Glycolysis Overview

• Glycolysis is a metabolic pathway that breaks down glucose to produce energy.
• Key phases of glycolysis include:
  • Priming
  • Cleavage
  • Energy Extraction

Priming Phase

• The initial step involves adding phosphates to glucose and fructose:
  • Hexose Kinase is responsible for phosphorylating glucose to form glucose six phosphate (G6P).
  • Glucose Pyrophosphorylase can also phosphorylate glucose as an alternative pathway.
• Isomerase is critical as it allows the conversion between G6P and fructose six phosphate (F6P).
• Phosphofructokinase adds another phosphate to F6P, creating fructose one,6-bisphosphate (F1,6BP).

Cleavage Phase

• Fructose one,6-bisphosphate is cleaved by Alolase, resulting in two three-carbon molecules:
  • Glyceraldehyde 3-phosphate (G3P)
  • Dihydroxyacetone phosphate (DHAP)
• G3P further converts to pyruvate in subsequent reactions.

Energy Extraction Phase

• The oxidation of G3P leads to ATP production:
  • First, oxidation converts G3P, facilitating electron collection to form NADH.
  • Next, the liberation of phosphates from G3P leads to substrate-level phosphorylation to produce ATP.
• The complete yield from a single cycle of glycolysis from 6 carbons:
  • 2 Pyruvate
  • 2 NADH
  • 2 ATP
  • Described as Rule of Twos by the lecturer: "For every 1 sucrose, the products are doubled (4 pyruvate, 4 NADH, 4 ATP)."

Comparison of Pathways for Sucrose Breakdown

• Sucrose can be broken down through:
  • Invertase pathway for ATP generation.
  • Sucrose Synthase pathway for producing intermediates instead of energy.

Detailed Glycolysis Steps

1. Phosphorylation of glucose and/or fructose (priming), using hexokinase or glucose pyrophosphorylase.
2. Conversion via isomerase leads to the formation of fructose bisphosphate (F1,6BP).
3. Cleavage by aldolase to form G3P and DHAP.
4. Isomerization of DHAP to G3P facilitates further reactions.
5. Oxidation of G3P to phosphoenolpyruvate (PEP) through several steps which include:
   • Conversion to glycolytic intermediates such as phosphoglycerate (3PG).
   • Eventually to PEP and subsequent conversion to pyruvate.
6. Energy Extraction happens as phosphates are transferred to ADP to form ATP through substrate-level phosphorylation.

Connection to Gluconeogenesis

• Gluconeogenesis is the reverse process of glycolysis but involves unique enzymes and steps to synthesize glucose from pyruvate and other precursors (like fats).
• Key enzymes in gluconeogenesis include:
  • Pyruvate Carboxylase and PEP Carboxykinase to produce PEP from oxaloacetate.
  • Fructose-1,6-bisphosphatase facilitates the conversion of fructose-1,6-bisphosphate back to fructose-6-phosphate.
• Gluconeogenesis is essential for maintaining blood glucose levels during fasting or energy depletion.

Final Notes on Pathways and Enzymes

• Factors influencing glycolysis and gluconeogenesis include substrate availability and energy status within the cell.
• In plants, carbohydrates can be formed from fats via gluconeogenesis, especially in seeds which rely on stored energy during periods without photosynthesis.

Important Molecules to Memorize

• Key intermediates:
  • G6P (Glucose 6 Phosphate)
  • F6P (Fructose 6 Phosphate)
  • F1,6BP (Fructose 1,6-bisphosphate)
  • G3P (Glyceraldehyde 3-Phosphate)
  • PEP (Phosphoenolpyruvate)
  • Pyruvate
• Enzymes:
  • Hexokinase
  • Glucose Pyrophosphorylase
  • Phosphofructokinase
  • Aldolase
  • Isomerase
  • Fructose-1,6-bisphosphatase
  • Pyruvate Carboxylase
  • PEP Carboxykinase