22 - Gluconeogenesis Flashcards
What is gluconeogenesis?
Making glucose from scratch in the liver
When does gluconeogenesis happen? What prompts it?
Gluconeogenesis occurs more and more after last meal. A decreasing amount of glycogen in the liver prompts it.
What stoichiometrically goes into gluconeogenesis? (6 things)
2 Pyruvate 2 NADH 4 H+ 4 ATP 2 GTP 6 H2O
What stoichiometrically comes out of gluconeogenesis?
1 Glucose 2 NAD+ 4 ADP 2 GDP 6 Pi
Why is gluconeogenesisi not a perfect reversal of glycolysis?
- The highly exergonic steps of glycolysis must be circumvented in gluconeogenesis
- Only the steps in the pathway that are operating near steady state are the same for glycolysis and gluconeogenesis
What glycolytic enzymes are NOT used in gluconeogenesis?
Hexokinase
Phosphofructokinase
Pyruvate kinase
What glucose enzymes are used in gluconeogenesis and not glycolysis? (4)
- Pyruvate carboxylase
- PEPCK (Phosphoenolpyruvate carboxykinase)
- Fructose bisphosphatase
- Glucose-6-phosphatase
Which enzymes do glycolysis and gluconeogenesis share (reversible enzymes) (6)
- Enolase
- Phosphoglycerate mutase
- Phosphoglycerate kinase
- glyceraldehyde-3-phosphate dehydrogenase
- triose phosphate isomerase
- aldolase
What two reactions does gluconeogenesis use to convert pyruvase to PEP? Which one of these is a anaplerotic reaction?
- Pyruvate carboxylase converts pyruvate into oxaloacetate
- PEP carboxykinase converts oxaloacetate into PEP
The conversion of pyruvate to oxaloacetate by pyruvate carboxylase is an anaplerotic reaction
What are all gluconeogenic precursors first converted to?
oxaloacetate, which is converted to PEP (phosphoenolpyruvate)
Why are fatty acids and acetyl-CoA not gluconeogenic?
Because acetyl-CoA is converted to CO2 when reacted with oxaloacetate (ie. it is not a TCA cycle intermediate)
Fatty acids are not TCA cycle intermediates either
What does pyruvate carboxylase require to convert pyruvate to oxaloacetate?
ATP
Biotin as prosthetic group
Allosterically activated by acetyl CoA (high acetyl-CoA = a need for TCA cycle intermediates)
What does PEPCK do and what does it require to do it?
PEPCK (phosphoenolpyruvate carboxykinase) uses GTP as a phosphorylating agent and decarboxylates oxaloacetate to form phosphoenolpyruvate (PEP)
PEPCK is a gluconeogenic enzyme that converts oxaloacetate to PEP (phosphoenolpyruvate), is it anaplerotic or cataplerotic?
Cataplerotic
What are the intermediates of gluconeogenesis (12, in order)
- Pyruvate and other substrates for:
- Oxaloacetate
- Phosphoenolpyruvate (PEP)
- 2-Phosphoglycerate
- 3-Phosphoglycerate
- 1,3-bisphosphoglycerate
- glyceraldehyde-3-phosphate
- Dihydroxyacetone phosphate
- Fructose-1,6-bisphosphate
- Fructose-6-phosphate
- glucose-6-phosphate
- Glucose
What are the enzymes of gluconeogenesis? (11, in order between the intermediates)
Pyruvate 1) Pyruvate carboxylase Oxaloacetate 2) PEPCK Phosphoenolpyruvate (PEP) 3) Enolase 2-Phosphoglycerate 4) phosphoglycerate mutase 3-Phosphoglycerate 5) phosphoglycerate kinase 1,3-bisphosphoglycerate 6) Glyceraldehyde-3-phosphate dehydrogenase glyceraldehyde-3-phosphate 7) triose phosphate isomerase Dihydroxyacetone phosphate 8) aldolase Fructose-1,6-bisphosphate 9) fructose bisphosphatase Fructose-6-phosphate 10) phosphoglucose isomerase glucose-6-phosphate 11) glucose-6-phosphatase Glucose
Between what two intermediates are enzymes the same in gluconeogenesis as they are in glycolysis?
Phosphoenolpyruvate (PEP) to glyceraldehyde-3-phosphate
Where is glucose-6-phosphatase expression restricted to in the body?
