Metabolism — glycolysis (step 5)
Triose phosphate isomerase: enediol isomerization
How the Triose phosphate isomerase 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), TIM (Glu165 base, His95 acid). Conditions: Aqueous, pH ≈ 7, 37 °C. Bonds formed: C=O at C1 (aldehyde); C–H at C2; bonds broken: C1–H; C2=O (ketone). Stereochemistry: Enzyme gives only (R)-glyceraldehyde 3-phosphate. Rate and selectivity: kcat/KM ≈ 10⁸–10⁹ M⁻¹s⁻¹, near the diffusion limit.
At a glance
Step 1: Glu165 removes the C1 proton
An active-site glutamate deprotonates the alpha carbon of dihydroxyacetone phosphate, forming a resonance-stabilized enediolate intermediate — the same alpha-deprotonation chemistry underlying every enolate covered throughout this library, here stabilized further by an adjacent phosphate.
Step 2: Enediolate reprotonated on the other carbon
This planar enediolate can be reprotonated at either of its two ends; the enzyme's active site geometry favors reprotonation at the other carbon from where the proton was originally removed.
Step 3: Glyceraldehyde 3-phosphate released
The product is glyceraldehyde-3-phosphate — this isomerization is so mechanistically simple (a single proton shuttled across a resonance-stabilized intermediate) that the enzyme catalyzing it, TIM, is famously cited as operating near the theoretical diffusion-controlled speed limit for an enzyme.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- M
- Metal. A generic metal centre.
How it compares with related reactions
| Triose phosphate isomerase | Enolase | GAPDH | |
|---|---|---|---|
| Reagents | Dihydroxyacetone phosphate (DHAP), TIM (Glu165 base, His95 acid) | 2-Phosphoglycerate, 2 Mg²⁺, enolase (Lys345 base, Glu211 acid) | Glyceraldehyde-3-phosphate (G3P), NAD⁺, inorganic phosphate (Pi), GAPDH (Cys149, His176) |
| Conditions | Aqueous, pH ≈ 7, 37 °C | Cytosol, pH ≈ 7, 37 °C | Aqueous, pH ≈ 7.4, 37 °C (cytosol) |
| Stereochemistry | Enzyme gives only (R)-glyceraldehyde 3-phosphate | Anti elimination of H and OH; reversible | Hydride delivered to one face of NAD⁺ (pro-R) |
| Rate / selectivity | kcat/KM ≈ 10⁸–10⁹ M⁻¹s⁻¹, near the diffusion limit | ΔG°′ ≈ +1.8 kJ/mol; PEP has a very high phosphoryl-transfer potential | Fast and reversible; couples oxidation to phosphate capture |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Triose phosphate isomerase?
Dihydroxyacetone phosphate (DHAP), TIM (Glu165 base, His95 acid). Conditions: Aqueous, pH ≈ 7, 37 °C.
Which bonds are formed and which are broken?
Formed: C=O at C1 (aldehyde); C–H at C2. Broken: C1–H; C2=O (ketone).
What is the stereochemical outcome?
Enzyme gives only (R)-glyceraldehyde 3-phosphate
What controls the rate or the selectivity?
kcat/KM ≈ 10⁸–10⁹ M⁻¹s⁻¹, near the diffusion limit
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make