Peptidase S1 family (Serine protease)

General Activity
Serine proteases (or serine proteinases) are enzymes that cleave peptide bonds in proteins, in which serine serves as the nucleophilic amino acid at the active site. In mammals, they are responsible for coordinating various physiological functions, including digestion, immune response, blood coagulation and reproduction.

In snake venom, serine proteases (SVSP) mainly affect the hemostatic system of the prey. They act on components of the coagulation cascade, fibrinolytic and kallikrein-kinin systems, and on platelets to cause an imbalance of the hemostatic system. In venom from other species, only a few serine proteases have been isolated and their function in the venom is not well characterized.

Active site
SVSPs act by a common catalytic mechanism that includes a highly reactive serine residue (Ser-202, according to batroxobin numbering) that plays a key role in the formation of a transient acyl-enzyme complex, which is stabilized by the presence of histidine and aspartic acid residues within the active site (Barrett and Rawlings, 1995 ).

Structure
SVSPs contain twelve cysteine residues that form six disulfide bonds. Most SVSPs are glycoproteins showing a variable number of putative N- or O-glycosylation sites in sequence positions that are not conserved. SVSPs are in general one-chain proteins (see proteins here ) with molecular mass varying between 26 and 67 kDa, depending on their carbohydrate content. Exceptions are some group C and D prothrombin activators from Australian snake venoms which are large protein complexes with multiple subunits (see proteins here ) or two-chain serine proteinases (see proteins here ), respectively.