What are the unique features of hepatocytes allowing them to have a key role in glucose metabolism?
- Hepatocytes have the GLUT2 transporter which allows the passive diffusion of glucose into and out of the cells
- Hepatocytes have hexokinase IV (glucokinase) which has a higher Km than other hexokinases meaning it has a lower affinity to glucose so if glucose levels are low it will not be converted to G6P so it can be sent back out to the body.
- Glucokinase also is not inhibited by G6P- so G6P can be synthesised continually (maintaining import of glucose into hepatocyte)
What are the fates of G6P in the liver?
- Dephosphorylated to make free glucose that can diffuse out of the liver and sent to other tissues if blood glucose levels are low
- Made into liver glycogen by glycogen synthase (if blood glucose is normal -> high)
- Enter glycolysis to form acetyl CoA and then ATP via oxidative phosphorylation (if hepatocyte ATP is low)
- Enter glycolysis to form acetyl CoA which is converted into fatty acids and then triacylglycerols (if hepatocyte ATP is high)
- Enter the pentose phosphate pathway to form NADPH and ribose 5 phosphate
Metabolism of fatty acids in the liver:
- Used to make liver lipids
- converted into acetyl CoA and NADH (by B-oxidation)- some of this acetyl CoA can go onto make ATP via oxidative phosphorylation while some excess acetyl CoA can be used to synthesise cholesterol
- Used to synthesise phospholipids
- Converted into TAGs for storage
- Transported to other tissues for B-oxidation
Where in the body are each of these glucose transporters found:
- GLUT 1:
- GLUT 2:
- GLUT 3:
- GLUT 4:
- GLUT 1: ubiquitous- red blood cells
- GLUT 2: liver and pancreas
- GLUT 3: brain
- GLUT 4: myocytes and adipocytes
Which glucose transporters are insulin independent?
- GLUT 1 (erythrocytes) and GLUT 3 (brain) are insulin independent and will take up glucose even when blood glucose is low (therefore GLUT1/GLUT3 have a low Km for glucose)
How does GLUT2 work?
- GLUT2 transporters are found on the liver and pancreas and they have a high Km for glucose meaning they will only bind and transport glucose into the cells when blood glucose concentration is high
- The pancreas responds to the glucose uptake by secreting insulin
How do GLUT4 transporters work?
- GLUT4 on myocytes and adipocytes is regulated by insulin
- When insulin levels are low (lower blood glucose) there are some GLUT4 transporters on the membrane but most are sequestered within vesicles
- When blood glucose (and therefore insulin) increases insulin binds to the surface these cells and triggers the incorporation of GLUT4 transporters into the membrane so uptake of glucose increases by 15x
- When insulin and blood glucose levels fall the transporters are sequestered back into intracellular vesicles
Which has a higher Km for glucose: hexokinase I (glucokinase) or hexokinase IV?
Which is inhibited by G6P?
- Hexokinase IV (glucokinase) has a higher Km and is only expressed in the liver; this means that glucokinase has a lower affinity for glucose so will only bind it and convert it to G6P when hepatocyte glucose levels are high
- Hexokinase I is inhibited by G6P
When is glycolysis favoured?
Occurs when cells need energy:
- ATP is low
- ADP is high
- AMP is high
- NADH/NAD+ ratio is low
When is gluconeogenesis favoured?
Occurs when cells have a good supply of energy:
- ATP is high
- ADP is low
- AMP is low
- NADH/NAD+ ratio is high
Which glycolytic reactions are irreversible?
Reactions 1,3 and 10
How are glycolytic and gluconeogenic pathway mediated?
- By the 3 irreversible steps of glycolysis meaning that there are 3 bypass reactions in gluconeogenesis to bypass these reactions
How is the first futile cycle of glycolysis regulated?
This first reaction of glucose -> glucose-6-phosphate is catalysed by hexokinase IV in hepatocytes; this means that it will only occur if glucose levels are high; if glucose levels are low the bypass reaction: glucose-6-phosphate -> glucose catalysed by glucose-6-phosphatase will be favoured
What is the second futile cycle? What conditions favour it?
What conditions inhibit it?
- The second futile cycle is the 3rd step of glycolysis involving tge conversion of fructose 6-phosphate -> fructose 1,6- bisphosphate by phosphofructokinase-1 (PFK1)
- PFK1 is stimulated by low energy molecules ADP and AMP and the presence of the allosteric modulator fructose 2,6-bisphosphate (F26BP)
- PFK1 is inhibited by high energy molecules ATP and citrate as well as insufficient F26BP levels
What is the second bypass reaction of gluconeogenesis?
What conditions favour it?
What conditions inhibit it?
