Glycolysis Pathway in Cellular Respiration

Glycolysis (Embden-Meyerhof Pathway)

• Glycolysis is the pathway where glucose is split into two 3-carbon pyruvate molecules under aerobic conditions, or lactate under anaerobic conditions, along with production of a small quantity of energy.
• The word "glycolysis" is derived from the Greek words, glykys = sweet; and lysis = splitting.

Site of Reactions

• All the reaction steps take place in the cytoplasm.

Significance of the Glycolysis Pathway

• It is the only pathway that takes place in all the cells of the body.
• Glycolysis is the only source of energy in erythrocytes.
• In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.
• The glycolytic pathway may be considered as the preliminary step before complete oxidation.
• The glycolytic pathway provides carbon skeletons for synthesis of non-essential amino acids as well as glycerol part of fat.
• Most of the reactions of the glycolytic pathway are reversible, which are also used for gluconeogenesis.

Glucose Entry into Cells

• Glucose transporter-4 (GluT4) transports glucose from the extracellular fluid to muscle cells and adipocytes.
• GluT4 is under the influence of insulin.
• In diabetes mellitus, insulin deficiency hinders the entry of glucose into the peripheral cells.
• GluT2 is the transporter in liver cells, and it is not under the control of insulin.

Steps of Glycolytic Pathway

Step 1 of Glycolysis

• Glucose is phosphorylated to glucose-6-phosphate.
• The enzyme is hexokinase (HK), which splits the ATP into ADP, and the Pi is added on to the glucose.
• The energy released by the hydrolysis of ATP is utilized for the forward reaction.
• Hexokinase is a key glycolytic enzyme and catalyses a regulatory step in glycolysis that is irreversible.
• Hexokinase and glucokinase may be considered as iso-enzymes.

Step 2 of Glycolysis

• Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase.
• This is readily reversible.

Step 3 of Glycolysis

• Fructose-6-phosphate is further phosphorylated to fructose-1,6-bisphosphate.
• The enzyme is phosphofructokinase (PFK), which is an allosteric, inducible, regulatory enzyme.
• PFK is an important key enzyme of this pathway and catalyses an irreversible step.
• This step is the rate limiting reaction in glycolysis.

Steps 4 and 4-A of Glycolysis

• The 6-carbon fructose-1,6-bisphosphate is cleaved into two 3-carbon units; one glyceraldehyde-3-phosphate and another molecule of dihydroxy acetone phosphate (DHAP).
• The enzyme is called aldolase, and this reaction is reversible.
• Dihydroxy acetone phosphate is isomerised to glyceraldehyde-3-phosphate by the enzyme phosphotriose isomerase.
• Thus, the net result is that glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate.

Learn about the Embden-Meyerhof pathway, where glucose is converted into pyruvate or lactate, producing energy in the process.