Graspases--a special group of serine proteases of the chymotrypsin family that has lost a conserved active site disulfide bond.

In this report we propose a new approach to classification of serine proteases of the chymotrypsin family. Comparative structure-function analysis has revealed two main groups of proteases: a group of trypsin-like enzymes and graspases (granule-associated proteases). The most important structural peculiarity of graspases is the absence of conservative "active site" disulfide bond Cys191-Cys220. The residue at position 226 in the S1-subsite of graspases is responsible for substrate specificity, whereas the residue crucial for specificity in classical serine proteases is located at position 189. We distinguish three types of graspases on the base of their substrate specificity: 1) chymozymes prefer uncharged substrates and contain an uncharged residue at position 226; 2) duozymes possess dual trypsin-like and chymotrypsin-like specificity and contain Asp or Glu at 226; 3) aspartases hydrolyze Asp-containing substrates and contain Arg residue at 226. The correctness of the proposed classification was confirmed by phylogenic analysis.