Gluconeogensis Flashcards
What is gluconeogenesis and where does it occur?
Gluconeogenesis is the process of making glucose from pyruvate
Most of the steps of glycolysis are retained
The regulated steps are changed for spontaneous reactions in the direction of sugar synthesis
Occurs mainly in the mitochondria of liver and kidney cells
Which glycolysis steps must be bypassed (and why) in order for gluconeogenesis to proceed?
Hexokinase, phosphofructokinase, and pyruvate kinase must be bypassed because of their large delta G
These reactions are essentially irreversible
What is pyruvate converted into and where does this conversion take place?
Pyruvate is converted to oxaloacetate in the mitochondria.
Pyruvate carboxylase is the enzyme responsible for this.
ADP + Pi + 2 H+ are also generated
What is the prosthetic group that is used by pyruvate carboxylase?
Biotin is the prosthetic group that is used by pyruvate carboxylase.
How is pyruvate carboxylated?
1) HCO3- is activated to carboxyphosphate
2) Activated CO2 is bonded to the biotin ring to form the carboxybiotin enzyme intermediate
CO2 - biotin - enzyme + H+ —> CO2 + biotin - enzyme
delta Go’ is -20 kj/mol-1
This large - delta Go’ indicates that carboxybiotin is able to transfer CO2 to acceptors
Where does the conversion of oxaloacetate to phosphoenolpyruvate (PEP) occur and what enzyme is responsible?
The conversion of oxaloacetate to PEP occurs in the cytosol
PEP carboxykinase is responsible
GTP is used in this step and it comes from the TCA
Picture: Oxaloacetate is made in cytosol.
Schematic of gluconeogensis.
Gluconeogensis pathway.
How are carboxylation and decarboxylation steps in gluconeogenesis powered?
The addition of a phosphoryl group to pyruvate is energetically unfavorable as is the formation of phosphoenolpyruvate (PEP)
ATP is used to power the addition of CO2 to pyruvate
The decarboxylation of pyruvate is used to power the formation of PEP
What is the purpose of gluconeogenesis?
The purpose is to form glucose from non-glucose precursors.
It occurs in the liver and kidney
The reactions take place in the cytosol of the cell
This is an anabolic process
What is the first reaction of gluconeogenesis?
Pyruvate to oxaloacetate
Takes place in mitochondria
Purpose: Add carbon to pyruvate in an ATP-dependent manner
Enzyme: Ligase (Carboxylase)
Biochemical process: Carboxylation
Free energy change: Large; this reaction is irreversible
NOTE: This is the first of four gluconeogenesis reactions that differ from the reverse glycolysis steps
What is the 2nd reaction of gluconeogensis?
Oxaloacetate to Phosphoenolpyruvate
Purpose: Remove a carbon and add a phosphate to oxaloacetate in a GTP-dependent manner
Enzyme: Lyase (Phosphoenolpyruvate carboxykinase)
Biochemical process: Decarboxylation and phosphorylation
delta G: - 31.8 kj/mol
This reaction is spontaneous and non-reversible
This is the 2nd of four reactions in gluconeogenesis that differ from reverse glycolysis steps
What is the 3rd reaction of gluconeogenesis?
Phosphoenolpyruvate to 2-Phosphoglycerate
Purpose: Add water to phosphoenolpyruvate
Enzyme: Lyase (Enolase or 2-phosphoglycerate dehydratase)
Biochemical process: Hydration
delta Go’ : -1.8 kj/mol
delta G: + 1.1 kj/mol
This reaction is spontaneous and reversible
What is the 4th reaction of gluconeogenesis?
2-Phosphoglycerate to 3-Phosphoglycerate
Purpose: Reshuffle the phosphate group
Enzyme: Isomerase (Phosphoglycerate mutase)
Biochemical process: Isomerisation
delta Go’ : - 4.4 kj/mol
delta G: - 0.8 kj/mol
This reaction is spontaneous and reversible
What is the 5th reaction of gluconeogenesis?
3-Phosphoglycerate to 1,3-Bisphosphoglycerate
Purpose: Add energy in an ATP-dependent manner
Enzyme: Transferase (3-phosphoglycerate kinase)
Biochemical process: Phosphorylation
delta Go’ : + 18.5 kj/mol
delta G: - 1.3 kj/mol
This reaction is spontaneous and reversible
What is the 6th reaction of gluconeogenesis?
