Glycolysis [detailed]

Steps of Glycolysis: Intro and Stage I

1. Stage I: Glucose → Fructose-1,6-Bisphosphate
2. Stage II: Fructose-1,6-Bisphosphate → Glyceraldehyde 3-phosphate
3. Stage III: Glyceraldehyde 3-phosphate → Pyruvate

Stage I: Glucose → Fructose-1,6-Bisphosphate

• Energy is consumed in this part of glycolysis.
• The phosphoryl groups of ATP are transferred to the hydroxyl groups on the sugar.
• This transfer is mediated by 2 kinases (phosphate transfer enzymes).

1st step: Glucose → Glucose-6-phosphate (G6P)

• Hexokinase transfers a phosphate from ATP to glucose;
• The hexokinase reaction traps glucose inside the cell;
• This reaction is considered irreversible.

2nd step: Interconversion to Fructose-6-phosphate (F6P)

• Conformational change (rearrangement) of G6P → F6P is catalyzed by phosphoglucose isomerase.
• An isomerase is an enzyme that catalyzes the change of a molecule into its isomer.

3rd step: Fructose-6-phosphate (F6P) → Fructose-1,6-Bisphosphate (F-1,6-BP)

• Phosphofructokinase transfers a phosphate from ATP to F6P.
• The product is fructose-1,6-bisphosphate (F-1,6-BP).
• Phosphofructokinase activity is highly regulated (considered a “rate limiting enzyme”).
• Less active when ADP is low and ATP is high (no need to make more ATP!).
• This reaction is also considered irreversible.

Steps of Glycolysis: Stages II and III

Stage II: Fructose-1,6-Bisphosphate → Glyceraldehyde 3-phosphate

• Cleavage of F-1,6-BP is mediated by aldolase.
• This cleavage generates 2 compounds:

1. Dihydroxyacetone phosphate (DHAP, a ketone)
2. Glyceraldehyde 3-phosphate (G3P, an aldehyde)

• Glyceraldehyde 3-phosphate can be inter-converted to and from dihydroxyacetone phosphate by triose phosphate isomerase.
• The interconversion is in equilibrium (therefore reversible);
• Only G3P can be used in glycolysis.

Stage III: Glyceraldehyde 3-phosphate → Pyruvate

• Happens twice for every glucose molecule (1 glucose makes 2 G3P).
• Produces 2 molecules of ATP per G3P.

• Catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
• Aldehyde group is oxidized. During oxidation, electrons are transferred to Pi, NAD+ to form NADH and H+.
• This is a potentially limiting step as NAD+ must be always regenerated to form NADH.
• A phosphate group also gets transferred to glyceraldehyde.

2nd step: Formation of ATP and 3-Phosphoglycerate

• Catalyzed by phosphoglycerate kinase.
• 1,3-biphosphoglycerate transfers phosphate group to ADP, generating ATP.

3rd step: Formation of 2-Phosphoglycerate

• Catalyzed by phosphoglycerate mutase.
• Mutase is an enzyme that catalyzes the transfer of a phosphoryl group from one hydroxyl group to another.
• The phosphate is moved to the 2nd carbon.

4th step: Formation of Phosphoenolpyruvate (PEP)

• Catalyzed by enolase.
• Enolase carries out the dehydration of the C3 and a double bond is formed.
• This double bond increases the potential energy of the molecule, making PEP the highest energy phosphate bond found in living things.

5th step: Formation of Pyruvate

• Pyruvate kinase catalyzes the transfer of the phosphoryl group from PEP to ADP to form ATP and pyruvate.
• Silly mnemonic: pyruvate is a quiet molecule, it doesn't say a PEeP!