Can immobilization be exploited to modify enzyme activity?

Immobilization has long been recognized as a useful tool for retaining enzymes in bioreactors and enabling the continuous operation of enzymic processes. However, the potential of immobilization for modifying enzyme activity in an advantageous, if not predictable, manner is less-well appreciated. This review summarizes selected studies that have used immobilization to tailor the catalytic properties of enzymes, and highlights the application of immobilization to the rational design of biocatalysts.

Random and site-specific immobilization of catalytic antibodies

The effects of immobilization on the immunologic and catalytic activity of a catalytic antibody were compared for randomly immobilized (via glutaraldehyde) whole antibody and site-specifically immobilized (via the reactive sulfhydryl group at the base of the fragment) Fab' fragments. Upon immobilization, the specific binding capacity (n) and the catalytic activity decreased significantly for both systems. Increases in the Michaelis constant (KM) were accompanied by corresponding decreases in the equilibrium binding constant determined through immunoassays. For the immobilized Fab', n decreased dramatically with increased protein loading, suggesting that, despite the site-specific attachment and favorable orientation, molecular crowding denatured the Fab' fragments. These results also show that there is an optimal surface coverage, not necessarily at the maximum loading, for both immunologic and catalytic activity. Finally, the combining/active site conformation was probed using electron paramagnetic resonance (EPR) spectroscopy. In all antibody samples, there was no spectral evidence of conformational changes in the antibody active site.

Surface-density and orientation effects on immobilized antibodies and antibody fragments

The binding parameters of randomly immobilized protein 315 and Fv fragments, as well as site-specifically immobilized Fab' fragments, have been measured for a small hapten (MW = 341 Daltons) and a large synthetic antigen (MW = 50 kD). Immobilized Fv fragments had the highest binding capacities; hence, removing unnecessary protein domains can be beneficial for improving the total capacity of an immunosorbent. For all immunosorbents, high protein loadings led to relatively low specific activities (n values). This effect was reversible, however, as the loss of immobilized antibody upon prolonged storage partially restored the specific activity. At high loadings the specific activity of immobilized whole antibody was lower for the large antigen than for the small hapten, whereas no effect of hapten size on n was evident for either immobilized Fab' or Fv fragments. Although a fraction of immobilized antibody was inactive at the higher loadings, EPR spectroscopy revealed no significant changes in the conformation of active immobilized antibody.

Structural and catalytic properties of enzymes in reverse micelles

Structural and catalytic properties of two enzymes--alpha-chymotrypsin and horse liver alcohol dehydrogenase (LADH)--are studied in bis(2-ethylhexyl) sodium sulfosuccinate (AOT)-isooctane reverse-micelle solutions. Circular dichroism (CD) and electron paramagnetic resonance spectroscopy (EPR) studies show little change in alpha-chymotrypsin structure upon incorporation into reverse micelles. These structural properties explain, in part, the observed activity of these two enzymes in reverse micelles. alpha-Chymotrypsin retains activity in reverse micelles and, in some cases, displays enhanced activity. A sixfold increase in the turnover number was observed in w0 = 10 reverse micelles. LADH has low activity in reverse micelles compared to that in aqueous solution. At w0 = 70, the turnover number of LADH is 18% of the aqueous value. Active-site titrations show a decrease in active enzyme concentration for both enzymes upon incorporation into reverse micelles. Little change in the structure of both LADH and alpha-chymotrypsin is observed with change of water content in the reverse-micelle system.