Five steps from threonine. The last four enzymes are the same ones that make valine
Route: Threonine → α-Ketobutyrate + Pyruvate → (2S)-2-Aceto-2-hydroxybutyrate → (2R,3R)-2,3-Dihydroxy-3-methylvalerate → (3S)-α-Keto-β-methylvalerate → Isoleucine
Isoleucine is an essential amino acid, so humans must eat it. Threonine deaminase, which needs pyridoxal phosphate, removes the amino group and the hydroxyl of threonine and releases ammonium, which leaves α-ketobutyrate. The other four enzymes also work in the valine pathway. Acetolactate synthase joins α-ketobutyrate to a pyruvate and releases CO₂ (the enzyme accepts α-ketobutyrate as well as a second pyruvate). Ketol-acid reductoisomerase rearranges and reduces the product with NADPH. Dihydroxy-acid dehydratase removes water to make a keto acid. Branched-chain aminotransferase adds the amino group from glutamate. Isoleucine inhibits threonine deaminase, and valine activates it. Isoleucine has two stereo centers; L-isoleucine is the 2S,3S form.
Practice this pathway All pathways
| Step | Enzyme | EC | In | Out |
|---|---|---|---|---|
| 1 | Threonine deaminase | 4.3.1.19 | NH₄⁺ | |
| 2 | Acetolactate synthase | 2.2.1.6 | CO₂ | |
| 3 | Ketol-acid reductoisomerase | 1.1.1.86 | NADPH | NADP⁺ |
| 4 | Dihydroxy-acid dehydratase | 4.2.1.9 | H₂O | |
| 5 | Branched-chain aminotransferase | 2.6.1.42 | Glutamate | α-KG |
Enzyme names and EC numbers are those used on the interactive board. Checked against sources: see the claim table in the repository (tools/pathway_data/checks).