Enzymatic multi-functionalization of microparticles under aqueous neutral conditions†
The immobilization of proteins and enzymes on microparticles enables innovative biofunctionalization and facilitates the recycling of, for example, catalysts in a biocatalytic process. When enzymes are used for the immobilization of proteins to material surfaces, site-specificity can be achieved, and the immobilization reaction can proceed under mild reaction conditions in an aqueous environment. By selectively oxidizing the exposed tyrosyl side chains of protein to reactive o-quinones using molecular oxygen as electron acceptor, tyrosinase promotes the formation of new covalent bonds via 1,4-addition of nucleophilic moieties to the o-quinones. The introduction of tyrosinase-susceptible tyrosine residues (Y-tag) to a protein by genetic engineering can thus enable site-specific crosslinking and site-specific protein immobilization. In this study, several variants of the fluorescent protein GFPuv were produced in order to investigate the tyrosinase-mediated crosslinking and immobilization reaction. The Y-tag was a target for rapid protein–protein crosslinking by tyrosinase catalysis. Moreover, low concentrations of tyrosinase were sufficient to obtain detectable fluorescent microparticles using the Y-tagged GFPuv variants, e.g. 0.4 μM (50 μg ml−1) protein concentrations. Eventually, we showed that this enzyme-based technology allows for the multiple functionalization of microparticles using different fluorescent proteins.