Threonine proteases use the secondary alcohol of their N-terminal threonine as a nucleophile to perform catalysis.[1][2] 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.[3][4]
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 (the acceptor) performs the second nucleophilic attack and so leaves with the acyl group.
^ abBrannigan JA, Dodson G, Duggleby HJ, Moody PC, Smith JL, Tomchick DR, Murzin AG (November 1995). "A protein catalytic framework with an N-terminal nucleophile is capable of self-activation". Nature. 378 (6555): 416–9. Bibcode:1995Natur.378..416B. doi:10.1038/378416a0. PMID7477383. S2CID4277904.