PA clan of proteases

The PA clan (Proteases of mixed nucleophile, superfamily A) is the largest group of proteases with common ancestry as identified by structural homology. Members have a chymotrypsin-like fold and similar proteolysis mechanisms but can have identity of <10%. The clan contains both cysteine and serine proteases (different nucleophiles). PA clan proteases can be found in plants, animals, fungi, eubacteria, archaea and viruses.

The common use of the catalytic triad for hydrolysis by multiple clans of proteases, including the PA clan, represents an example of convergent evolution. The differences in the catalytic triad within the PA clan is also an example of divergent evolution of active sites in enzymes.