Active sites of serine protease contain
Histidine
Lysine or threonine
Arginine
Serine
Serine proteases name stems from the nucleophilic Ser (serine) in the enzyme active site Nucleophilicity of the catalytic Ser is typically dependent on a catalytic triad of Asp, His, and Ser residues, commonly referred to as the charge relay system Ref: biochemistry by U. Satyanarayana 3rd edition Pgno : 171
Structure and function of protein