Glycolysis Overview and Stages

Overview of Glycolysis

• Glycolysis is the initial metabolic pathway for glucose breakdown.
• It occurs in the cytoplasm of cells.
• It doesn't require oxygen.
• It is a crucial pathway for energy production, especially in tissues with limited oxygen access.
• It converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons).
• The net gain from glycolysis is 2 ATP and 2 NADH.

Stages of Glycolysis

• Glycolysis can be divided into two main phases:
  • Energy investment phase:
    • Glucose is phosphorylated twice requiring 2 ATP, forming fructose-1,6-bisphosphate.
    • This phase prepares glucose for cleavage.
  • Energy payoff phase:
    • Fructose-1,6-bisphosphate is cleaved, and energy is released in the form of ATP and NADH.
    • This phase yields a net gain of 2 ATP and 2 NADH.

Steps of Glycolysis (in more detail)

• Step 1-3: Phosphorylation and isomerizations: Glucose is phosphorylated, isomerized, and further phosphorylated.
• Step 4: Cleavage: Fructose-1,6-bisphosphate is cleaved into two 3-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate).
• Step 5: Isomerization: Dihydroxyacetone phosphate is isomerized into glyceraldehyde-3-phosphate so both 3-carbon molecules follow the same pathway.
• Step 6-9: Oxidations, ATP formation, and rearrangements: Glyceraldehyde-3-phosphate is oxidized, and energy released is used to produce ATP and NADH. The remaining steps are dedicated to producing more ATP and rearranging the molecules to form pyruvate.

Key Enzymes involved in Glycolysis

• Hexokinase
• Phosphoglucose isomerase
• Phosphofructokinase
• Aldolase
• Triosephosphate isomerase
• Glyceraldehyde-3-phosphate dehydrogenase
• Phosphoglycerate kinase
• Phosphoglycerate mutase
• Enolase
• Pyruvate kinase

Regulation of Glycolysis

• The activity of key glycolytic enzymes is tightly regulated to maintain energy homeostasis.
• Key enzymes like phosphofructokinase are subject to allosteric regulation and hormonal control by factors like ATP, AMP, and citrate.
• Hormonal signals such as insulin and glucagon also play a role in regulating glycolytic activity. This regulation ensures the appropriate supply of energy based on cellular needs.

Fate of Pyruvate

• The fate of pyruvate depends on the availability of oxygen.
• If oxygen is present, pyruvate enters the mitochondria for further oxidation via the citric acid cycle.
• If oxygen is absent, pyruvate is converted to lactate (in animals) or ethanol and CO2 (in some microorganisms) via fermentation. This anaerobic process regenerates NAD+ for glycolysis to continue, even without oxygen.

Summary of Glycolytic Products

• Two molecules of pyruvate are produced per initial glucose molecule.
• Two molecules of ATP are generated per glucose during the energy payoff phase, with a net gain of 2 ATP.
• Two molecules of NADH are also produced, which are crucial for energy production via the electron transport chain in aerobic conditions, or are used in fermentation reactions.

Explore the metabolic pathway of glycolysis, where glucose is broken down to produce energy. This quiz covers the essential phases and steps, including the energy investment and payoff stages, highlighting the significance of this process in energy production. Test your knowledge on the conversion of glucose to pyruvate and the overall ATP yield.