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Overview of Carbohydrate Metabolism

  • Chapters 14 and 15 cover carbohydrate metabolism focusing on:
    • Glycolysis
    • Gluconeogenesis
    • Regulation of metabolic pathways
  • Content is integrated due to overlapping topics and detailed pathways.

Key Pathways in Carbohydrate Metabolism

  • All carbohydrate pathways originate from glucose.
    • Key conversions include:
    • Glucose to pyruvate (glycolysis)
    • Glucose to glycogen (storage form)
    • Glucose to starch or sucrose (in plants)
    • Glucose to ribose 5-phosphate (used for nucleotide synthesis)
    • Conversion of glucose into various polysaccharides (structural polymers)
  • Focus will primarily be on glycolysis and gluconeogenesis, along with the pentose phosphate pathway and glycogen metabolism.

Glycolysis

General Information

  • Definition: Glycolysis is a universal pathway present in nearly all living cells, especially human cells.
  • It is the most carbon-intensive metabolic pathway, with a significant flow of carbon atoms.
  • Consists of 10 reactions, varying in rate according to tissue (e.g., liver, muscle, brain).
  • Divided into two main phases:
    • Preparatory Phase: Energy-consuming phase converting glucose into two molecules of glyceraldehyde 3-phosphate (G3P).
    • Payoff Phase: Energy-generating phase producing ATP and NADH from G3P and yielding pyruvate.

Phase Breakdown

  1. Preparatory Phase:

    • ATP is invested to phosphorylate glucose and fructose 6-phosphate.
    • Conversion of glucose to 2 molecules of G3P.
    • Key reactions include:
      • Conversion of glucose to glucose 6-phosphate (using ATP).
      • Isomerization of glucose 6-phosphate to fructose 6-phosphate.
      • Phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate.
      • Splitting of fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and G3P.

  2. Energy Generation Phase:

    • Involves conversion of G3P to pyruvate, generating energy.
    • Reactions include:
      • Oxidation of G3P to 1,3-bisphosphoglycerate, producing NADH.
      • Transfer of phosphate from 1,3-bisphosphoglycerate to ADP forming ATP.
      • Further reactions lead to formation of phosphoenolpyruvate (PEP) and subsequent conversion to pyruvate.
    • Net Output for one glucose:
      • 2 pyruvate, 2 ATP, 2 NADH.

Importance of Pyruvate

  • Pyruvate can undergo different fates post-glycolysis, influenced by oxygen availability.
    • Under anaerobic conditions:
    • May convert to lactic acid in muscles or ethanol in yeast.
    • Under aerobic conditions:
    • Enters the citric acid cycle for further energy extraction.

Reaction Dynamics in Glycolysis

Specific Pathway Steps

  • Initial Phosphorylation:

    • ATP transfers a phosphate group to carbon 6 of glucose, forming glucose 6-phosphate:
    • Important for trapping glucose inside the cell (negatively charged) and preventing its exit.

  • Isomerization Reaction:

    • Conversion of glucose 6-phosphate to fructose 6-phosphate follows:
    • Rearrangement of the carbonyl group to enable phosphorylation at carbon 1.

  • Cleavage of Fructose 1,6-bisphosphate:

    • Fructose 1,6-bisphosphate is split into DHAP and G3P by breaking the bond between carbons 3 and 4.

  • Interconversion of DHAP and G3P:

    • DHAP is isomerized to G3P, allowing for a uniform continuation through the glycolytic pathway.

  • Key Oxidation and Energy Capture:

    • Conversion of aldehyde in G3P to a carboxylic acid releases energy:
    • High-energy mixed anhydride bond created with the addition of phosphate (1,3-bisphosphoglycerate).
    • This compound is crucial for ATP generation (substrate-level phosphorylation).

  • Formation of Phosphoenolpyruvate (PEP):

    • Removal of water creates a double bond in 2-phosphoglycerate leading to PEP, which is highly energetic and ready to donate a phosphate to ADP, forming ATP.

Chemical Logic of Glycolysis

  • The pathway demonstrates a clear "chemical logic," where each reaction facilitates the next.
  • Importance of phosphorylating glucose and other intermediates ensures:
    • Metabolite confinement within the cell.
    • Availability of high-energy intermediates for subsequent ATP production.
  • Hydrolysis of ATP and high-energy phosphate groups crucial for providing energy to drive reactions forward.

Connection to Citric Acid Cycle

  • Glycolysis accounts for only a small fraction of the total energy yield from glucose (approximately 5%).
  • Subsequent journey of pyruvate into the citric acid cycle leads to significantly higher energy yields through further oxidation reactions.
  • Citric acid cycle integrates with glycolysis to maximize energy extraction from glucose.