Gluconeogenesis Flashcards
in what conditions does gluconeogenesis occur?
1) starvation
2) intense exercise
3) diabetes
What is the criteria for the usage of FA to make glucose?
Must use the last 3 FA of uneven chains
what is gluconeogenesis? what are the main sources?
synthesis of glucose from non-carb sources
-Glycerol, amino acids and lactate (90%)
how many glycerol molecules are needed to make glucose?
2
where does OAA enter into gluconeogenesis?
DHAP
what is the most important amino acid used to make glucose?
alanine
how many pathways do glycolysis and gluconeogenesis share?
7
what is pyruvate converted to prior to PEP? what is needed for this reaction
oxaloacetate
-ATP is needed
why does the conversion of Pyruvate to PEP require 2 reactions?
PEP is a very high energy molecule so in order to convert pyruvate to PEP, 2 high energy molecules are needed (ATP/GTP)
what enzyme catalyzes the conversion of pyruvate to oxaloacetate? what is special about this enzyme?
pyruvate carboxylase
-only found in mitochondria, tightly regulated and irreversible
why is the conversion of pyruvate to oxaloacetate sometimes considered an anaplerotic reaction?
it is considered an anaplerotic reaction if oxaloacetate is being used in the CAC following the rxn
in gluconeogenesis, what kind of molecule is oxaloacetate considered to be?
an activated form of pyruvate
what is the role of biotin in the conversion of pyruvate to oxaloacetate? what 3 steps are involved?
Biotin functions similar to the lipoamide swinging arm, transferring CO2 to pyruvate, which is then transferred to oxaloacetate
1) addition
2) binding
3) transfer
what is the relation of Acetyl-coA in the carboxylase rxn converting pyruvate to oxaloacetate?
allosteric activator
how is PEPCK regulated?
it does not have any allosteric regulators and is not a tightly regulated enzyme
how is oxaloacetate converted to PEP? what is needed for this reaction to occur?
1) CO2 removed from oxaloacetate
2) outer phosphate from GTP removed and transferred to enol form of oxaloacetate to make PEP
what forms of the PEPCK enzyme exist?
mitochondrial and cytosolic
How is F-1,6-BP converted to F6P in gluconeogenesis? what enzyme does this oppose?
Hydrolysis rxn using the enzyme fructose bisphosphatase to remove a phosphate group
opposes PFK-1
-not as tightly regulated
How is glucose formed from G6P? where is this reaction not present and why? what enzymes does this oppose?
a regulated, irreversible hydrolysis rxn
never found in muscle cells because if G6P was converted to glucose it would be shipped out of the muscle cell to be used elsewhere
opposes hexokinase and is regulated by substrate availability
where do majority of the reactions of gluconeogenesis occur?
in the cytosol
in the fed state, what is the source used to create glucose?
NADH
How does NADH production affect gluconeogenesis in the fed vs starving state?
in the starving state, no NADH is produced because lactate is not converted to pyruvate, therefore NADH is not made
this means that there is no NADH production to be used in the GAP dehydrogenase rxn in gluconeogenesis
- alanine is converted to pyruvate so there is no source of electrons from there
why is OAA converted to malate before leaving the cytosol?
there are no transporters to transport OAA out of the mitochondria so it must be converted to OAA first
what enzyme converts OAA to malate in the mitochondria during gluconeogenesis?
mitochondrial malate dehydrogenase
what enzyme converts malate to OAA in the cytosol in gluconeogenesis?
cytosolic malate dehydrogenase
what enzyme converts Pyruvate to OAA in gluconeogenesis? where does this rxn occur?
Pyruvate carboxylase
-occurs in the mitochondrial matrix
can NADH be transported directly across the mitochondrial membrane? how is this done?
no the membrane is impermeable to large molecules like NADH
-electrons are shuttled from the matrix to the cytosol using electron shuttle systems
- malate is used to shuttle electrons in gluconeogenesis
how does PEPCK-C and PEPCK-M differ in terms of their activity?
PEPCK-C is inducible while PEPCK-M is always on in the mitochondria
what hormone increases expression of the lactate pathway in gluconeogenesis?
lactate
why does gluconeogenesis not occur in muscle cells?
They lack the enzyme to convert G6P to glucose
-Glucose-6-phosphotase
how are glycolysis and glucose regulated? which enzyme catalyzing the conversion of F-1,6-BP to F6P is more tightly regulated?
reciprocally
-PFK-1 is more tightly regulated
what are allosteric regulators of PFK-1?
inhibitors: ATP and Citrate
activators: AMP
what are allosteric regulators of fructose bisphosphatase?
AMP is an inhibitor
what is the main function of fructose 2,6BP?
mirrors glucose levels and acts as an important regulator for glycolysis and gluconeogenesis
-potent allosteric activator of PFK and an inhibitor of FBase
what kind of enzyme is PFK-2? what does this mean?
bi-functional enzyme
-it can act as a kinase or phosphatase
what enzyme is responsible for converting F6P to F-2,6-BP? what is the purpose of F-2,6-BP?
PFK-2
-F-2,6-BP is not involved in glycolysis directly but acts as a potent allosteric activator of
PFK-1
what 3 domains are present on PFK-2?
regulatory - gets phosphorylated / dephosphorylated
Kinase - phosphorylates
Phosphatase - dephosphorylates
what effect does glucagon have on FPK-2? how does this effect glycolysis?
increased glucagon levels will cause phosphorylation of PFK-2, activating its phosphatase domain in order to remove phosphate group from F-2,6-BP, converting it to F6P
The reduction in F-2,6BP will slow down glycolysis as this mirrors insulin levels and tells the body glycolysis does not need to be activated
what kind of enzyme does PFK-2 act as when it is phosphorylated? and dephosphorylated?
acts as a phosphatase when phosphorylated
acts as a kinase when dephosphorylated
what is the name of the enzyme that responds to glucagon levels to phosphorylate PFK-2?
