Aspartic proteases are a catalytic type of protease enzymes that use an activated water molecule bound to one or more aspartate residues for catalysis of their peptide substrates. In general, they have two highly conserved aspartates in the active site and are optimally active at acidic pH.
These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Cysteine proteases are commonly encountered in fruits including the papaya, pineapple, fig and kiwifruit. The proportion of protease tends to be higher when the fruit is unripe.
The serine proteases differ in their sequence and in their substrate specificity: the bacterial protease subtilisin (one of the major enzymes that resulted in the advertising slogan “the cleaning power of enzymes!”) will cleave essentially any
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.