To tissues with gluconeogenesis, mostly in the liver, some kidney.
Phosphorylated glucose can or cannot be transported out of the cell?
Cannot
Besides using different enzymes, what makes the reactions between fructose-1,6-bisphosphate/fructose-6-phosphate and glucose-6-phosphate/glucose different?
In glycolysis, the reactions between these four intermediates use ATP and convert it to ADP. in gluconeogenesis, an H2O molecule is used by the phosphatases to convert the intermediates, producing an inorganic phosphate (Pi) in the process
Why does glucose not get broken down to pyruvate and then that pyruvate gets resynthesized into glucose?
Because these futile cycles would cost energy and therefore must be avoided
What does Pfk-2 do to regulated glycolysis? (phosphofructokinase 2)
Catalyzes phosphorylation of fructose-6-phosphate to fructose-2,6-bisphosphate, which acts as an allosteric regulator of Pfk-1 (increases activity). Fructose-2,6-BP also inhibits FBPase-1 activity
What does Pfk-1 do in glycolysis?
Phosphofructokinase 1 catalyzes phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis
Why is phosphofructokinase-2 (Pfk-2) considered a bifunctional enzyme? What regulates this property of it?
Because it has Pfk-2 activity in glycolysis
And FBPase-2 (Fructose-bisphosphatase) activity in gluconeogenesis
This is because it has seperate domains for these two pathways that are regulated by phosphorylation
What does FBPase-1 do? In which pathway? What regulates it?
FBPase-1 catalyzes the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate in gluconeogenesis.
It is a domain part in Pfk-1, and inhibited by fructose-2,6-bisphoshate, which is produced by Pfk-2
What does FBPase-2 activity of Pfk-2 do?
FBPase-2 converts fructose-2,6-BP (which inhibits gluconeogenesis and stimulates glycolysis) to fructose-6-phosphate.
FBPase activity promotes gluconeogenesis and inhibits glycolysis
What leads to the inhibition of glycolysis and the stimulation of gluconeogenesis?
In response to low blood glucose, glucagon stimulates protein kinase A, which activates FBPase-2,
FBPase-2 stimulates conversion of fructose-2,6-bisphophate to fructose-6-phosphate, stopping the F-2,6-BP’s inhibition of gluconeogenesis and stimulation of glycolysis
What leads to the stimulation of glycolysis and inhibition of gluconeogenesis?
High levels of fructose-6-phosphate stimulate phosphoprotein phosphatases, these activate Pfk-2, which catalyzes the synthesis of fructose-2,6-bisphosphate
Fructose-2,6-bisphosphate stimulates PFK to convert fructose-6-phosphate to fructose-1,6-bisphosphate (glycolysis) and inhibits FBPase-1 activity of the reverse reaction (gluconeogenesis)
When glucose is abundant in the blood:
Glycolysis is dominant pathway
GLuconeogenesis is the dominant pathway
Glycolysis is dominant, stimulated by high levels of fructose-6-phosphate.
Gluconeogenesis only becomes dominant when levels of glucose in the blood are low
What things inhibit phosphofructokinase activity of glycolysis (2) and stimulate it (2)
Stimulate
- Fructose-2,6-bisphosphate
- AMP
Inhibit
- ATP
- Citrate
What things inhibit fructose-1,6-bisphosphatase activity of gluconeogenesis (2) and stimulate it (1)
Inhibit
- Fructose-2,6-bisphosphate
- AMP
Stimulate
- Citrate
What is the feed-forward enzyme in the reciprocal regulation of glycolysis and gluconeogenesis? What activates it?
Pyruvate kinase
Activated by fructose-1,6-bisphosphate
What inhibits pyruvate kinase (4)? Activates (1)?
Inhibits
- ATP
- Alanine
- Acetyl-CoA
- cAMP-dependent phosphorylation
Activates
- Fructose-1,6-bisphosphate (feed-forward)
What regulates phosphoenolpyruvate carboxykinase?
Under transcriptional control under hormonal regulation by glucagon and insulin
Glucagon (+)
Insulin (-)
What activates pyruvate carboxylase?
Acetyl-CoA
How does breaking down glucose in glycolysis and resynthesizing it in gluconeogenesis cost energy?