PKA (Protein Kinase A)
what is the enzyme responsible for de-phosphorylating PFK-2 in response to insulin in the liver?
PP-1 (protein phosphatase-1)
explain the metabolic effects of low blood glucose on the PFK-2 pathway in the liver:
explain the metabolic effects of high blood glucose on the PFK-2 pathway in the liver:
what kind of control is regulation of PFK-2 subject to in the muscle cells? why?
allosteric control
- it doesn’t respond to glucagon and is not regulated by phosphorylation
what is the effect of high levels of F6P on glucose uptake in skeletal muscle cells?
High levels of F6P (allosteric inhibitor) will:
1) inhibits phosphatase domain on PFK-2
2) increases production of F-2,6-BP
3) enhances glycolysis
what is the effect of Acetyl-coA on gluconeogenesis? when does this occur? why is Acetyl coA being used?
high levels will stimulate gluconeogenesis
-starvation and intense exercise
-Fatty acids are being oxidized to produce Acetyl-coA
how many FA yield how many Acetyl coA? why?
10 FA = 5 Acetyl coA
There are 20C in FA and 2C in Acetyl
What effect does Acetyl coA have on pyruvate carboxylase?
Allosteric activator
-converts pyruvate to OAA
during starvation vs exercise, what sources are typically used to create pyruvate?
starvation: uses alanine to make pyruvate
exercise: oxidizes FA to acetyl coA which stimulates production of OAA
-gluconeogenesis is not turned on because glucagon is most likely not released
what determines whether OAA goes into the CAC or gluconeogenesis?
glucagon signals
-most important factor is if energy is needed for brain, if so it will be sent to CAC
what is a common effect of glucagon vs insulin in regards to phosphorylation?
glucagon typically phosphorylates and insulin typically de-phosphorylates
what effect does glucagon have on pyruvate kinase (PK)?
inhibits
what effect does Acetyl-coA have on pyruvate kinase (PK) in the liver vs muscle?
inhibits PK in the muscle
-unlikely to build up enough in the liver as it will be used to make FA or shipped of to CAC
what effect does ATP have on pyruvate kinase (PK)?
inhibits it
what effect does F-1,6-BP have on pyruvate kinase (PK)?
activates it (feed forward activator)
what effect does Acetyl CoA have on pyruvate carboxylase?
activates it to convert pyruvate to OAA
what effect does citrate have on PFK-1?
inhibits it
what effect does ATP have on PFK-1?
inhibits
what effect does ADP/AMP have on PFK-1?
activates it
what effect does F-2,6-BP have on PFK-1?
what effect does F-2,6-BP have on Fructose-1,6-BPase?
inhibits it
what effect does AMP have on Fructose-1,6-BPase?
inhibits it
what effect does G6P have on glucose-6-phosphotase?
activates it
what effect does glucose have on glucokinase?
activates it
what does Fructose-1-phosphate signal to the liver? why?
It is a potent indicator that we’re in a well fed state
- when fructose is consumed it is converted into Fructose-1-phosphate, which can only be made by ingesting fructose
what makes fructose metabolism dangerous to the body?
when fructose is consumed it is converted to fructose-1-phosphate which is then broken down to DHAP and glyceraldehyde
Since DHAP is a glycolysis intermediate that is created after the PFK-1 step, this means it can bypass the regulatory step and go through glycolysis unregulated
metabolism of fructose promotes fat metabolism as the intermediates produced from the breakdown of fructose can be used for lipogenesis
how is glucokinase (GK) regulated in the liver?
1) When blood glucose levels are low, GK is bound to glucokinase receptor protein (inactive state) in the nucleus
2) as blood glucose levels rise, GLUT 2 is inserted into the cell membrane and glucose levels rise in the cell
GK can be released from GKRP in two ways:
a) Fructose-1-phosphate is metabolised in the liver from ingested fructose which aids in the displacement of GK from GKRP
b) Direct binding of glucose to GK causing conformational change and releasing it from GKRP
3) the free GK moves to the cytosol where it binds with glucose and is converted to G6P
4) glycolysis is then carried out and glucose / fructose levels decrease and GK is bound back to GKRP
what is the effect of F6P on the regulation of glucokinase (GK) in the liver? what does this tell us about metabolism in the body?
Fructose-6-Phosphate will enhance the binding of GK to the GKRP
High levels of F6P will signal that gluconeogenesis is active and we are in the starving state
what is the effect of F1P on the regulation of glucokinase (GK) on the liver? what does this tell us about metabolism in the body?
Fructose-1-phosphate will decrease binding of GK to the GKRP
this signals that glycolysis is active and we are in the well fed state
how do fructose-1-phosphate and fructose-6-phosphate regulate glucokinase (GK) in the liver?
they compete at the same binding site for the GKRP
F1P will decrease binding
F6P will increase binding
what do high levels of F6P signal in muscle cells? why is this?
High levels of F6P in the muscle cells signal that glycolysis is active
muscle cells are only considered with making energy and they do not undergo gluconeogenesis, therefore, high levels of F6P signal in influx of glucose or a slowing of glycolysis due to sufficient energy needs
how does AMPK affect GKRP?
AMPK will phosphorylate GKRP when AMP levels are high, the phosphorylation will reduce the binding of GK to the GKRP
what do high levels of AMP signal and where? what do high levels of ADP signal and where?
high AMP signal low energy in the cytosol and high ADP signals low energy in the mitochondrial matrix
