CARBOHYDRATE CATABOLISM - GLYCOLYSIS

 GLYCOLYSIS

• Glycolysis, the oxidation of Glucose to Pyruvic acid, is usually the first stage in Carbohydrate catabolism which takes place in Cytoplasm of the cell.
• The word Glycolysis means splitting of sugar, and this is exactly what happens (Greek, glycol – sugar; lysis – loosening).
• Glycolysis pathway was described by Embden, Meyerhof and Parnas in 1940. Hence, it is also called as Embden - Meyerhof pathway.
• Glycolysis occurs in the absence of oxygen (Anaerobic) or in the presence of oxygen (Aerobic). Lactate is the end product under Anaerobic condition. In the Aerobic condition, Pyruvate is formed, which is then oxidized to CO₂ and H₂O.
• The Glycolysis pathway is operated by Yeast to produce Alcohol and Lactic acid bacteria to produce Lactic acid and several Organic acids, gases, Fatty acids and Alcohols.
• Glycolysis is a major pathway for ATP synthesis in tissues lacking Mitochondria.
• The enzymes of Glycolysis catalyze the splitting of Glucose (six - carbon sugar) into two three-carbon sugars. These sugars are then oxidized, releasing energy, and their atoms are rearranged to form two molecules of Pyruvic acid.
• During Glycolysis NAD⁺ is reduced to NADH, and there is a net production of two ATP molecules by Substrate level phosphorylation.

Glycolysis (1 to 4 – Preparatory stage; 5 to 10 – Energy conserving stage)

• STEP - 1: Glucose enters the cell and is Phosphorylated. A molecule of ATP is invested. The product is Glucose-6-phosphate.
• STEP - 2: Glucose-6-phosphate is rearranged to form Fructose-6-phosphate.
• STEP - 3: The Phosphate group from another ATP is used to produce Fructose 1,6-diphosphate, still a six-carbon compound (Note - The total investment of two ATP molecules up to this point).
• STEP - 4:  An enzyme Fructose bisphosphate aldolase cleaves the sugar into two three-carbon molecules: Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (GP).
• STEP - 5:  DHAP is readily converted to GP (the reverse action may also occur).
• STEP - 6: The next enzyme converts each GP to another three-carbon compound. 1,3 -diphosphoglyceric acid. Because each DHAP molecule can be converted to GP and each GP to 1.3-diphosphoglyceric acid. The result is two molecules of 1,3 - diphosphoglyceric acid for each initial molecule of glucose. GP is oxidized by the transfer of two hydrogen atoms to NAD⁺ to form NADH. The enzyme couples this reaction with the creation of a high-energy bond between the sugar and a Phosphate group. The three - carbon sugar now has two Phosphate group.
• STEP - 7: The high energy Phosphate group is moved to ADP, forming ATP, the first ATP production of Glycolysis (Since the sugar splitting in step 4, all products are doubled. Therefore, this step actually repays the earlier investment of two ATP molecules).
• STEP - 8: An enzyme relocates the remaining Phosphate group of 3-phosphoglyceric acid to form 2-phosphoglyceric acid in preparation for the next step.
• STEP - 9: By the loss of a water molecule, 2-phosphoglyceric acid is converted to Phosphoenol pyruvic acid (PEP). In the process, the phosphate bond is upgraded to a high - energy bond.
• STEP - 10: This high-energy Phosphate group is transferred from PEP to ADP, forming ATP. For each initial glucose molecule, the result of this step is two molecules of ATP and two molecules of a three-carbon compound called Pyruvic acid.