Hexokinase and PFK use ATP and produce ADP = exergonic
Fructose-1,6-bisphosphate and glucose-6-phosphatase produce inorganic phosphate = exergonic
Overall, in those steps, glycolysis uses high energy ATP and gluconeogenesis produces low energy inorganic phophates = loss of energy from both things
Where does gluconeogenesis take place (in cells)
In the cytosol
Which materials must be transported into cytosol for gluconeogenesis? From Where?
PEP must be transported from mitochondria directly or malate (from TCA cycle) can be transported and then converted to oxaloacetate (NAD+ to NADH), which is then converted to PEP.
Pyruvate can be converted into oxaloacetate, which can be transported across as malate, or converted to PEP, which can be transported across.
How is oxaloacetate transported across the mitochondrial membrane for gluconeogenesis?
It is converted to malate by the oxidation of NADH to NAD+ in the mitochondrial matrix.
Malate is then transported across the membrane before being re-converted to oxaloacetate by the reduction of NAD+ to NADH (leading to production of NADH in the cytosol).
How does ethanol cause glycemia?
Ethanol inhibits gluconeogenesis, so with no food intake, hypoglycemia can occur
How does ethanol inhibit gluconeogenesis?
It is metabolized in two steps
- Alcohol dehydrogenase converts it to acetaldehyde (generating NADH in the process)
- Aldehyde dehydrogenase converts acetaldehyde to acetate (generating NADH in the process)
- Acetate is converted to Acetyl CoA
Lactate dehydrogenase and oxaloacetate transport into cytoso (generating NADH in cytosol)l is inhibited when cytosolic NADH is too high
NADH also inhibits TCA and fatty acids oxidation in mitochondria
What is the Cori cycle?
- Lactate that is made under anaerobic conditions in muscle is transported to liver.
- Liver uses lactate for gluconeogenesis
- Muscles can use that glucose to prolong activity under anaerobic conditions
- Lactate made in red blood cells (no mitochondria) is metabolized the same way
What does the liver use for gluconeogenesis in anaerobic conditions?
Lactate
Where is lactate made? Under what conditions?
Lactate is made in muscles under anaerobic conditions.
In the Cori cycle, it us shuttles to the liver, where it is used for gluconeogenesis.
How is lactate made in red blood cells metabolized?
Cori Cycle, it is used for gluconeogenesis in the liver and then that glucose is transported to muscles to prolong activity under anaerobic conditions
What is the glucose alanine cycle in muscle? What is the consequence of this?
Pyruvate and glutamate react to α-ketoglutarate and alanine.
Alanine is transported to the liver and enters gluconeogenesis
Outcome: Nitrogen from protein degradation is transported to the liver (urea cycle). During long fasting muscle protein can be used for glucose production
Blood glucose must be maintained between what narrow range (in mM)
4 - 7 mM
What is hypoglycemia?
Low blood glucose, where the brain does not get enough glucose to function
What is hyperglycemia?
High blood glucose, long term detrimental effects, when very high there are acute effects such as fainting
The brain and red blood cells can only use ____ as fuel, not ______
The brain and red blood cells can only use glucose as fuel, not fatty acids
What are four sources of glucose?
- Directly after a meal (fed state), glucose coming from diet
- Several hours after a meal (post-absorptive): no dietary glucose, liver breaks down stored glycogen to supply the rest of the body
- As liver glycogen gets used up, liver gluconeogenesis becomes more and more important as a source of glucose for the body
- The liver starts a process called ketogenesis really long after a meal, where energy molecules that the brain can use, called ketone bodies, are made from acetyl-CoA. This saves glucose, because the brain can get by with less glucose
What are four consequences of ethanol consumption?
- Inhibition of oxaloacetate transport into cytosol and impaired gluconeogenesis
- Lactate dehydrogenases works in direction of pyruvate to lactate, leading to less gluconeogenesis and to lactic acidosis
- Increased NADH inhibits fatty acids oxidation, may increase fatty acid synthesis leading to fatty liver with chronic ethanol ingestion
- NADH inhibits isocitrate dehydrogenases and a ketoglutarate dehydrogenase in the citric acid cycle. Acetyl-CoA can accumulate and a) leade to ketone body synthesis and further acidosis and b) acetaldehyde can accumulate and damage proteins
