name the substrates in order.
Glycolysis Substrates: Pyruvate Oxaloacetate Phosphoenolpyruvate (PEP) 2-Phosphoglycerate 3-Phosphoglycerate 1,3-Bisphosphoglycerate Glyceraldehyde-3-phosphate Dihydroxyacteone phosphate Fructose-1,6-bisphosphate Fructose-6-phosphate Glucose-6-phosphate Glucose
Which of the reactions in glycolysis are irreversible?
Glucose —> Glucose-6-phosphate
Fructose-6-phosphate –> Fructose-1,6-bisphosphate
Phosphoenolpruvate —> Pyruvate
Give the enzymes which catalyse gluconeogenesis, in order.
pyruvate carboxylase Phosphoenolpyruvate (PEP) carboxykinase enolase phosphoglycerate mutase phosphoglycerate kinase glyceraldehyde phosphate dehydrogenase triose phosphate isomerase (x2) aldolase fructose-1,-6-bisphosphatase phosphohexose isomerase glucose-6-phosphatase
Give the enzyme which catalyse glycolysis, in order.
hexokinase phosphohexose isomerase phosphofructokinase 1 aldolase triose phosphate isomerase glyceraldehyde phosphate dehydrogenase phosphoglycerate kinase phosphoglycerate mutase enolase pyruvate kinase
Which two reactions in gluconeogenesis are exergonic?
Pyruvate (+ bicarbonate) —> Oxaloacetate (uses GTP)
Oxaloacetate —> Phosphoenolpyruvate (uses ATP)
What is an anaplerotic reaction?
A ‘filling up’ reaction, which supplies a substrate to ‘fill up’ /keep going a metabolic pathway, usually the TCA cycle.
Give an example of an anaplerotic reaction in gluceoneogenesis.
Pyruvate + bicarbonate (+ATP) —> Oxaloacetate (+ADP +Pi)
Requires cofactor biotin (form vitamin b7) and enzyme pyruvate carboxylate
This is a carboxylation reaction too.
In the reaction converting Oxaloacetate to Phosphoenolpyruvate, what else is used/produced?
Used: GTP
Produced: GDP and CO2
Why is biotin required in the first step of gluconeogenesis?
Biotin can carry CO2. Pyruvate carboxylase, the enzyme which catalyses the first step of gluconeogenesis, is biotin-dependent. Biotin moves the CO2 from site 1 to site 2.
How is lactate converted to pyruvate?
Lactate + NAD+ —> Pyruvate + NADH + H+
Requires enzyme lactate dehydrogenase
When are the two different forms of Phosphoenolpyruvate carboxykinase used?
One form is used in the mitochondria when the carbons come from lactate.
The other form is used in the cytoplasm when the carbons come from amino acids.
What happens to pyruvate in the mitochondria?
Pyruvate is converted to oxaloacetate by pyruvate carboxylase with the addition of CO2. Oxaloacetate is then converted to either PEP (Phosphoenolpyruvate) or Malate. PEP leaves the mitochondria and gluconeogenesis continues. Malate leaves the mitochondria and in the cytoplasm is converted back to oxaloacetate, using NAD+ and releasing NADH and H+, by the enzyme cytosolic malate dehydrogenase. Then Oxaloacetate is converted to PEP by cytosolic PEP carboxykinase, releasing CO2. So overall, pyruvate enters the mitochondria and PEP is released/made into/in the cytoplasm.
Why is oxaloacetate made in the mitochondria, converted to malate and then made again in the cytoplasm after malate has left the mitochondria?
There are no transporters for oxaloacetate in the mitochondria, but there are for malate.
Under what conditions does the Cori Cycle proceed?
Anaerobic conditions
When is the Cori cycle important?
In anaerobic conditions
In rapidly contracting muscles
What is the purpose of the Cori cycle?
To remove lactate from rapidly contracting muscles
What happens in the Cori cycle?
In rapidly contracting muscles, glucose undergoes glycolysis, releasing pyruvate and ATP. Due to the anaerobic conditions in the muscle, the pyruvate is then converted to lactate.
The lactate then leaves the muscle and travels, via the bloodstream, to the liver, where it is converted to pyruvate and then glucose, using up 6 ATP by gluconeogenesis. The glucose then leaves the liver and travels, via the bloodstream, to the muscles to provide more ATP for the rapidly contracting muscle.
What is significant about the ATP cost in the Cori Cycle?
The 6 ATP cost is in the liver, not in the muscle.
Why should glycolysis and gluconeogenesis not occur at the same place at the same time?
If both were occurring at the same place at the same time, this would be a futile cycle (also known as a substrate cycle). This is the hydrolysis of ATP with no useful metabolic reaction occurring, which is very wasteful if both set of enzymes are operating at a high rate. The products of one reaction are the substrates of the other reaction and vice versa.
What are the allosteric activators/inhibitors of pyruvate kinase and the reaction it catalyses in glycolysis.
Activators: Fructose-1,6-bisphosphate
Inhibitors: ATP and alanine
Catalyses Phosphoenolpyruvate (PEP) —> pyruvate
Give the allosteric activators/inhibitors of pyruvate carboxylase and the reaction it catalyses in gluconeogenesis.
Activators: Acetyl CoA
Inhibitors: ADP
Catalyses Pyruvate —> Oxaloacetate
Give the allosteric activators/inhibitors of Phosphoenolpyruvate (PEP) carboxykinase and the reaction it catalyses in gluconeogenesis.
Inhibitors: ADP
Catalyses Oxaloacetate —> Phosphoenolpyruvate (PEP)
Give the allosteric activators/inhibitors of phosphofructokinase and the reaction it catalyses in glycolysis.
Activators: Fructose-2,6-bisphosphate and AMP
Inhibitors: ATP, citrate and H+
What is Fructose-2,6-bisphosphate?
A specially synthesised regulatory molecule which activates PFK1 and inhibits FBPase 1 (phosphofructokinase 1 and fructose-1,6-bisphosphate 1), stimulating glycolysis and inhibiting gluconeogenesis.
Give the allosteric activators/inhibitors of fructose-1,6-bisphosphate and the reaction it catalyses in gluconeogenesis.
Activators: citrate
Inhibitors: Fructose-2,6-bisphosphate and AMP
Catalyses Fructose-1,6-bisphosphate —> Fructose-6-phosphate
How is Fructose-2,6-bisphosphate regulated?
Fructose-2,6-bisphosphate is converted to Fructose-6-phosphate by FBPase 2 (fructose-1,6-bisphosphate 2), with the release of Pi and vice versa by PFK 2 (phosphofructokinase 2), using ATP and releasing ADP.
Why is fructose-1,6-bisphosphate allosterically inhibited by AMP?
If AMP is present, ATP is low, so gluconeogenesis should stop, because it used lots of ATP.
What is the result when the bifunctional protein has PFK 2 active and FBPase 2 inactive?
Gluconeogenesis is inhibited and glycolysis is stimulated.
What is the result when the bifunctional protein has PFK 2 inactive and FBPase 2 active?
Glycolysis is inhibited and gluconeogenesis is stimulated.
How is the bifunctional protein changed from PFK 2 active to PFK 2 inactive?
The Serine group is phosphorylated by cAMP-dependent kinase, using ATP and releasing ADP, stimulated by glucagon (inhibits glycolysis as blood glucose is already low).
How is the bifunctional protein changed from FBPase 2 active to FBPase 2 inactive?
The protein is dephosphorylated by phosphatase, using H2O and releasing Pi, which is stimulated by insulin (inhibits gluconeogenesis as blood glucose is already high)
