Improving the activity of immobilized subtilisin by site-directed attachment through a genetically engineered affinity tag

An octapeptide affinity tag, Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys (termed FLAG), was genetically fused to the C-terminus of subtilisin BPN' (SBT) from Bacillus amyloliquefaciens. The fusion protein SBT-FLAG was immobilized to nonporous polystyrene and silica beads both in a site-directed and a random fashion. Site-directed immobilization was achieved by employing the interaction between protein A and a monoclonal antibody specific for the FLAG peptide, while random immobilization was obtained by using glutaraldehyde as a cross-linking reagent. The activity of the immobilized enzymes was compared. It was found that the site-directed subtilisin had higher catalytic efficiency, k_cat/K_M, which was more than 7-fold of that of the randomly immobilized enzyme. It was also noted that the site-directly immobilized enzyme had superior storage stability over the homogeneous enzyme.