Major concepts:

1. How is the structure of glucose changed by the reactions in the preparation phase of glycolysis?
2. Which glycolysis reaction is the committed step, and what does "committed step" mean?
3. How is glyceraldehyde-3-phosphate changed during the reaction catalyzed by GAPDH?
How is it changed, overall, during the reactions of the payoff phase of glycolysis?
4. What happens to pyruvate during fermentation? Why?
5. Why are the fermentation products of skeletal muscle cells different from yeast cells?

Core knowledge:

Overview:
glucose + 2 ADP + 2 P_i + 2 NAD⁺ → 2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O
occurs in the cytosol in two phases: preparation (reactions 1–5) + pay-off (reactions 6–10)
involves phosphorylated intermediates
     higher energy (more activated), prevents diffusion out of the cell,
      lower ΔG^† because binding phosphate is favored.

Preparation reactions (5): C₆ → 2 C₃, costing energy

Hexokinase catalyzes transfer of phosphate from ATP to glucose → glucose-6-P + ADP
requires Mg²⁺ to stabilize ATP                                                                          ΔG°′ = − 16.7 kJ/mol

Phosphohexose isomerase converts glucose-6-P ↔ fructose-6-P.                     ΔG°′ = 1.7 kJ/mol

   Reaction mechanism = acid-base, with Glu⁻ functioning as a base to remove H⁺ from C2.

Phosphofructokinase (PFK-1)
transfers P from ATP to fructose-6-P → fructose-1,6-bis-P + ADP               ΔG°′ = − 14.2 kJ/mol
requires Mg²⁺ to stabilize ATP
major regulated enzyme ( committed step of glycolysis)

Aldolase catalyzes
fructose-1,6-bis-P ↔ dihydroxyacetone phosphate + glyceraldehyde-3-phosphate  
ΔG°′ = 23.8 kJ/mol

   Aldolase (a lyase) occur in two classes: type I uses a Schiff base mechanism (acid-base),
     and type II uses a metal ion.

Triose phosphate isomerase converts dihydroxyacetone-phosphate to glyceraldehyde-phosphate, so overall in the preparation phase 1 glucose → 2 glyceraldehyde-phosphates.
"perfect" enzyme                                                                                           ΔG°′ = 7.5 kJ/mol
Uses acid-base catalysis similar to the reaction catalyzed by phosphohexose isomerase.

Preparation phase summary: 1 glucose + 2 ATP → 2 glyceraldehyde-phosphates + 2 ADP

Pay-off reactions (5): 2 C₃-phosphates → 2 pyruvates; oxidation + substrate phosphorylation

Glyceraldehyde-3-phosphate dehydrogenase oxidizes glyceraldehyde-3-phosphate, reduces NAD⁺, and adds P_i to C1 of glyceraldehyde-3-phosphate.
first productive reaction of glycolysis                                                                  ΔG°′ = 6.3 kJ/mol

   Reaction mechanism involves covalent catalysis, requires Cys and His in the active site.

Phosphoglycerate kinase transfers the P from C1 of 1,3-BPG to ADP:
1,3-bis-phosphoglycerate + ADP ↔ 3-phosphoglycerate + ATP
first ATP production = substrate level phosphorylation                              ΔG°′ = − 18.5 kJ/mol
This reaction pulls the previous reaction.

Phosphoglycerate mutase converts 3-phosphoglycerate ↔ 2-phosphoglycerate
                                ΔG°′ = 4.4 kJ/mol
This involves transfer of P from P–His–enzyme → 2,3-BPG intermediate.
This occasionally diffuses away from the enzyme, which must then be regenerated.
2,3-BPG is important in red blood cells as a modulator of Hb conformation.

Enolase removes H₂O from 2-phosphoglycerate ↔ phosphoenolpyruvate (PEP)
                                 ΔG°′ = 7.5 kJ/mol
converts the phosphate into a high energy phosphate.

Pyruvate kinase transfers phosphoryl from PEP to ADP → pyruvate + ATP.
ΔG°′ = − 31.4 kJ/mol
Requires K⁺ and Mg²⁺ (or Mn²⁺) to stabilize negative charges during transfer.
Two-step reaction: transfer phosphoryl, then convert enol → keto (more stable)
Second substrate level phosphorylation
Irreversible , making the pathway irreversible; a regulated step

Pay-off phase summary: 2 glyceraldehyde-phosphates + 2 NAD⁺ + 2 P_i + 4 ADP →
              2 pyruvates + 2 NADH + 2 H⁺ + 4 ATP

Overall energy yield from glycolysis per glucose: 2 ATP + 2 NADH + 2 pyruvate

Multi-enzyme complexes channel pathway intermediates, increasing pathway efficiency.

Other sugars and glycolysis
Fructose: phosphorylation by hexokinase → fructose-6-phosphate
         phosphorylation by fructokinase → fructose-1-phosphate
Galactose (from lactose) → → → glucose-1-phosphate → glucose-6-phosphate
Mannose: hexokinase → mannose-6-phosphate → fructose-6-phosphate
         isomerization by phosphomannose isomerase

Pyruvate and NADH
aerobically NADH is recycled using electron transport; requires O₂ as e⁻ acceptor
      pyruvate → acetyl-CoA → CO₂ (citric acid cycle) + more energy (30-32 ATP)
anaerobically NADH must be recycled by another method, usually involving pyruvate

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