Glycolysis

Question 1

Major types of chemical reactions in glycolysis

Answer 1

1. Degradation of the carbon skeleton of glucose to yield pyruvate
2. Substrate level phosphorylation = Phosphorylation of ADP to ATP by high-energy phosphate compounds formed during glycolysis
3. Generation of NADH which can be used in the electron transport chain to produce ATP if the cell contains mitochondria and oxygen is present

Question 2

Key steps in glycolysis (aerobic)

Answer 2

1. Glucose ==> cytpolasm
2. Glucose + P ==> Glucose-6-P
   
   1. catalyzed by hexokinase or glucokinase
3. Glucose -6-P <==> Fructose-6-P
4. Fructose-6-P + P ==> Fructose-1,6-bis-P
   
   1. catalyzed by phosphofructokinase1 (PFK-1)
5. Several intermediate steps ==> Phosphoenolpyruvate
   
   1. NAD⁺ ==> NADH
   2. ADP ==> ATP
6. Phosphoenolpyruvate ==> Pyruvate + ATP
   
   1. catalyzed by pyruvate kinase

Question 3

Enzyme kinetics of glucokinase and hexokinase

Answer 3

• glucokinase =
  
  • selective for glucose
  • present in liver & pancreatic beta-cells
  • high Km and Vmax
    
    • produces lots of product quickly @ higher concentrations of glucose
  • inhibited by fructose-6-P
• hexokinase
  
  • not very selective
  • present in all cells
  • low Km and Vmax
    
    • low Km = high affinity for sugars
    • drives rxn at lower concentrations of glucose
  • inhibited by glucose-6-P

Question 4

Regulation of glucokinase and hexokinase

Answer 4

• Hexokinase is inhibited by G6P
• Glucokinase is inhibited by F6P indirectly, because it is in equilibrium with G6P, and indirectly stimulated by glucose

Question 5

Regulation of phosphofructokinase 1 (PFK-1) (general)

Answer 5

• Rate limiting, irreversible, most tightly regulated step of glycolysis
• Allosteric inhibitors: ATP, citrate from TCA
• Allosteric activators: AMP, fructose 2,6-bisphosphate
• F 2,6-B is the PFK-1 trump card activator, it is able to activate the enzyme even when ATP levels are high. It is formed by PFK-2

Question 6

Regulation of glycolysis vs. glyconeogenesis

Answer 6

• Fasting State: ↑ Glucagon → ↑ cAMP → ↑ Protein Kinase A → ↑ FBPase-2, ↓ PFK-2 ==> Gluconeogenesis
• Fed State: ↑ Insulin → ↓ cAMP → ↓ Protein Kinase A → ↓ FBPase-2, ↑ PFK-2 ==> glycolysis

Question 7

Regulation of pyruvate kinase

Answer 7

• Pyruvate kinase runs when insulin is high; if glucagon is high, the enzyme is phosphorylated by protein kinase A → inactive
  
  • Irreversible
• Stimulated by F1,6 BP
• Inhibited by ATP, alanine & Protein Kinase A

Question 8

Regulation of flux through glycolysis

Answer 8

• amount of substrate available: ↑ glucose = ↑ flux, and vice versa
• Availability/levels of enzymes: eg ↑ phosphofructokinase = ↑ flux, and vice versa
• Allosteric regulation: eg ↑ Fructose 2,6 bisphosphate = ↑ Phosphofructokinase activity = ↑ flux
• Covalent modification of a key enzyme: eg glucagon activates protein kinase A which phosphorylates and covalently modifies PFK-2 to inhibit its function.

Question 9

Final products of aerobic metabolism

Answer 9

• pyruvate
• NADH
• ATP
• Eventually aerobic metabolism of glucose produces 32 ATP via malate-aspartate shuttle (heart and liver) or 30 ATP via glycerol-3-phosphate shuttle (muscle)

Question 10

Final products of anaerobic glycolysis

Answer 10

• anaerobic glycolysis which is a major pathway in RBC’s + sperm (also…leukocytes, kidney, medulla, lens, testes, and cornea)
• final product = lactate
  
  • Pyruvate + NADH ==> Lactate + NAD+