Kinetic studies of site-specifically and randomly immobilized alkaline phosphatase on functionalized membranes

A comparison of enzyme activities has been made between a site-specifically immobilized and a randomly immobilized bacterial alkaline phosphatase (BAP) on macroporous membranes. An octapeptide tag (FLAG) was attached at the N-terminus of alkaline phosphatase by recombinant DNA techniques (gene fusion) to yield BAP that is modified in a site-directed fashion (SDBAP). The corresponding antibody (antiFLAG) was immobilized on an aldehyde-modified polyethersulfone (MPS) membrane via protein A. Immobilization of SDBAP on this membrane result in a membrane-protein A-antiFLAG-SDBAP linkage. This site-specifically immobilized enzyme demonstrated a relative activity (RA), defined as the ratio of immobilized activity (V_max) to the corresponding homogeneous enzymatic activity, of 85% as compared with the randomly immobilized BAP which had an RA of 0·8%. BAP, when chemically conjugated to the FLAG peptide and immobilized via anti-FLAG and protein A on the MPS membrane, showed an RA of only 1·9%, demonstrating the effectiveness of site-directed immobilization. SDBAP was also immobilized on the MPS membrane in the absence of protein A. In this case, the RA dropped to 22%, further explaining the effectiveness of ordered immobilizations as compared with random immobilizations. The ratio of immobilized enzyme activity to the activity in the absence of added phosphate inhibitor for the immobilized BAP was three-fold higher than the corresponding homogeneous ratio, showing a reduction in product inhibition for the immobilized enzyme.