Week 5: Gluconeogenesis

Question 1

Where does gluconeogenesis occur?

Answer 1

• 90% in the liver
• 10% in the kidney to supply itself

Question 2

Gluconeogenesis sources of pyruvate

Answer 2

• RBCs & Skeletal muscle generate lactate which goes into the blood

In the Liver:

transporters to take up the lactate

lactate + NAD+ -Lactate DH-> pyruvate + NADH

• Alanine mostly from skeletal muscle

In The Liver:

Alanine -Alanine Transaminase-> Pyruvate

Question 3

Gluconeogenesis from pyruvate

Answer 3

2 potential directions in the mitochondria

1. Pyruvate + NAD+ -Pyruvate Dehydrogenase-> acetyl CoA + NADH + CO2
2. Pyruvate -Pyruvate carboxylase-> Oxaloacetate

Which pathway is decided by the [Acetyl CoA]

In the fasting state, the [Acetyl CoA] is high in the liver mitochondrial matrix from the fasting state is designated by Glucagon which stimulates adipose to release FAs which enter the bloodstream and enter the liver mitochondria by Carnitine transporter and undergo β-oxidation generating Acetyl-CoA

High [Acetyl-CoA] inhibits Pyruvate DH so under these conditions pyruvate carboxylase will be activated converting pyruvate to oxaloacetate

Oxaloacetate <-malate dehydrogenase-> Malate (reverse reaction is favorable G= -7.1)

Malate then exits the mitochondria into the cytoplasm

Malate + NAD+ -cytosolic malate dehydrogenase-> Oxaloacetate + NADH

Oxaloacetate - PEP Carboxykinase (PEPCK)-> phosphoenolpyruvate (PEP)

A futile loop could go here because in glycolysis PEP +ADP -pyruvate kinase-> Pyruvate + ATP (But Glucagon signals through Gαs adenylate cyclase cAMP PKA -Pi-> Pyruvate kinase inactivating it preventing the futile loop)

Question 4

Metformin Drug target

Answer 4

PEPCK to prevent gluconeogenesis

PEPCK catalyzes oxaloacetate to PEP in the gluconeogenic pathway

Question 5

PEP Gluconeogenesis

Answer 5

In glycolysis from PEP backwards through Fructose-1, 6-bisphosphate are reversible

So we go backwards from PEP to 1, 3-bisphosphoglycerate

1, 3-bisphosphoglycerate + NADH -glyceraldehyde-3-phosphate DH-> glyceraldehyde-3-phosphate + NAD+

glyceraldehyde-3-phosphate + DHAP -Aldolase-> gfructose-1, 6-bisphosphate (DHAP from β-oxidation)

Fructose-1, 6-bisphosphate + ADP <-PFK1-x Fructose-6-phosphate + ATP (*IS NOT REVERSIBLE SO WE HAVE TO GO AROUND*)

Fructose-1, 6-bisphosphate -fructose-1, 6-bisphosphatase-> Fructose-6-phosphate + Pi

high {ATP} inhibits PFK1, which is high in the fasting state because of the β-oxidation

Question 6

PFK2 structure

Answer 6

has 2 sites

1 has kinase activity

1 has phosphatase activity

Converts fructose-6-phosphate -PFK2-> Fructose-2, 6-bisphosphate which is an activator of PFK-1 when there is a lot of fructose-6-phosphate in the fed state

However, in gluconeogenesis, PFK2 uses its phosphatase activity regulated by glucagon

PFK2 is phosphorylated through the PKA pathway inactivating PFK2’s kinase activity and activates the phosphatase activity

Fructose-2, 6-bisphosphate is dephosphorylated by PFK2 and further inactivates PFK1

Question 7

gluconeogenesis from Fructose-6-phosphate

Answer 7

Fructose-6-phosphate -phosphoglucoisomerase-> glucose-6-phosphate

glucose-6-phosphate <-glucokinase-X Glucose (IRREVERSIBLE)

glucose-6-phosphate -glucose-6-phosphatase-> glucose + Pi

controlled by substrate concentration (in the fasting state, [glucose] low, [glucose-6-phosphate] will be high) glucokinase is a low affinity enzyme so there is not enough glucose for it to work

Question 8

Gluconeogenesis regulatory steps

Answer 8

I think there is 4 maybe more

Question 9

Main sources of pyruvate for gluconeogenesis

Answer 9

• Lactate from skeletal muscle and RBC’s
• Alanine mostly from skeletal muscle

Question 10

What enzyme converts lactate to pyruvate?

Answer 10

Lactate dehydrogenase

Question 11

What enzyme converts alanine to pyruvate?

Answer 11

alanine transaminase

Question 12

Fate of pyruvate in the liver depends on?

Answer 12

• the concentration of Acetyl CoA
• During thfed state their is low Acetyl CoA because it is used in TCA cycle
• In the fasting state their is high Acetyl CoA from beta oxidation and pyruvate is sent into gluconeogenesis

Question 13

What prevents PEP from being catalyzed into pyruvate and go back down glycolysis in gluconeogenesis?

Answer 13

Glucagon signals through GalphaS adenylate cyclase, cAMP and PKA

PKA phosphorylates pyruvate kinase preventing this futile loop

Question 14

what prevents fructose-6-phosphate from being catalyzed back into Fructose-1, 6-bisphosphate and return down glycoylsis in gluconeogenesis?

Answer 14

• In the fasting state ATP levels are high in the liver which inhibits PFK1
• Also, PFK 2 has both kinase and phosphorylase capabilities and converts Fructose-6-phosphate into Fructose-2, 6-phosphate which is an activator of PFK1 to get glycolysis moving, however in the fasting state in the liver, glucagon will activate the PHOSPHOTASE activity of PFK2 to remove any fructose-2, 6-bisphosphate to prevent activation of PFK1

Question 15

What prevents glucose from being catalyzed back into glucose-6-phosphate by glucokinase in gluconeogenesis?

Answer 15

ATP is high in the fasting state in the liver and inhibits glucokinase

Question 16

glucose-6-phosphatase regulation

Answer 16

regulated by substrate concentration of glucose-6-phosphate to glucose

Question 17

DHAP’s role in gluconeogenesis

Answer 17

• A source of G3P for gluconeogenesis from glycerol
• glycerol -glycerol kinase glycerol-3-phosphate -glycerol-3-phosphate DH-> DHAP -triose phosphate isomerase -> Glyceraldehyde-3-phosphate