Define gluconeogenesis
synthesizes glucose from non-carbohydrates and releases free glucose into the blood
When does gluconeogenesis occur?
during fasting and flight and fight
What stimulates gluconeogenesis?
glucagon, epinephrine and cortisol
Purpose of gluconeogenesis?
provision of blood glucose for cells that need glucose as metabolic fuel
When is gluconeogenesis the sole provider of serum glucose?
when glycogen stores are depleted 24 hours after fasting
Where does gluconeogenesis occur in the body?
liver and to a small extent in the kidney
Which cells perform gluconeogenesis in the beginning of a fast, which cells add to this process during prolonged starvation?
At the beginning of a fast, the hepatocytes perform most of the gluconeogenesis. During prolonged starvation, the kidney cortex cells contribute.
Percentage of gluconeogenesis in liver vs kidney during overnight fast?
90% liver, 10% kidney
Percentage of gluconeogenesis in liver vs kidney during prolonged fast?
60% liver, 40% kidney
What occurs in the alanine-glucose cycle?
Alanine is formed in muscle using pyruvate and in the liver, alanine is used to form pyruvate
Where are the enzymes for gluconeogenesis in cells?
cytosol, except for pyruvate carboxylase which is found in the mito, PEP carboxykinase in mito and cytosol
Why are mito needed for gluconeogenesis?
needed to use lactate, pyruvate and glucogenic amino acids as substrates for gluconeogenesis
Is gluconeogenesis a reversal of glycolysis?
no
Can gluconeogenesis and glycolysis take place at the same time?
no
Can glycogen degradation and gluconeogenesis take place at the same time?
yes
Describe the Cori-cycle. What is transported in blood?
The Cori-cycle describes the release of lactate from muscle (and RBC) into the blood and uptake into the liver. Liver uses this lactate for gluconeogenesis and releases glucose into the blood where it can be taken up by muscle (and RBC).
What is the advantage of the Cori-cycle?
The Cori-cycle allows anaerobic glycolysis and release of lactate without lactate accumulation in blood. It also saves lactate for gluconeogenesis for generation of glucose.
Compare gluconeogenesis to the alanine-glucose cycle.
The Alanine-glucose cycle describes the release of alanine from the muscle into the blood and the usage of alanine in the liver for gluconeogenesis. [Alanine is formed in muscle using pyruvate and in the liver, alanine is used to form pyruvate]
Lactate in the blood is taken up by the liver and the heart. Describe the usage of lactate in the liver and in the heart.
Lactate is taken up into the liver and heart and lactate dehydrogenase forms pyuvate in both tissues. When insulin is ruling, pyruvate in the liver is substrate for PDH and the formed acetyl-CoA joins the TCA cycle. When glucagon is ruling, pyruvate is su
Which amino acids are mainly transported in the blood to the liver and can be used for the urea cycle and gluconeogenesis?
The two mainly transported amino acids in the blood are alanine and glutamine
How are alanine and glutamine used in gluconeogenesis?
Alanine can be used for gluconeogenesis as it generates pyruvate in the liver. Glutamine is also used for gluconeogenesis via glutamate and ?-ketoglutarate.
How many nitrogens are transported by alanine and glutamine for use in urea synthesis?
Alanine transports one nitrogen to the liver, glutamine transports two nitrogens to the liver where they can be used for urea synthesis
Which specific amino acid can be used for gluconeogenesis and inhibits at the same time glycolysis at the level of pyruvate kinase? Explain.
It is alanine that can be used for gluconeogenesis and inhibits at the same time the glycolytic enzyme pyruvate kinase. High levels of alanine from the blood signal protein degradation in muscle during fasting.
Describe what happens under high alanine conditions
Under these conditions, in order to fulfill the alanine-glucose cycle, the glycolytic enzyme pyruvate kinase shall be allosterically inhibited. The inhibition of pyruvate kinase saves PEP. Alanine itself is used to form pyruvate which is used for gluconeo
How many glucose molecules can be formed during gluconeogenesis using one alanine as substrate
None, as we need two alanines leading to two pyruvates. Eventually you need one glyceraldehyde 3-P and one DHAP in order to form fructose 1,6-bisphosphate.
When is liver glycogen depleted?
after 24hours of fasting
Why does glycogen degradation occur before gluconeogenesis during the beginning of a fast?
provides most of the glucose because it is faster than gluconeogenesis
Role of G6P in gluconeogenesis
product of gluconeogenesis and glycogen degradation
Function of G6 phosphatase?
in liver and kidney, bound to ER membrane and acts toward ER lumen, cleaves G6P to glucose which are both transported into cytosol by transporters
How does glucagon act to increase gluconeogenesis enzymes?
activates G-protein, which activates cAMP, which activates PKA that phosphorylates key enzymes of gluconeogenesis pathway and induces gluconeogenesis enzyme transcription
What is the effect of glucagon on glycolysis?
inhibits glycolysis by PKA mediated inactivation of pyruvate kinase
What happens to phosphenol pyruvate if pyruvate kinase is inhibited?
it is used for gluconeogenesis in the reverse direction of glycolysis
What enzyme of gluconeogenesis circumvents the irreversible step of PFK1?
fructose-1,6-bisphosphatase catalyzes the irreversible step converting fructose 1,6 bisphosphate to fructose-6-P
What enzyme of gluconeogenesis circumvents the irreversible step of glucokinase?
glucose-6-phosphatase
What enzyme of gluconeogenesis circumvents the irreversible step of pyruvate kinase?
pyruvate carboxylase and PEP carboxykinase
Function of pyruvate carboxylase?
forms oxaloacetate in mito from pyruvate and CO2, using biotin as a coenzyme and ATP is cleaved, REQUIRES acetyl CoA
What is the other function of pyruvate carboxylase?
can refill the TCA cycle with oxaloacetate if it is needed
How is acetyl CoA formed for pyruvate carboxylase?
generated by beta-oxidateion of fatty acids NOT by pyruvate dehydrogenase
What is the effect of acetyl CoA on pyruvate dehydrogenase?
inhibits it
Where are the two ways oxaloacetate can be formed for gluconeogenesis?
by pyruvate carboxylase or degradation of glucogenic amino acids, cytosolic malate is oxidized to OAA
How does OAA enter mito?
