Gluconeogenesis Flashcards
What is gluconeogenesis
The formation of new glucose from non-carbohydrate sources
What is the purpose of gluconeogenesis?
To provide glucose for export to other tissues when glycogen stores are exhausted ad when no dietary glucose is available
Where does gluconeogenesis occur?
In the liver and kidneys (to a lesser extent)
What hormones regulate gluconeogenesis?
- Glucagon
- Insulin
What are some glucogenic precursors that can be utilized in gluconeogenesis?
- Pyruvate
- Lactate
- Amino acids (alanine)
- Glycerol
Is gluconeogenesis glycolysis in reverse?
No!
- There are three irreversible steps in glycolysis (due to high -ΔG)
- These steps must be bypassed in gluconeogenesis
What is the starting material for gluconeogenesis?
Any compound that can be converted to either pyruvate or oxaloacetate
Of the 10 reactions of gluconeogenesis, how many are the reverse of glycolysis?
7
What are the three reactions that must be bypassed in gluconeogenesis?
- Conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase
- The phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase-1 (PFK-1)
- Conversion of glucose to glucose 6-phosphate by hexokinase
Describe the first bypass reaction in gluconeogenesis
- Conversion of phosphoenolpyruvate (PEP) to pyruvate by pyruvate kinase
- Last step of glycolysis
- 2 steps
Step 1: Pyruvate → Oxaloacetate
- Occurs in the mitochondria
- Pyruvate is transported into the mitochondria via the malate shuttle
- Reaction is catalyzed by pyruvate carboxylase (2 active sites)
- Biotin (coenzyme) has long arms that can access both sites 1 and 2
- Site 1 - HCO3- uses energy from hydrolysis to form CO2 which is covalently added to biotin
- Site 2 - Biotin donates CO2 to pyruvate
Pyruvate + HCO3- + ATP → Oxaloacetate + ADP +Pi
Step 2: Oxaloacetate → Phosphoenolpyruvate (PEP)
- Occurs in the cytosol
- Utilize malate shuttle
- Catalyzed by PEP carboxykinase
Oxaloacetate + GTP ⇌ PEP + CO2 + GDP
Why does the second part of the first bypass reaction need to utilize the malate shuttle?
- The second step in the first bypass reaction takes place in the cytosol BUT Oxaloacetate CANNOT freely transport across the mitochondria membrane
- Oxaloacetate gets converted to malate which can freely cross the mitochondria membrane by the malate transporter
- Malate gets reconverted to oxaloacetate in the cytosol of the mitochondria
- Catalyzed by malate dehydrogenase
What is the significance of the malate shuttle?
- NADH is low in the cytosol compared to in the mitochondria
- The malate shuttle transports NADH from the mitochondria to the cytosol so that it can be used in subsequent gluconeogenesis reactions (conversion of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate)
Mitochondria: Oxaloacetate NADH + H+ ⇌ Malate + NAD+
Cytosol: Malate + NAD+ → Oxaloacetate + NADH + H+
The first bypass reaction in which pyruvate → PEP described above is predominant when pyruvate is the glucogenic precursor BUT, lactate can also be used as a glucogenic precursor. How does the by pass reaction differ when lactate is the precursor?
- Lactate is able to bypass the malate shuttle
- Oxaloacetate is directly converted to PEP by using a mitochondrial version of PEP carboxykinase
Describe the second and third bypass reaction in gluconeogenesis
- The phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase-1 (PFK-1)
- Conversion of glucose 6-phosphate to glucose by hexokinase
These are both hydrolysis reactions that utilizes water
- Step 2 = catalyzed by fructose 1,6-bisphosphatase (FBPase-1)
Fructose 1,6-phosphate + H2O → Fructose 6-phosphate +Pi - Step 3 = catalyzed by Glucose 6-phosphatase
Glucose 6-phosphate + H2O → glucose + Pi
Gluconeogenesis is very energetically expensive. So, why does the cell carry out this reaction?
- There are many tissues (ex: brain) that rely heavily on glucose
- Irreversibility of gluconeogenesis ensures that gluconeogenesis and glycolysis do NOT occur simultaneously
(Doing both at the same time is very wasteful!) - Prevents the excretion (complete waste) of pyruvate
- Many molecules feed into gluconeogenesis
How and why do we prevent gluconeogenesis and glycolysis from running at the same time?
PFK-1 and FBPase-1 reciprocally regulate glycolysis and gluconeogenesis to prevent both pathways from running at the same time (which is wasteful)
PFK-1 catalyzes the reaction that commits glucose 6-phosphate to glycolysis
Describe the reciprocal regulation of PFK-1 and FBPase-1
- Both enzymes catalyze the reaction that converts Fructose 6-phosphate to Fructose 1,6-bisphosphate (reverse in gluconeogenesis)
- When ATP (or citrate) = high, PFK-1 is inhibited (ATP production is higher than consumption)
- ADP or AMP will relieve this inhibition (activate PFK-1) (ATP consumption is higher than ATP production)
- AMP inhibits FBPase-1
What is responsible for the reciprocal regulation of PFK-1 and FBPase-1?
- Fructose 2,6-bisphosphate
How does Fructose 2,6-bisphosphate allosterically regulate PFK-1 and FBPase-1
- PFK-1 = allosterically activated by Fructose 2,6-bisphosphate (reduced affinity for inhibitors citrate / ATP)
- FBPase-1 (second bypass step / gluconeogenesis equivalent of PFK-1) = allosterically inhibited by Fructose 2,6-bisphosphate
In other words….
- If Fructose 2,6-bisphosphate is present: PFK-1 activity is high and FBPase-1 activity = low
Where does Fructose 2,6-bisphosphate come from?
Fructose 6-phosphate
What catalyzes the reaction that creates fructose 6-phosphate?
PFK-2
What regulates the breakdown of fructose 6-phosphate?
FBPase-2
How are PFK-2 and FBPase-2 regulated?
- Phosphorylation
- PFK-2 and FBPase-2 are the same protein molecule. When the enzyme is phosphorylated FBPase-2 is active. When the enzyme is NOT phosphorylated PFK-2 is active.
- cAMP-dependent protein kinase phosphorylates enzyme activating FBPase-2
- Phospho-protein phosphatase removes phosphoryl group from enzyme activating PFK-2
What role does glucagon play in the regulation of fructose 6-phosphate?
- Stimulates adenylyl cyclase to synthesize cAMP which in turn activates cAMP-dependent protein kinase
- Lowers cellular levels of fructose 2,6-bisphosphate (FBPase-2)
- Inhibits glycolysis and stimulates gluconeogenesis
What role does insulin play in the regulation of fructose 6-phosphate?
- Stimulates the activity of phosphorus-protein phosphatase
- Increases cellular levels of fructose 2,6-bisphosphate (PFK-2)
- Inhibits gluconeogenosis
How else can PFK-2 and FBPase-2 be regulated?
- Xylulose 5-phosphate
- Dephosphorylates PFK-2 / FBPase-2 enzyme activating PFK-2 / inhibiting FBPase-2 inhibiting gluconeogenesis and stimulating glycolysis
How does the spatial separation of glycolysis and gluconeogenesis prevent both processes from occurring at the same time?
- Glucose 6-phosphatase (last bypass reaction) is exclusively found on the ER membrane of the liver and kidneys
- The active site of glucose 6-phosphatase faces the ER lumen
- Glucose 6-phosphate = converted to glucose in the ER lumen
- Glucose is then transported to the cytosol (very briefly) before it is transported back out of the cytosol and into the bloodstream
Because glucose is NOT in the cytosol for a long period of time it cannot enter glycolysis
