9.2-Glycolysis

Introduction to Glycolysis

Glycolysis is a crucial metabolic pathway that plays a foundational role in carbohydrate metabolism. It consists of a series of enzymatic reactions that convert glucose and other sugars into pyruvate while generating energy in the form of ATP and reducing equivalents in the form of NADH. Throughout this process, pyruvate serves as a significant intermediate, funneling into the Krebs cycle (Citric Acid Cycle) to further produce energy in aerobic conditions. Glycolysis can be divided into three main stages, which sequentially lead to pyruvate production, encompassing energy investment, splitting, and energy generation.

Stages of Glycolysis

Stage 1: Energy Investment Phase

In the energy investment phase, glucose is phosphorylated and converted into fructose 1,6-bisphosphate through a series of reactions that involve the investment of two ATP molecules.

  • Conversion: The conversion begins with the phosphorylation of glucose to glucose 6-phosphate, catalyzed by the enzyme hexokinase. This reaction is crucial as it prevents glucose from diffusing out of the cell.

  • Key Intermediates: Important intermediates during this stage include glucose 6-phosphate and fructose 6-phosphate, which are essential for subsequent reactions.


Stage 2: Splitting Phase

The second stage involves the splitting of fructose 1,6-bisphosphate into two three-carbon molecules.

  • Products: The splitting produces glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP), with GAP being the primary molecule that proceeds through glycolysis.

  • Enzyme: The enzyme aldolase catalyzes this cleavage, while the interconversion of GAP and DHAP is facilitated by triose phosphate isomerase.

Stage 3: Energy Generation Phase

In this final stage, the two GAP molecules are processed into pyruvate, leading to the production of ATP and NADH.

  • Key Intermediates: The reaction progresses through intermediates such as 1,3-bisphosphoglycerate and 3-phosphoglycerate.

  • ATP Production: This stage yields a net production of two ATP molecules via substrate-level phosphorylation, which is a critical energy source for the cell.

Detailed Steps of Glycolysis

  1. Phosphorylation of Glucose:

    • Enzyme: Hexokinase

    • ATP donates a phosphate group, rendering glucose 6-phosphate (irreversible reaction).

  2. Isomerization of Glucose 6-Phosphate:

    • Enzyme: Phosphoglucose isomerase.

    • Interconverts glucose 6-phosphate to fructose 6-phosphate. (Reversible).

  3. Phosphorylation of Fructose 6-Phosphate:

    • Enzyme: Phosphofructokinase.

    • Another ATP donation converts fructose 6-phosphate to fructose 1,6-bisphosphate.

  4. Splitting of Fructose 1,6-Bisphosphate:

    • Enzyme: Aldolase.

    • This step yields GAP and DHAP.

  5. Interconversion of Triose Phosphates:

    • Enzyme: Triose phosphate isomerase.

    • Facilitates the interconversion between GAP and DHAP.

  6. Formation of 1,3-Bisphosphoglycerate:

    • Enzyme: Glyceraldehyde 3-phosphate dehydrogenase (NAD reduced to NADH).

  7. ATP Generation:

    • Enzyme: Phosphoglycerate kinase.

    • Converts 1,3-bisphosphoglycerate to 3-phosphoglycerate, generating ATP via substrate-level phosphorylation. Transfer phosphate to adp—> atp

  8. Phosphate Transfer:

    • Enzyme: Phosphoglycerate mutase.

    • Converts 3-phosphoglycerate to 2-phosphoglycerate (reversible).

  9. Formation of Phosphoenolpyruvate (PEP):

    • Enzyme: Enolase.

    • Converts 2-phosphoglycerate to PEP through the removal of water.

  10. Formation of Pyruvate and ATP:

  • Enzyme: Pyruvate kinase (irreversible reaction).

  • Final step converting PEP to pyruvate, yielding ATP.

Overall Reaction of Glycolysis

  • Inputs: 1 glucose, 2 inorganic phosphates, 2 ADP, 2 NAD+.

  • Outputs: 2 pyruvate, 2 ATP, 2 NADH, 2 protons, and 2 water molecules.

  • Net Yield of ATP: +2 ATP (4 produced - 2 used in investment phase).

Alternative Carbohydrate Substrates

Other monosaccharides such as galactose and fructose can integrate into glycolysis.

  • Galactose: Converted to glucose 6-phosphate.

  • Fructose: Converted to either GAP or DHAP, facilitating their entry into the glycolytic pathway.

Regulation of Glycolysis

Key regulating enzymes ensure the glycolytic pathway is appropriately modulated based on the cell's energy requirements:

  • Hexokinase: Inhibited by its product, glucose 6-phosphate, preventing excessive glucose conversion.

  • Phosphofructokinase (PFK): Allosterically inhibited by ATP (indicating high energy) and citrate (indicating citric acid cycle activity) while activated by AMP (indicating low energy).

  • Pyruvate Kinase: Activated by fructose 1,6-bisphosphate (a feedforward activation), inhibited by low glucose and high ATP levels, ensuring balanced flow through glycolysis.

Fate of Pyruvate

The produced pyruvate can follow different metabolic pathways based on oxygen availability:

  • Aerobic Conditions: Pyruvate is converted into acetyl CoA, which is directed into the Krebs cycle for further ATP production.

  • Anaerobic Conditions: Results in the conversion of pyruvate to lactate (in animals) or ethanol (in yeast), processes that regenerate NAD+ essential for sustaining glycolysis during oxygen deprivation.

NAD+ Regeneration

This regeneration is vital for maintaining ongoing glycolytic activity.

  • Aerobic: NADH is reoxidized back to NAD+ through electron transport chains that utilize oxygen.

  • Anaerobic: The enzyme lactate dehydrogenase catalyzes the conversion of pyruvate to lactate, simultaneously regenerating NAD+ to keep glycolysis functioning under low-oxygen conditions.