Five steps from aspartate through homoserine. Humans cannot do this; most microorganisms can
Route: Aspartate → β-Aspartyl phosphate → Aspartate semialdehyde → Homoserine → O-Phosphohomoserine → Threonine
Threonine is an essential amino acid, so humans must eat it. This is the bacterial route. Aspartate kinase puts a phosphate from ATP on the side-chain carboxyl of aspartate. Aspartate-semialdehyde dehydrogenase reduces it with NADPH and releases the phosphate, leaving the aldehyde aspartate semialdehyde. This is the fork of the aspartate family: lysine branches off here, while homoserine dehydrogenase reduces the aldehyde to a hydroxyl and makes homoserine. Homoserine kinase then phosphorylates that hydroxyl with ATP. The last enzyme, threonine synthase, uses pyridoxal phosphate to remove the phosphate from phosphohomoserine and add water at the beta carbon, which builds threonine. In E. coli the first and third enzymes can sit on one polypeptide chain. Homoserine is also where the methionine pathway starts.
Practice this pathway All pathways
| Step | Enzyme | EC | In | Out |
|---|---|---|---|---|
| 1 | Aspartate kinase | 2.7.2.4 | ATP | ADP |
| 2 | Aspartate-semialdehyde dehydrogenase | 1.2.1.11 | NADPH | NADP⁺, Pᵢ |
| 3 | Homoserine dehydrogenase | 1.1.1.3 | NADPH | NADP⁺ |
| 4 | Homoserine kinase | 2.7.1.39 | ATP | ADP |
| 5 | Threonine synthase | 4.2.3.1 | H₂O | Pᵢ |
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).