PA clan

PA clan of proteases
The double β-barrels that characterise the PA clan are highlighted in red. (TEV protease, PDB: 1lvm​)
Identifiers
Symbol	N/A
Pfam	CL0124
InterPro	IPR009003
SCOP	50494
SUPERFAMILY	50494
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

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 sequence 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 PA clan represents an example of convergent evolution to the use of a catalytic triad for hydrolysis. It also is an example of extreme divergent evolution of active sites in enzymes.

In the 1960s, the sequence similarity of several proteases indicated that they were evolutionarily related. These were grouped into the chymotrypsin-like serine proteases (now called the S1 family). As the structures of these, and other proteases were solved by X-ray crystallography in the 1970s and 80s, it was noticed that several viral proteases such as Tobacco Etch Virus protease showed structural homology despite no discernible sequence similarity and even a different nucleophile. Based on structural homology, a superfamily was defined and later named the PA clan (by the MEROPS classification system). As more structures are solved, more protease families have been added to the PA clan superfamily.