Serine

Serine proteases are ubiquitous in nature and present in all kingdoms of life along with the viruses. They are endopeptidases that cleaves peptide bonds like other proteases having serine as a nucleophile. The serine proteases in addition to proteolytic activity also perform the other functions such as protein metabolism, digestion, regulation of development, fertilisation, blood coagulation, immunity, and apoptosis (Di Cera, 2009, Gohara and Di Cera, 2011, Patel, 2017). Two step hydrolytic reaction is carried out by serine proteases: one acylation producing a covalently linked enzyme-peptide intermediate (Fastrezx and Fersht, 1973), followed by de-acylation process mediated by a nucleophilic attack on the intermediate by water causing the peptide hydrolysis (Rao et al., 1998). Catalytic serine nucleophilicity depends on the triad Asp, His, and Ser. The catalytic triad is utilized by trypsin, prolyl oligopeptidase, subtilisin, and ClpP, however, the other proteases employ a simpler dyad mechanism where Lys or His is paired with the catalytic Ser. Serine proteases based on their primary substrate specificity can be classified into three groups: a) trypsin-like, which cleave after positively charged residues; (ii) chymotrypsin-like, which cleave after large hydrophobic residues; and (iii) elastase-like, which cleave after small hydrophobic residues (Rao et al., 1998).