Characterisation of tyrosinase immobilised onto spacer-arm attached glycidyl methacrylate-based reactive microbeads

Abstract

Immobilisation of tyrosinase onto modified poly(methyl methacrylate–glycidyl methacrylate–divinyl benzene), poly(MMA–GMA–DVB), microbeads was studied. The epoxy group containing poly(MMA–MMA–DVB) microbeads were prepared by suspension polymerisation. The epoxy groups of the poly(MMA–GMA–DVB) microbeads was converted into amino groups with either ammonia or 1,6-diaminohexane (i.e., spacer-arm). Tyrosinase was then covalently immobilised on aminated and the spacer-arm-attached poly(MMA–GMA–DVB) microbeads using glutaric dialdehyde as a coupling agent. Incorporation of the spacer-arm resulted an increase in the apparent activity of the immobilised tyrosinase with respect to the enzyme immobilised on the aminated microbeads. The activity yield of the immobilised tyrosinase on the spacer-arm-attached poly(MMA–GMA–DVB) microbeads was 68%, and this was 51% for the enzyme, which was immobilised on the aminated microbeads. Both immobilised tyrosinase preparation has resistance to temperature inactivation as compared to that of the free form. The temperature profiles were broader for both immobilised preparations than that of the free enzyme. Kinetic parameters were determined for immobilised tyrosinase preparations as well as for the free enzyme. The values of the Michaels constants ( K m) for all the immobilised tyrosinase preparations were significantly larger, indicating decreased affinity by the enzyme for its substrate, whereas V max values were smaller for the both immobilised tyrosinase preparations. In a 40 h continuous operation with spacer-arm-attached poly(MMA–GMA–DVB) microbeads at 30 °C, only 3% of immobilised tyrosinase activity was lost. The operational inactivation rate constant ( k opi) of the immobilised tyrosinase was 1.25×10 −5 min −1.