- The second bypass reaction is the conversion of fructose 1,6-bisphosphate to fructose-6-phosphate by fructose 1,6-bisphosphatase1 (FBPase1)
- FBPase 1 is indirectly favoured by low glucose levels which result in low insulin levels and thus low F26BP production and low activity of the opposing glycolytic enzyme PFK1
- FBPase1 is inhibited by low energy molecules such as AMP
What is F26BP? What is it produced in response to?
- Fructose 2,6, bisphosphate (F26BP) is an allosteric modulator of the PFK1 enzyme of the second futile cycle of glycolysis
- It is produced in response to high glucose (and thus high insulin) levels
How is F26BP formed?
Fructose-6-phosphate is converted to F26BP by the enzyme phosphofructokinase 2 (PFK2) which is active in its dephosphorylated form. In order for F26BP to be formed the opposing enzyme FBPase 2 which breaks F26BP down to F6P must be inactive- the desphorylated form.
- PFK2 and FBPase 2 are dephosphorylated in response to high insulin
Give a summary of how insulin signalling leads to the favouring of the 2nd futile cycle glycolysis reaction:
Increase in glucose -> increase in insulin -> insulin activates the phospho-protein phosphatase that dephosphorylates PFK2 and FBPase2 -> FBPase2 is active in this form; PFK2 is not active -> Fructose-6-phosphate is converted to F26BP by active PFK2 -> Higher F26BP levels allosterically up-regulate PFK1 (along with activation from ADP and AMP) -> Fructose-6-phosphate is converted to Fructose-1,6-bisphosphate
Give a summary of how glucagon signalling leads to the favouring of the second bypass reaction of gluconeogenesis:
Decrease in glucose -> increase in glucagon -> glucagon activates cAMP dependent protein kinase -> this protein kinase phosphorylates FBPase-2 and PFK-2 -> FBPase- 2 is now in its inactive form and PFK-2 is in its active form -> PFK-2 converts fructose-2,6-bisphosphate (F26BP) to Fructose-6-phosphate -> a lack of F26BP means that the glycolytic enzyme PFK1 loses activity and the gluconeogenic reaction is favoured -> FBPase1 converts fructose 1,6-bisphosphate to fructose-6-phosphate
What is the glycolytic reaction in the third futile cycle?
How is it regulated?
- The glycolytic reaction of the third futile cycle is: conversion of 2x phosphoenol pyruvate -> 2x pyruvate
- This reaction is catalysed by pyruvate kinase
- It is regulated by glucagon levels; as when glucagon levels are high (glucose is low), glucagon activates a cAMP dependent protein kinase that phosphorylates pyruvate kinase and makes it inactive and therefore down regulates the glycolytic reaction.
- If glucose levels are high (glucagon levels are low) a protein phosphatase will cleave the phosphate group off the pyruvate kinase activating it so it can convert the 2x phosphoenol pyruvate molecules -> 2x pyruvate molecules
What is the 1st bypass reaction of gluconeogenesis? How is is regulated?
- This first reaction is the conversion of 2x pyruvate -> 2x oxaloactate -> 2x phosphoenol pyruvate by the enzymes pyruvate carboxylase and PEP carboxylase respectively
- This pathway is favoured by low glucose which leads to glucagon secretion which activates a protein kinase enzyme that inactivates the opposing enzyme pyruvate kinase
How is glycogen synthesis (glycogenesis) activated? What conditions favour it?
Glycogenesis occurs in the fed state (liver)/resting state (muscles).
- The main regulatory enzyme is glycogen synthase
- When glucose levels in the blood are high; insulin levels in the cells will increase
- High insulin levels activate the phosphatase enzymes that dephosphorylate glycogen phosphorylase a and glycogen synthase b into glycogen phosphorylase b and glycogen synthase a
- due to this phosphorylation glycogen synthase is in its active form- a; and glycogen phosphorylase is in its inactive form- b
- Glycogen synthesis (glycogenesis) is favoured)
How is glycogen breakdown activated? How is it regulated?
- Glycogen breakdown is activated in the fasting state (liver) and the active state (muscle)
- During these conditions insulin and epinephrin levels are increased which activates the phosphokinases that phosphorylate glycogen synthase a and glycogen phosphorylase b
- the phosphorylation of the enzymes results in glycogen synthase being in form b- less active, and glycogen phosphorylase being in form a- more active
- The glycogen phosphoylase cleaves the glycosidic linkages in glycogen releasing glucose-1-phosphate
What are the two major processes involved in amino acid metabolism in the liver?
- Transamination (transfer of a amino group from an amino acid to a keto-acid)
- Deamination (release of an amino group)
