L14: Gluconeogenesis

Question 1

LO1: List three sources of blood glucose

Answer 1

Glycogenolysis
Gluconeogenesis
Glycolysis (diet/carbs)

Question 2

LO2: Name three liver enzymes whose activity is essential to buffering the blood glucose concentration

Answer 2

1. Glucose-6-Phosphatase, used in gluconeogenesis
2. Glucokinase, used in glycolysis, glycogen Synthesis
3. Glycogen phosphorylase, used in glycogenolysis

Question 3

LO3: How is gluconeogenesis dependent on glycolysis, the TCA cycle (citric acid cycle), and fatty acid oxidation?

Answer 3

Glycolysis: anerobic version provides lactate, which can be used to resynthesize glucose in gluconeogenesis (recycling)

TCA cycle: provides oxaloacetate, which is modified to make glucose

Fatty acid oxidation: provides the 4ATP+2GTP needed to drive gluconeogenesis

Question 4

LO4: List the four enzymes that are unique to gluconeogenesis, their tissue specificity, and how they are regulated.

Answer 4

IN LIVER AND KIDNEY
Glucose-6-phosphorylase: absent in adipose and skeletal muscle, but otherwise located in ER, synthesis decreased by insulin and increased by glucagon

Fructose-1,6-bisphosphatase (FBPase1): reaction occurs in cytosol; allosterically activated by ATP and glucagon, and inhibited by AMP, F-2,6-P2

PEPCK: cytosolic form in glucogenogenesis while mitochondrial form does other things, transcription of PEPCK gene stimulated by cAMP (increased by glucagon, epinephrine, glucocorticoids, repressed after carb feeding/insulin release)

Pyruvate carboxylase- reaction occurs in mitochondria, requires biotin and ATP, allosterically activated by acetyl-CoA

Question 5

LO5: Reaction catalyzed by pyruvate carboxylase? what activates it?

Answer 5

pyruvate+CO2+ATP+H20—-pyruvate carboxylase, biotin—->oxaloacetate +ADP +Pi

-activated by acetyl CoA

Question 6

LO5: Reaction catalyzed by PEPCK?

Answer 6

oxaloacetate+GTP—-PEPCK—–> posphoenolpyruvate+GDP+Pi+CO2

• stimulated by increased cAMP due to glucagon, epinephrine, glucocorticoids
• repressed after carb. feeding/insulin release

Question 7

LO5: Reaction catalyzed by FBPase1?

Answer 7

fructose-1,6-P2+H20—–FBPase1——>fructose-6-P+Pi

• activated by ATP
• inhibited by AMP+F-2,6-P2

Question 8

LO5: Reaction catalyzed by G-6Pase?

Answer 8

Glucose-6-phosphate+H2O—–G-6Pase—-> glucose+Pi

• highly expressed in liver and kidney, somewhat expressed in intestine and pancreas
• not expressed in adipose/skeletal muscle (reason why glucose stays trapped in glycogen in this tissue, as in can’t be used for blood glucose)

Question 9

LO6: Interplay between mitochondria+cytosol in gluconeogenesis

Answer 9

1. Pyruvate carboxylase (in mitochondria) converts pyruvate to oxaloacetate
2. Malate oxaloacetate shuttle required to transport OAA (converted to malate in mitochondria, malate is transported to cytoplasm and oxidized back to oxaloacetate once back in cytosol)
3. OAA can then be converted to PEP for next step in cytosol

Question 10

LO7: List three sources of carbon skeletons that are used for gluconeogenesis

Answer 10

1. glycerol (converted to DHAP by liver)
2. lactate (converted to pyruvate)
3. glucogenic amino acids (alanine=major, all AAs except lysine and leucine)
4. proprionyl-CoA (via degredation of Val, Ile, Thr, Met and odd-chained FAs)

Question 11

LO8: Describe the Cori cycle and the role it plays in glucose homeostasis

Answer 11

• provides a mechanism by which the end products of anerobic glycolysis (muscle, RBCs) can be transported to liver/used to resynthesize glucose (recycling only; no net glucose synthesis)
• lactate from muscle/RBCs—->blood—–>liver—–>pyruvate—–>glucose——>blood——>muscle/RBCs—–>pyruvate——->lactate again

Question 12

LO9: Predict the effects of deficiencies in biotin and B12 on gluconeogenesis

Answer 12

BIOTIN

• required for pyruvate carboxylase to make OAA from pyruvate (OAA couldn’t form and gluconeogenesis wouldn’t occur)
• also required for propionyl-CoA carboxylase to add carboxyl onto propionyl-CoA to make methyl malonyl CoA (methyl malonyl CoA wouldn’t then be converted to succinyl-CoA, an intermediate of TCA cycle that leads to replenishment of OAA)

B12
-required for conversion of methyl malonyl-CoA to succinyl CoA (see above)

Question 13

LO9: Predict the effects of deficiencies in B6 and niacin on gluconeogenesis

Answer 13

B6
-precursor to pyridoxal phosphate, which is a required coenzyme for transamination of AAs (catabolism; would result in decreased carbon skeletons available for gluconeogenesis)

NIACIN
-used in many ox/redox reactions (NAD/NADH; NADP/NADPH) (would cause problems with malate-OAA shuttle and any other pathway that requires these factors)

Question 14

LO10: Why would a deficiency in FBPase1 result in lactic acidosis?

Answer 14

• normally generates fructose-6-P + Pi
• gluconeogenesis will be inhibited, so pyruvate will be diverted to undergo reduction by lactate dehydrogenase instead
• lactate will accumulate, resulting in lactic acidosis

Question 15

LO11: Discuss the role of fructose-2,6-bisphosphate in the coordinate regulation of gluconeogenesis and glycolysis in the fed. vs. fasted states

Answer 15

• Fructose-2,6-bisphosphate=signalling molecule, regulates FBPase1 and PFK-1
• it is coordinated by the bifunctional enzyme (PFK-2/FBPase-2)
• when phosphorylated (fasted state), FBPase-2 is active and PFK-2 is inactive, so F-2,6-P2 levels are decreased and FBPase-1 activity is increased (gluconeogenesis activated) and PFK-1 activity is inhibited (glycolysis inhibited)
• when dephosphorylated (fed state), PFK-2 is active and FBPase-2 is inactive, so F-2,6-P2 levels are increased and FBPase-1 activity is decreased (no gluconeogenesis) and PFK-1 activity is activated (glycolysis activated)