Biochem Lecture 11

Introduction to Glycolysis

• Glycolysis: A metabolic pathway that breaks down glucose to harvest energy in the form of ATP.

• The importance of glycolysis has historical implications in biochemistry, first described in the early 20th century.

• Instructor's personal experience in winemaking linked to glycolysis, illustrating its practical applications.

Overview of Metabolism

• Metabolism is divided into two categories:

  • Catabolism: Reactions that break down molecules to yield energy (e.g., the degradation of food).

  • Anabolism: Biosynthetic pathways that require energy to build complex molecules (e.g., synthesis of proteins from amino acids).

• Pathways involve a series of chemical transformations leading to specific reactants and products.

• Interconnection between pathways allows for metabolic flexibility.

Glycolysis: Key Features

• Glycolysis occurs in the cytosol of cells and is universal among living organisms.

• Comprised of ten reactions, each catalyzed by a specific enzyme, functioning together to convert glucose into pyruvate.

• Main Outcomes:

  • Net production of 2 ATP molecules per glucose molecule.

  • Production of NADH, which can be used in further ATP generation in aerobic conditions.

• The glycolytic pathway can be divided into two main phases:

  • Priming Phase (Reactions 1-5): Consumes ATP to phosphorylate glucose, preparing it for breakdown.

  • Pay-off Phase (Reactions 6-10): Generates ATP and NADH through oxidation of intermediates.

Details of Glycolysis Phases

Priming Phase

1. Hexokinase Reaction:

   • Glucose is phosphorylated to form glucose-6-phosphate, consuming one ATP.

   • Hexokinase is regulated by feedback inhibition of its product, glucose-6-phosphate.

   • Phosphorylation prevents glucose from exiting the cell and prepares it for further metabolism.

2. Isomerization:

   • Glucose-6-phosphate is converted to fructose-6-phosphate via phosphoglucose isomerase.

3. Phosphorylation of Fructose:

   • Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate by phosphofructokinase (PFK-1), a key regulatory step.

   • PFK-1 activity is adversely affected by high ATP levels (feedback inhibition) and activated by ADP and AMP (low energy indicators).

4. Aldol Cleavage:

   • Fructose-1,6-bisphosphate is split into two triosephosphates: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

5. Isomerization of Triosephosphates:

   • Dihydroxyacetone phosphate is converted into glyceraldehyde-3-phosphate by triose phosphate isomerase, ensuring two molecules of glyceraldehyde-3-phosphate proceed to the next phase.

Pay-off Phase

6. Oxidation and NADH Formation:

   • Glyceraldehyde-3-phosphate is oxidized and phosphorylated to form 1,3-bisphosphoglycerate, producing NADH.

7. ATP Generation (First Substrate-Level Phosphorylation):

   • 1,3-bisphosphoglycerate donates a phosphate group to ADP to generate ATP, catalyzed by phosphoglycerate kinase.

8. Mutase Reaction:

   • The phosphate group is relocated from the 3rd to the 2nd position to form 2-phosphoglycerate via phosphoglycerate mutase.

9. Dehydration:

   • 2-phosphoglycerate undergoes dehydration to create phosphoenolpyruvate (PEP) via enolase, which is a high-energy intermediate.

10. Final ATP Generation:

• PEP donates its high-energy phosphate to ADP to form ATP, resulting in the production of pyruvate, catalyzed by pyruvate kinase.

• This reaction is also a regulatory step, inhibited by ATP and activated by fructose-1,6-bisphosphate (feed-forward activation).

Summary of Glycolytic Output

• Net Yield: 2 ATP (4 produced - 2 consumed) and 2 NADH per molecule of glucose.

• Pyruvate serves as a key metabolite that can undergo further reactions (e.g., TCA cycle under aerobic conditions).

• Glycolysis represents an ancient energy-harvesting pathway crucial for both prokaryotic and eukaryotic organisms.

Related Pathways

• Pentose Phosphate Pathway: A metabolic pathway primarily for anabolic processes, yielding NADPH and ribose sugars necessary for nucleotide synthesis.

  • Distinct roles of NADH and NADPH in catabolism and anabolism, respectively, emphasizing the importance of maintaining separate pools of these electron carriers.