29.5 Catabolism of Carbohydrates: Glycolysis

Glycolysis is a crucial metabolic pathway that breaks down glucose into pyruvate. This 10-step process occurs in the cell's cytoplasm and doesn't require oxygen, making it a versatile energy-producing mechanism for various organisms.

The pathway is divided into two phases: preparatory and payoff. While the preparatory phase consumes ATP, the payoff phase generates a net gain of 2 ATP molecules per glucose. Key reactions include phosphorylation, isomerization, and oxidation.

Glycolysis Overview

Steps of glycolysis

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• 10-step metabolic pathway breaks down glucose ($C_6H_{12}O_6$) into two pyruvate ($CH_3COCOO^-$) molecules
  • Occurs in cytoplasm of cells
  • Does not require oxygen (anaerobic process)
• Preparatory phase (steps 1-5)
  1. Glucose phosphorylated twice, consuming 2 ATP molecules
  • First phosphorylation catalyzed by hexokinase
  1. Fructose-1,6-bisphosphate split into two 3-carbon molecules: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP)
  • Splitting reaction catalyzed by aldolase
  1. DHAP isomerized to GAP, resulting in two GAP molecules
• Payoff phase (steps 6-10)
  1. Each GAP molecule undergoes oxidation and phosphorylation to form 1,3-bisphosphoglycerate (1,3-BPG)
  • Catalyzed by glyceraldehyde 3-phosphate dehydrogenase, producing NADH
  1. 1,3-BPG converted to 3-phosphoglycerate (3-PG), generating 2 ATP molecules per glucose
  2. 3-PG converted to 2-phosphoglycerate (2-PG) through isomerization reaction
  3. Dehydration of 2-PG forms phosphoenolpyruvate (PEP)
  4. PEP converted to pyruvate, generating additional 2 ATP molecules per glucose
  • Final step catalyzed by pyruvate kinase

ATP production in glycolysis

• Substrate-level phosphorylation directly transfers phosphate group from high-energy substrate to ADP, forming ATP
  • Occurs twice in payoff phase of glycolysis
    • 1,3-BPG transfers phosphate group to ADP, forming ATP and 3-PG
    • PEP transfers phosphate group to ADP, forming ATP and pyruvate
  • Each substrate-level phosphorylation event produces 2 ATP molecules per glucose molecule (4 ATP total)
• Net ATP production in glycolysis
  • Gross ATP production: 4 ATP (2 from each substrate-level phosphorylation event)
  • ATP consumed in preparatory phase: 2 ATP
  • Net ATP yield: 2 ATP per glucose molecule (gross production minus consumption)

Organic reactions of glycolysis

• Phosphorylation: Addition of phosphate group ($PO_4^{3-}$) to molecule
  • Glucose phosphorylated twice in preparatory phase (steps 1 and 3)
  • Second phosphorylation catalyzed by phosphofructokinase
• Isomerization: Rearrangement of atoms within molecule without changing molecular formula
  • Glucose-6-phosphate isomerized to fructose-6-phosphate (step 2)
  • DHAP isomerized to GAP (step 5)
  • 3-PG isomerized to 2-PG (step 8)
• Aldol cleavage: Splitting of 6-carbon molecule into two 3-carbon molecules
  • Fructose-1,6-bisphosphate cleaved into GAP and DHAP (step 4)
• Oxidation: Loss of electrons from molecule
  • GAP oxidized to form 1,3-BPG (step 6)
• Dehydration: Removal of water molecule from substrate
  • 2-PG dehydrated to form PEP (step 9)