Zeolitic imidazolate framework-8 (ZIF-8) is widely used as a protective coating to encapsulate proteins via biomimetic mineralization. The formation of nucleation centers and further biocomposite crystal growth is entirely governed by the pure electrostatic interactions between the protein’s surface and the positively charged Zn(II) metal ions. It was previously shown that enhancing these electrostatic interactions by a chemical modification of surface amino acid residues can lead to a rapid biocomposite crystal formation. However, a chemical modification of carbohydrate components by periodate oxidation for glycoproteins can serve as an alternative strategy. In the present study, an industrially important enzyme glucose oxidase (GOx) was selected as a model system. Periodate oxidation of GOx by 2.5 mM sodium periodate increased negative charge on the enzyme molecule, from −10.2 to −36.9 mV, as shown by zeta potential measurements and native PAGE electrophoresis. Biomineralization experiments with oxidized GOx resulted in higher specific activity, effectiveness factor, and higher thermostability of the ZIF-8 biocomposites. Periodate oxidation of carbohydrate components for glycoproteins can serve as a facile and general method for facilitating the biomimetic mineralization of other industrially relevant glycoproteins.

Cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium can be used in lactobionic acid production, biosensor for lactose, biofuel cells, lignocellulose degradation, and wound-healing applications. To make it a better biocatalyst, CDH with higher activity in an immobilized form is desirable. For this purpose, CDH was expressed for the first time on the surface of S. cerevisiae EBY100 cells in an active form as a triple mutant tmCDH (D20N, A64T, V592M) and evolved further for higher activity using resazurin-based fluorescent assay. In order to decrease blank reaction of resazurin with yeast cells and to have linear correlation between enzyme activity on the cell surface and fluorescence signal, the assay was optimized with respect to resazurin concentration (0.1 mM), substrate concentration (10 mM lactose and 0.08 mM cellobiose), and pH (6.0). Using optimized assay an error prone PCR gene library of tmCDH was screened. Two mutants with 5 (H5) and 7 mutations (H9) were found having two times higher activity than the parent tmCDH enzyme that already had improved activity compared to wild type CDH whose activity could not be detected on the surface of yeast cells.

Many articles in the literature deal with horseradish peroxidase (HRP) biomineralization, but none pay attention to the isoenzyme composition of commercial HRP or the influence of the carbohydrate component of the protein molecule on the biomineralization process. To study the impact of these factors, we performed periodate oxidation of commercial HRP and a purified HRP-C isoform for biomineralization within ZIF-8. With purified HRP, enzyme@ZIF-8 biocomposites with higher activity were obtained, while periodate oxidation of the carbohydrate component of both commercial HRP and purified HRP-C yields biocomposites with very high activity in acetate buffer that does not degrade the ZIF-8 structure. Using acetate instead of phosphate buffer can prevent the false high activity of HRP@ZIF-8 biocomposites caused by the degradation of ZIF-8 coating. At the same time, purification and especially oxidation of the carbohydrate component of enzymes prior to biomineralization lead to significantly improved activity of the biocomposites.

Production of soluble cellobiose dehydrogenase (CDH) mutant proteins previously evolved on the surface of S. cerevisiae yeast cells was established for use in biosensors and biofuel cells. For this purpose, mutant cdh genes tm (D20N, A64T, V592M), H5 (D20N, V22A, A64T, V592M) and H9 (D20N, A64T, T84A, A261P, V592M, E674G, N715S) were cloned to pPICZ? plasmid and transformed into Pichia pastoris KM71H strain for high expression in a soluble form and kinetic characterization. After 6 days of expression under methanol induction, the CDHs were purified by ultrafiltration, ion- -exchange chromatography and gel filtration. Sodium dodecyl sulfate electrophoresis confirmed the purity and presence of a single protein band at a molecular weight of 100 kDa. Kinetic characterization showed that the H5 mutant had the highest catalytic constant of 43.5 s-1 for lactose, while the mutant H9 showed the highest specificity constant for lactose of 132 mM-1 s-1. All three mutant proteins did not change the pH optimum that was between 4.5 and 5.5. Compared to the previously obtained wild types and mutants of CDH from Phanerochaete chrysosporium, the variants reported in this article had higher activity and specificity that together with high protein expression rate in P. pastoris, makes them good candidates for use in biotechnology for lactobionic acid production and biosensor manufacture.