Biosynthesis — cofactor-based imine chemistry
Pyridoxal phosphate (PLP) transamination
How the Pyridoxal phosphate (PLP) transamination 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: PLP cofactor, amino acid, α-keto acid acceptor, aminotransferase (Lys-bound PLP). Conditions: Aqueous, pH ~7.5, 37 °C. Bonds formed: C=N (Schiff base); keto-acid C=O; pyridoxamine C–N; bonds broken: Cα–H; C=N (hydrolysis). Stereochemistry: Enzyme reprotonates the quinonoid on one face (stereospecific). Rate and selectivity: Cα deprotonation is facilitated by the pyridinium electron sink.
At a glance
Step 1: Schiff base (external aldimine)
PLP's aldehyde condenses with an amino acid's amine to form an imine (a Schiff base) — the identical carbonyl/amine condensation covered throughout this library, here anchored to a vitamin B6-derived cofactor.
Step 2: Cα–H removal gives the quinonoid
The pyridine ring's electron-withdrawing, protonated nitrogen dramatically stabilizes negative charge at the adjacent alpha carbon, letting a base remove that proton to form a resonance-delocalized quinonoid intermediate — PLP's whole catalytic purpose is providing this electron sink, the same conceptual role the ester groups play in stabilizing a malonate enolate.
Step 3: Reprotonation at C4′ and hydrolysis
Reprotonation at a different position (the aldehyde carbon instead of the original alpha carbon) and hydrolysis exchange which molecule carries the amine — an amino acid is converted to its corresponding keto-acid, while PLP itself is converted to pyridoxamine, ready to transfer that amine onto a different keto-acid acceptor in the reverse half-reaction.
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.
- Lys
- Lysine. Basic amino acid with a nucleophilic –NH₂.
How it compares with related reactions
| Pyridoxal phosphate (PLP) transamination | Thiamine pyrophosphate (TPP) catalysis | Cytochrome P450 | |
|---|---|---|---|
| Reagents | PLP cofactor, amino acid, α-keto acid acceptor, aminotransferase (Lys-bound PLP) | Thiamine pyrophosphate (TPP/ThDP), pyruvate decarboxylase, Mg²⁺, pyruvate | Cytochrome P450 (heme-thiolate), O₂, NADPH (2 e⁻ via reductase), 2 H⁺, substrate R–H |
| Conditions | Aqueous, pH ~7.5, 37 °C | Aqueous, pH ~6, 25–37 °C, enzyme active site | Aqueous, pH ~7.4, 37 °C, membrane-bound or soluble enzyme |
| Stereochemistry | Enzyme reprotonates the quinonoid on one face (stereospecific) | No stereocentre in the product (acetaldehyde) | Rebound is fast, so configuration is often largely retained |
| Rate / selectivity | Cα deprotonation is facilitated by the pyridinium electron sink | C2–H deprotonation (pKa ~18 free, lowered in the enzyme) and decarboxylation are fast | Compound I formation fast; C–H abstraction is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Pyridoxal phosphate (PLP) transamination?
PLP cofactor, amino acid, α-keto acid acceptor, aminotransferase (Lys-bound PLP). Conditions: Aqueous, pH ~7.5, 37 °C.
Which bonds are formed and which are broken?
Formed: C=N (Schiff base); keto-acid C=O; pyridoxamine C–N. Broken: Cα–H; C=N (hydrolysis).
What is the stereochemical outcome?
Enzyme reprotonates the quinonoid on one face (stereospecific)
What controls the rate or the selectivity?
Cα deprotonation is facilitated by the pyridinium electron sink
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