Threonine protease
Threonine proteases are a family of proteolytic enzymes harbouring a threonine residue within the active site. The prototype members of this class of enzymes are the catalytic subunits of the proteasome, however the acyltransferases convergently evolved the same active site geometry and mechanism.
Mechanism
Threonine proteases use the secondary alcohol of their N-terminal threonine as a nucleophile to perform catalysis. The threonine must be N-terminal since the terminal amine of the same residue acts as a general base by polarising an ordered water which deprotonates the alcohol to increase its reactivity as a nucleophile.Catalysis takes place in two steps:
- Firstly the nucleophile attacks the substrate to form a covalent acyl-enzyme intermediate, releasing the first product.
- Secondly the intermediate is hydrolysed by water to regenerate the free enzyme and release the second product.
- * In ornithine acyltransferase, instead of water, the substrate ornithine performs the second nucleophilic attack and so leaves with the acyl group.
Classification and evolution
| Superfamily | Threonine protease families | Examples |
| PB clan | T1, T2, T3, T6 | archaean proteasome, beta component |
| PE clan | T5 | ornithine acetyltransferase |