Enzyme catalysis — Schiff base aldol
Aldolase: enzyme-catalyzed aldol via enamine
How the Aldolase works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Dihydroxyacetone phosphate (DHAP), glyceraldehyde 3-phosphate (G3P), Class I aldolase (Lys229). Conditions: Aqueous, pH ≈ 7.5. Bonds formed: C–C bond (C3–C4 of fructose 1,6-bisphosphate); bonds broken: Lys–C=N (hydrolysis); α C–H. Stereochemistry: Enzyme controls the face: gives the D-threo (3S,4R) product. Rate and selectivity: Enamine formation/C–C bond formation rate-limiting.
At a glance
Step 1: Lysine forms the Schiff base
A lysine in the active site condenses with the substrate ketone to form a Schiff base (iminium ion) — the same imine chemistry covered earlier, now used to activate the substrate instead of a metal enolate.
Step 2: Enamine forms and attacks the aldehyde
Deprotonation of the alpha carbon gives a resonance-stabilized enamine — the nitrogen analog of the enolates used in ordinary base-catalyzed aldol reactions — which then attacks a second aldehyde's carbonyl carbon.
Step 3: Hydrolysis releases the product
Hydrolysis of the resulting iminium releases the aldol product and regenerates the free lysine — the enzyme replaces the metal-enolate strategy of ordinary aldol chemistry with covalent Schiff-base activation instead.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Asp
- Aspartate. Acidic amino acid.
- Lys
- Lysine. Basic amino acid with a nucleophilic –NH₂.
How it compares with related reactions
| Aldolase | DNA polymerase | RNase A | |
|---|---|---|---|
| Reagents | Dihydroxyacetone phosphate (DHAP), glyceraldehyde 3-phosphate (G3P), Class I aldolase (Lys229) | Primer–3′-OH, dNTP, 2 Mg²⁺, DNA polymerase (Asp carboxylates) | RNA substrate, RNase A (His12 base, His119 acid, Lys41) |
| Conditions | Aqueous, pH ≈ 7.5 | Aqueous, pH ≈ 7.5, 37 °C | Aqueous, pH ≈ 6, 37 °C |
| Stereochemistry | Enzyme controls the face: gives the D-threo (3S,4R) product | In-line attack, inversion at Pα | In-line attack, inversion at P |
| Rate / selectivity | Enamine formation/C–C bond formation rate-limiting | kpol ≈ 10–300 s⁻¹ (chemistry step partly rate-limiting) | kcat/KM ≈ 10⁷ M⁻¹ s⁻¹; about 10¹¹-fold over uncatalysed |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Aldolase?
Dihydroxyacetone phosphate (DHAP), glyceraldehyde 3-phosphate (G3P), Class I aldolase (Lys229). Conditions: Aqueous, pH ≈ 7.5.
Which bonds are formed and which are broken?
Formed: C–C bond (C3–C4 of fructose 1,6-bisphosphate). Broken: Lys–C=N (hydrolysis); α C–H.
What is the stereochemical outcome?
Enzyme controls the face: gives the D-threo (3S,4R) product
What controls the rate or the selectivity?
Enamine formation/C–C bond formation rate-limiting
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make