Interaction of polymer-coated gold nanoparticles with cellobiose dehydrogenase: The role of surface charges

Studying the interaction of functional proteins such as enzymes and nanoparticles (NPs) includes the important topic of investigating any possible changes in stability and function of enzymes in nanostructured environments. The effects of NPs on the enzyme activity and stability are governed by their physical and chemical properties such as structure, shape, size, surface chemistry and their surface charges. In this study, the influence of negatively and positively charged AuNPs are investigated on the activity of immobilized Myriococcum thermophilum cellobiose dehydrogenase (MtCDH) and its electron transfer rate with graphite electrodes modified with positively and negatively AuNPs. The MtCDH modified graphite electrode premodified with positively charged AuNPs showed an alkaline shift in the pH of maximum activity from pH 5.5 to 8. No change in the pH of maximum activity was observed when MtCDH graphite electrodes were premodified with negatively charged AuNPs. The results clearly demonstrated the effect of surface charge of AuNPs on the activity of the enzyme. The catalytic current density and the KMapp value for MtCDH graphite electrode premodified with positively charged AuNPs were enhanced with up to 66 and 8 times, respectively. Two spectroscopic assays were also performed in solution to investigate the influence of the presence of positively or negatively charged AuNPs on the activity of MtCDH in homogeneous solution. The results clearly demonstrated that not only the rate of the heterogeneous electron transfer between the immobilized MtCDH and the electrode but also the rate of the homogeneous electron transfer between soluble MtCDH and the acceptor was highly dependent on the type of surface charge of the AuNPs.