Glycolysis Cycle

GLYCOLYSIS (EMBDEN- MEYERHOF PATHWAY)

• Glycolysis is conversion of glucose into pyruvate (aerobic) or lactate (anaerobic)
• Glycolysis takes place in all cells of the body.
• The only metabolic fuel for mature erythrocytes in fed & starving state is glucose.
• Glycolysis occurs in cytosol as all the enzymes are present here.
• Steps involved in the cycle are-

1. Step 1- Glucose to glucose-6- phosphate (first rate limiting step)

• Hexokinase– found in all tissues
• Irreversible reaction.
• No effect by feeding or insulin or starvation nor specific for glucose metabolism.
• Glucokinase-
• Found in liver and pancreatic β cells.
• High K_m for glucose & specific glucose.
• Not inhibited by glucose-6-phosphate.
• Induced by Insulin following a meal and important role in regulation of blood glucose
• Glucose-6-phosphate has many fates as- Glycolysis, glycogenesis, gluconeogenesis, pentose phosphate pathway.

2. Step 2- Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase

3. Step 3- Fructose-6-phosphate is further phosphorylated to fructose 1, 6- biphosphate by phosphofructokinase

• This is an irreversible.
• It is a rate limiting step & committed step and 1 ATP utilized.
• Phosphofructokinase- major regulatory enzyme
• Phosphofructokinase– allosterically inhibited by citrate

             The 1^st, 2^nd & 3^rd steps are the energy investment phase.

4. Step 4- Fructose 1, 6 biphosphate splits into glyceraldehyde 3-phophate & dihydroxyacetone phosphate

• Catalyzed by enzyme Aldolase.
• Aldolase is Lyase.
• Step 4 is called as Splitting phase.

5. Step 5-Glyceraldehydes 3- phosphate is oxidised to 1, 3- bisphospho glycerate  

• Enzyme glyceraldehyde – 3- phosphate dehydrogenase.
• An inorganic phosphate is added. (Iodoacetate & arsenate inhibits enzyme)

6. Step 6-  1, 3- biphosphoglycerate forms 3 –phosphoglycerate

• Enzyme phosphoglycerate kinase 
• Synthesis ATP.
• This is an example of substrate level phosphorylation

7. Step 7- 3- phosphoglycerate converted to 2- phosphoglycerate 

• Enzyme phosphogluco mutase (irreversible reaction)

8. Step 8- 2- phosphoglycerate is converted to phosphoenol pyruvate 

• Enzyme enolase which requires Mg²⁺ or Mn²⁺inhibited by fluoride.

9. Step 9- Phosphophenol Pyruvate (PEP) is dephosphorylated to pyruvate 

• Enzyme pyruvate kinase (ATP formed)
• This is an irreversible step.

10. Step 10- In anaerobic condition, pyruvate is reduced to lactate

• Enzyme lactate dehydrogenase.
• 6^th, 7^th, 8^th, 9^th, 10^th step is called energy generation phase.

Significance of Gycolysis-

• In strenuous exercise, anaerobic glycolysis forms the major source of energy for muscles.
• Tissues depending mainly on glucose as metabolic fuel- White fibres of skeletal muscle, erythrocytes, brain, GIT, renal medulla, skin.

Production of ATP in glycolysis-

• Under anaerobic conditions, 2 ATP are synthesized.
• Under aerobic conditions, 8 or 6 (7) ATP are synthesized.
• Glycolysis under glycogen, 1 ATP is generated.
• In anaerobic glycolysis, 3 ATP from glycogen.
• The inhibition of glycolysis by oxygen is known as Pasteur effects.

Exam Important

Don’t Forget to Solve all the previous Year Question asked on Glycolysis Cycle