1,3-Bisphosphoglycerate to Glyceraldehyde-3-phosphate
Purpose: Remove a phosphate & hydrogenate 1,3-Bisphosphoglycerate using NADH + H+
NOTE: One of the products is inorganic phosphate
Enzyme: Oxidoreductase (Glyceraldehyde-3-phosphate dehydrogenase)
Biochemical process: Dephosphorylation & hydrogenation
delta Go’ : -6.3 kj/mol
delta G: +1.3 kj/mol
This reaction is spontaneous and reversible
What is the 7th reaction of gluconeogenesis?
Glyceraldehyde-3-phosphate to Dihydroxyacetone phosphate
Purpose: Convert one molecule of Glyceraldehyde-3-phosphate to dihydroxyacetone phosphate
Enzyme: Isomerase (Triose phosphate isomerase)
Biochemical process: Isomerisation
delta Go’ : + 7.6 kj/mol
Rxn is spontaneous and reversible
What is the 8th reaction of gluconeogenesis?
Glyceraldehyde-3-phosphate & DHAP to Fructose-1,6-Bisphosphate
Purpose: Join two three carbon compounds into Fructose-1,6-Bisphosphate (6 carbons)
Enzyme: Lyase (Aldolase) in reverse direction
Biochemical process: Ligation in forward direction and lysis in reverse direction
delta Go’ : - 23.8 kj/mol
delta G: + 1.3 kj/mol
This reaction is spontaneous and reversible
What is the 9th reaction of glycolysis?
Fructose-1,6-bisphosphate to Fructose-6-phosphate
Purpose: To remove a phosphate from fructose-1,6-bisphosphate using water
Enzyme: Hydrolase (Fructose-1,6-bisphosphatase)
Biochemical process: Dephosphorylation and hydrolysis
delta Go’ : -16.7 kj/mol
delta G: -8.6 kj/mol
This reaction is spontaneous and irreversible
What is the 10th reaction of gluconeogenesis?
Fructose-6-phosphate to Glucose-6-phosphate
Purpose: Reshuffle the phosphate group
Enzyme: Isomerase (Phosphohexose isomerase)
Biochemical process: Isomerisation
delta Go’ : -1.7 kj/mol
delta G: + 2.9 kj/mol
The rxn is reversible and spontaneous
What is the 11th reaction of gluconeogenesis?
Glucose-6-phosphate to Glucose
Purpose: Remove a phosphate from glucose-6-phosphate using water
Enzyme: Hydrolase (Glucose-6-phosphatase)
Biochemical process: Dephosphorylation and hydrolysis
delta Go’ : -13.8 kj/mol
delta G: - 5.1 kj/mol
This reaction is irreversible and spontaneous
What is the fate of lactate generated while exercising?
Lactate is converted into pyruvate and enters gluconeogenesis.
What are the two regulated steps of gluconeogenesis?
1) Pyruvate carboxylase
2) Fructose-1,6-phosphatase
What is pyruvate carboxylase regulated by?
Increased Acetyl CoA in the liver
What is Fructose 1,6-bisphosphatase regulated by?
Increased cAMP
Increased Protein Kinase A (PKA)
Decreased Fructose 2,6-bisphosphate
Decreased pyruvate kinase activity
Explain how the pyruvate kinase reaction is bypassed in gluconeogensis.
- Pyruvate is converted to OAA by pyruvate carboxylase (biotin dependent)
- OAA is transported into the cytosol as either malate (malate dehydrogenase) or aspartate (ATT)
- In the cytosol OAA is regenerated and decarboxylated using a GTP to PEP via PEP carboxykinase
List 4 substrates that can be used for gluconeogenesis.
lactate, pyruvate, glycerol, alpha keto acids
Explain how the hexokinase/ glucokinase step is bypassed in gluconeogenesis. Where does this occur?
This step only occurs in the liver because we are trying to maintain blood glucose levels.
Glucose 6 phosphatase convets G6P to glucose
How is the PFK1 step of glycolysis bypassed in gluconeogenesis?
Fructose 1,6 bisphosphate is converted to fructose 6 phosphate by fructose 1,6 bisphosphatase.
Explain how ATP and AMP regulate fructose 1,6 bisphosphatase in gluconeogenesis.
Inhibited by high AMP because the liver doesn’t want to commit suicide by giving up all of it’s energy when it’s own suppy is low.
ATP activates F 1,6BPase because it signals that the liver energy is good and is can give away glucose
