Regulation of Glycolysis and Gluconeogenesis

Question 1

describe the irreversible steps of glycolysis and gluconeogenesis

Answer 1

Question 2

describe how glycolysis in skeletal muscle is coupled to glycogen degradation

Answer 2

• glycolysis starting with glucose from the blood needs hexokinase and ATP to form G6P
• glycolysis can follow glycogen degradation and use the generated glucose 1-P to form G6P without usage of ATP
• starting with G6P saves 1 ATP in the glycolysis investment phase and the overall gain of 3 ATP for anaerobic glycolysis is achieved

Question 3

describe the regulation of glycolysis in skeletal muscle

Answer 3

• the first regulated enzyme is hexokinase which is activated by glucose provided by GLUT-4 (insulin) and which is inhibited by its accumulating product G6P
• the committed step of glycolysis is performed by PFK-1 which is inhibited by ATP
  
  • the inhibition of PFK-1 is overcome by AMP and F2,6-BP formed by PFK-2 activity of the bifunctional enzyme
• pyruvate kinase is always highly active and does not need feed-forward activation by the committed step of glycolysis

Question 4

contrast hexokinase and glucokinase

Answer 4

• hexokinase has a high affinity for glucose
  
  • very efficient for small amounts of glucose that is saturated with glucose during hypoglycemia
• glucokinase has a low affinity for glucose
  
  • large Vmax which makes it very efficient to phosphorylate large amounts of glucose

Question 5

how is insulin release regulated?

Answer 5

• B-cells take up glucose (GLUT-2) and glucokinase forms G6P which is used in oxidative metabolism for ATP formation
  
  • glucokinase is not product inhibited
• Blood glucose levels >5 mM generate a high level of ATP which leads to the closing of ATP-sensitive K channels, which leads to membrane depolarization, calcium influx and triggers insulin release

Question 6

describe glucokinase deficiency in B-cells of pancreas

Answer 6

• Genetic deficiency of glucokinase can lead to maturity onset diabetes of the young type 2 (MODY-2) which is characterized by impaired insulin secretion from B-cells
• results in chronic mild hyperglycemia
• in MODY-2 patients, a higher blood glucose level (7-8 mM) is needed for insulin release

Question 7

how is hepatic glucokinase regulated?

Answer 7

• the hepatic glucokinase regulatory protein (GKRP) transports glucokinase into the nucleus at high levels of F6P and back into the cytosol at high free glucose levels
• glucokinase is induced by insulin and is not inhibited by its product G6P

Question 8

how is PFK-1 regulated

Answer 8

• PFK-1 catalyzes the committed step of glycolysis and is allosterically inhibited at normal physiological ATP levels in all cells
  
  • in many cells, ATP is end goal of glycolysis so if ATP levels achieved, it inhibits glycolysis
• glycolysis provides 1,3-BPG and PEP for SLP and is essential for ATP formation during anoxia or for cells without mt

Question 9

how is it possible to perform glycolysis in hepatocytes at normal ATP levels

Answer 9

• the liver has to perform glycolysis after a meal in order to reduce high blood glucose level
• insulin stimulates glycolysis as it leads to the dephosphorylation of the hepatic bifunctional enzyme
• PFK-2 is active and forms F2,6-BP that overcomes the inhibition of PFK-1 by ATP

Question 10

the allosteric inhibition of PFK-1 by ATP is overcome by 1) _____ and 2) ______

Answer 10

1. AMP
   
   • formed in skeletal muscle during exercise
     
     • glycolysis + glycogenolysis are activated
   • is formed in hepatocytes when ATP declines
     
     • glycolysis is activated and gluconeogenesis inhibited
2. Fructose 2,6-BP (only the liver)
   
   • formed by the bifunctional enzyme in skeletal muscle, heart and liver

Question 11

describe the feed-forward activation of pyruvate kinase during glycolysis

Answer 11

• PFK-1 forms F1,6 BP which is a feed-forward activator for pyruvate kinase
• F1,6-BP continues in glycolysis but some molecules will allosterically bind to pyruvate kinase
• pyruvate kinase is normally allosterically inhibited by ATP but F1,6-BP overcomes this inhibition

Question 12

describe hepatic pyruvate kinase inactivation during gluconeogenesis

Answer 12

• hepatic pyruvate kinase is inhibited by phosphorylation during gluconeogenesis
  
  • glucagon -> cAMP -> PKA -> phosphorylates pyruvate kinase leading to inhibition (you want gluconeogenesis, therefore turn off glycolysis enzymes)
• the inhibition now saves PEP for gluconeogenesis

Question 13

how is gluconeogenesis regulated?

Answer 13

• gluconeogenesis is regulated by the availability of energy for the pathway which is mostly provided by degradation of fatty acids
  
  • B-oxidation provides NADH and acetyl CoA for regulations to inhibit PDH and TCA cycle and stimulate gluconeogenesis
    
    • does not provide carbons for glucose formation

Question 14

describe the substrates for gluconeogenesis

Answer 14

• glycerol
• lactate
• alanine
• glutamine

Question 15

describe insulin vs glucagon regulation of the bifunctional enzyme

Answer 15

Question 16

name the enzymes that insulin induces the synthesis of

Answer 16

• the irreversible steps:
  
  • glucokinase
  • PFK-1
  • pyruvate kinase

Question 17

name the enzymes that glucagon induces the synthesis of

Answer 17

• PEP carboxykinase
• Fructose 1,6 -bisphosphatase
• glucose 6 phosphatase
• pyruvate carboxylase is needed for gluconeogenesis but is NOT INDUCED by glucagon

Question 18

give a summary of allosteric effectors of glycolysis and gluconeogenesis

Answer 18