Glycolysis

• Glucose is a very adaptable metabolite that is able to meet both the material and energy requirements of the cell. Unlike fats which mostly only meet energy requirements.
• Reasons why the whole 2834 kJ/mol (from the complete oxidation of glucose) is not released as heat


  • Biological systems cannot utilise heat as a source of energy
  • No single reaction of metabolism requires this amount of energy to be released in one step
  • Always need to overcome the activation energy
• Enzymes are capable of effecting only small changes when they catalyse biological reactions, releasing the energy in steps.
• During the catabolism of glucose the molecule is broken down in small steps and the energy is released in usable amounts (~ -30 kJ/mol) in the form of chemical energy, with ATP being the energy carrier.
• Ten steps to glycolysis


  1. Glucose

  2. 2 Pyruvate

  3. 2 ATP

  4. 2 NADH + 2H+

  5. Energy investment

  6. Energy payoff
• Glycolysis occurs in the cytosol
• Coenzymes in glucose catabolism


  • ATP/ADP deliver/receive phosphates
  • NADH/NAD+ involved in oxidation/reduction reactions
• ATP is also involved in transferring energy between reactions.
• Aldehyde


  • Carboxylate
  • A ketone with 2 methyls = acetone
• General Enzyme Naming Convention

  • Enzymes named after their substrate plus the enzymatic activity
  • Kinases phosphorylate substrates
  • Phosphatases dephosphorylate substrates
  • Not all enzymes follow this convention because of how their functions were originally discovered
• Knowing the substrates and the steps involved, allows you to also name the enzymes.
• Glycolysis - STEP 1

  1. ATP

  2. ADP

  3. Hexokinase

  4. Mg2+

  5. GLUCOSE

  6. GLUCOSE-6-PHOSPHATE (G6P)
• Glucose-6-phosphate (G6P)


  • Aldehyde at top
  • 6 carbon compound
  • Hydroxyl groups on the same side except for third carbon from top
• Glucose by itself cannot cross a membrane. Uses transport proteins e.g., GLUT2 to move from a high to a low concentration. This process does not require energy.
• Phosphorylation prevents the glucose from Returning. Remember G -> G6P is irreversible, powered by the energy from ATP hydrolysis using this enzyme hexokinase. Can be reversed but using a different enzyme.
• Glycolysis - STEP 2


  1. GLUCOSE-6-PHOSPHATE (G6P)

  2. FRUCTOSE-6-PHOSPHATE (F6P)

  3. Phosphoglucose isomerase
• Glycolysis - STEP 3


  1. FRUCTOSE-6-PHOSPHATE (F6P)

  2. FRUCTOSE-1,6-BISPHOSPHATE (F1,6P)

  3. ATP

  4. ADP

  5. Phosphofructokinase
• Fructose-1,6-bisphosphate (F1,6P)


  Bisphosphate means two phosphate groups added to different places
• Glycolysis - STEP 4


  1. FRUCTOSE-1,6-BISPHOSPHATE (F1,6P)

  2. Dihydroxyacetone phosphate (DHAP)

  3. Glyceraldehyde-3-phosphate (GAP)

  4. Aldolase
• This reaction is readily reversible and is catalysed by aldolase. The enzyme derives its name from the reverse reaction, an aldol condensation (reaction between an aldehyde and a alcohol).
• Glycolysis - STEP 5


  1. DHAP

  2. GAP

  3. Triose phosphate isomerase
• Glycolysis - STEP 6


  1. Glyceraldehyde-3-phosphate (GAP)

  2. 1,3-bisphosphoglycerate

  3. NAD+

  4. NADH + H+

  5. GAP dehydrogenase
• Dehydrogenase means it is an oxidation step in which an aldehyde is converted to a carboxylic acid.
• An usual phosphorylation step as the Pi comes from solution and NOT from ATP. What is driving this reaction is the oxidation step which is exergonic.
• Glycolysis - STEP 7


  1. 1,3-bisphosphoglycerate

  2. 3-phosphoglycerate

  3. ADP

  4. ATP

  5. Phosphoglycerate kinase
• 1,3-bisphosphoglycerate


  A carboxylic acid linked to a phosphate = phospho-anhydride
• An acid from the phosphate and an acid from the carboxylic acid results in a greater negative deltaG. Phosphoester bond as a result of the reaction between an acid and an alcohol. This has a negative deltaG.
• Glycolysis - STEP 8

  1. 3-phosphoglycerate

  2. 2-phosphoglycerate

  3. Phosphoglycerate mutase
• The phosphoester is not sufficiently exergonic to produce ATP from ADP but this rearrangement allows, in the next step, for such a compound to be formed.
• Glycolysis - STEP 9


  1. 2-phosphoglycerate

  2. Phosphoenol-pyruvate

  3. Enolase
• Phosphoenol-pyruvate

  Enol is an alkene with a hydroxyl group
• Donation of the phosphate group allows the enol to convert into the mores stable ketone (pyruvate).
• Glycolysis - STEP 10

  1. Phosphoenol-pyruvate

  2. Pyruvate

  3. ADP

  4. ATP

  5. Pyruvate kinase
• Glycolysis


  • Three irreversible steps
  • The hexose sugar is first phosphorylated at both ends before splitting
  • Conversation to GAP, followed by oxidation and phosphorylation with Pi results in the triose phosphorylated at both ends
  • The four phosphates are used to generate 4 ATP
• 2 ATPs used to prime glycolysis.
• 4 ATPs produced from the glycolytic pathway as a result of substrate level phosphorylation 2x(step 7 & 10).
• 2 NADHs generated (equivalent to 3 ATPs per NADH through oxidation via the electron transport chain).
• 8 ATPs net gained from the conversion of glucose to pyruvate and the re-oxidation of the 2 NADHs produced.
• Acetone


  A ketone with 2 methyls
• Glycolysis occurs in plants, animal cells, fungi and microbes.