NADH is used to covert OAA to malate, NAD is produced also
What happens to malate in mito?
converted to OAA
Function of PEP carboxykinase?
PEP and GDP fromed by oxaloacetate and GTP
Where is PEP carboxykinase found in humans?
mitochondria and cytosol
What is the function of glycerol kinase in gluconeogenesis?
phosphorylates glycerol to G3P which is then oxidized to DHAP
What coenzyme is needed for the oxidation of lactate, malate and G3P?
NAD, at high cytosolic NADH these enzymes are inhibited (in liver with high alcohol consumption)
How is gluconeogenesis regulated?
energy and substrate availability, ultimately fructose 1,6 bisphosphatase: inhibited by fructose 2,6 bisphosphate and AMP
When is fructose 2,6 bisphosphate formed?
bifunctional enzyme at insulin ruling and high F6P levels
Describe the function of the bifunctional enzyme
froms fructose 2,6 bisP with PFK2 activity and depgrades fructose 2,6 bisP with its BPase-2 activity, needs to be folded in a way to allow only one activity
Function of insulin on bifunctional enzyme?
leads to unphosphorylated enzyme that forms fructose-2,6 bisP - active PFK2
Function of glucagon on bifunctional enzyme?
leads to phosphorylated enzyme that degrades fructose-2,6 bisP - active BPase-2
What are the substrates for gluconeogenesis found inside the liver?
momentary intermediates of glycolysis and some lactate, degradation of amino acids that lead to pyruvate, degradation of glucogenic amino acids that lead to OAA
What is continuously provided via the blood for gluconeogenesis?
lactate from muscle and lactate from RBC (Cori), alanine and glutamine, glycerol from fat cells
How is degradation of TAGs in fat cells connected to gluconeogenesis?
low insulin/glucagon ratio and epinephrine activate lipase in fat cells, glycerol and free fatty acids released, glycerol=gluconeogenesis and fatty acids=beta oxidation
What provides energy for gluconeogenesis?
degradation of fatty acids leading to ATP formation in ETC occurs at the same time as gluconeogenesis
How many ATP needed for gluconeogenesis from 2 pyruvates?
4 ATP & 2 GTP
Why is the formation of acetyl-CoA by beta-oxidation needed?
gluconeogenesis needs it in mito for inhibition of pyruvate dehydrogenase, activation of pyruvate carboxylase
Are the carbons from acetyl-CoA used in gluconeogenesis?
NO
What happens when you eat unripe ackee fruit?
inhibition of fatty acid degradation leading to hypoglycemia.
Do you need pyruvate kinase and PEP carboxykinase for gluconeogenesis starting with glycerol
No, pyruvate kinase and PEP carboxykinase are not needed, glycerol joins eventually at the level of DHAP.
Which substrate is mostly used during starvation and where does it come from?
Amino acids provide the main substrates for gluconeogenesis during starvation. Absence of insulin leads to protein degradation in muscle. The generated free amino acids are changed to mainly alanine and glutamine, which are released into the blood and are
In which steps of gluconeogenesis is NAD+ needed?
Lactate dehydrogenase needs NAD+ in order to form pyruvate from lactate. Malate dehydrogenase in cytosol needs NAD+ in order to provide oxaloacetate for cytosolic PEP carboxykinase. Glycerol 3-P dehydrogenase needs NAD+ for formation of DHAP.
Name the four irreversible reactions catalyzed by enzymes during gluconeogenesis starting with pyruvate
Pyruvate to oxaloacetate by mitochondrial pyruvate carboxylase. Oxaloacetate to PEP by mitochondrial or cytosolic PEP carboxykinase. Fructose 1,6-bisphosphate to fructose 6-P by fructose 1,6-bisphosphatase. Glucose 6-P to glucose by glucose 6-phosphatase
Why can acetyl-CoA or fatty acyl-CoA not be used for gluconeogenesis?
When acetyl-CoA and oxaloacetate enter the TCA cycle, we find two decarboxylations in this cycle and we end up again with oxaloacetate. No molecule was generated in addition to the cycle that could be used for gluconeogenesis.
The liver cannot use ketone bodies for gluconeogenesis. Why? Explain.
Only the liver forms ketone bodies during fasting from extensive ?-oxidation of fatty acids. The ketone bodies are released into the blood. The liver does not contain an enzyme that could be used to fuel ketone bodies into the TCA or into gluconeogenesis.
Describe the release of cortisol, epinephrine and glucagon starting with ACTH.
Stress leads to release of ACTH (adrenocorticotropic hormone, it stimulates the adrenal cortex). The adrenal cortex releases cortisol. While cortisol passes by the adrenal medulla, it leads to release of epinephrine. Epinephrine reduces release of insulin