Immobilization of urease via adsorption onto l-histidine–Ni(II) complexed poly(HEMA-MAH) microspheres: Preparation and characterization

Poly(2-hydroxyethyl methacrylate-co-N-methacryloly-l-histidinemethylester) poly(HEMA-MAH) microspheres was prepared via suspension polymerization. l-Histidine groups of the poly(HEMA-MAH) microspheres were then chelated with Ni(II) ions (poly(HEMA-MAH)–Ni(II)). Urease immobilization onto the poly(HEMA-MAH) and poly(HEMA-MAH)–Ni(II) microspheres from aqueous solutions was investigated in a batch system. The amount of immobilized urease on the poly(HEMA-MAH) and poly(HEMA-MAH)–Ni(II) was 47.8 and 66.1 mg/g support, respectively. The values of Michaelis constants Km for both immobilized urease preparations were significant higher than free enzyme, indicating decreased affinity by the enzyme for its substrate, whereas Vmax values were smaller for both immobilized urease preparations compared to free enzyme. However, the urease-immobilized onto the poly(HEMA-MAH)–Ni(II) resulted in an increase in enzyme stability with time. Optimum operational temperature for both immobilized preparations was 5.0 °C higher than that of the free enzyme and the temperature profiles of the immobilized preparations were significantly broader. It was observed that enzyme could be repeatedly adsorbed and desorbed on the poly(HEMA-MAH) and poly(HEMA-MAH)–Ni(II) microspheres without loss of adsorption capacity or enzymic activity. Finally, a packed bed enzyme-reactor with/urease-immobilized poly(HEMA-MAH)–Ni(II) microspheres, were used for degradation of urea in the continuous operation mode. The enzyme-reactor operated continuously at 35 °C for 40 h without significant loss of performance.