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 (the acceptor) performs the second nucleophilic attack and so leaves with the acyl group. ==Classification and evolution==