Alignment of the primary structures of proteinase K subfamily enzymes and subtilisin BPN’. AQN, aqualysin I; VPR, protease from Vibrio sp. PA-44; SPRK, proteinase K-like enzyme from Serratia sp.; PK, proteinase K from Tritirachium album Limber; BPN, subtilisin BPN’ from Bacillus amyloliquefaciens. “#” denotes the catalytic residues Asp, His and Ser. The positions of common (closed circles), AQN-intrinsic (open arrowheads) and VPR-intrinsic (closed arrowheads) salt bridges are represented.