Modulation of molecular and kinetic properties of enzymes upon immobilization

Pea seedling glutamate dehydrogenase(EC 1.4.1.2) and Jack bean urease (EC 3.5.1.5) were immobilized on aminoethyl cellulose by cross-linking in a two-step reaction with glutaral dehyde. Specific activities of the immobilized dehydrogenase and urease were about 14% and 87%, respectively, of the original material in free solution. Both immobilized enzymes show no appreciable change in their pH dependence, whereas a less efficient binding of the substrates is suggested by the increased apparent Michaelis constants (K_m app.). Sigmoid kinetics were observed for both enzymes when reactions were carried out in a packed bed. Diffusional effects are considered responsible for producing these anomalous kinetics. The implications of these perturbations in terms of the catalytic efficiency of the enzymes, as well as the practical problems involved in the analysis of the kinetic data, are discussed.