Abstract
Four bacterial EPSs extracted from Rhizobium leguminosarum bv. trifolii Rt24.2, Sinorhizobium meliloti Rm1021, Bradyrhizobium japonicum USDA110, and Bradyrhizobium elkanii USDA76 were determined towards their metal ion adsorption properties and possible modification of Cerrena unicolor laccase properties. The highest magnesium and iron ion-sorption capacity (~ 42 and ~ 14.5%, respectively) was observed for EPS isolated from B. japonicum USDA110. An evident influence of EPSs on the stability of laccase compared to the control values (without EPSs) was shown after 30-day incubation at 25 °C. The residual activity of laccases was obtained in the presence of Rh76EPS and Rh1021EPS, i.e., 49.5 and 41.5% of the initial catalytic activity, respectively. This result was confirmed by native PAGE electrophoresis. The EPS effect on laccase stability at different pH (from 3.8 to 7.0) was also estimated. The most significant changes at the optimum pH value (pH 5.8) was observed in samples of laccase stabilized by Rh76EPS and Rh1021EPS. Cyclic voltamperometry was used for analysis of electrochemical parameters of laccase stabilized by bacterial EPS and immobilized on single-walled carbon nanotubes (SWCNTs) with aryl residues. Laccases with Rh76EPS and Rh1021EPS had an evident shift of the value of the redox potential compared to the control without EPS addition. In conclusion, the results obtained in this work present a new potential use of bacterial EPSs as a metal-binding component and a modulator of laccase properties especially stability of enzyme activity, which can be a very effective tool in biotechnology and industrial applications.
Highlights
Rhizobiaceae are a very diverse family of symbiotic Gramnegative bacteria occurring in the soil as free-living microorganisms or existing in special structures inside the roots
It has been shown that certain bacterial EPSs have antioxidant, antitumor, and immunomodulatory properties; they can be used as pharmaceuticals [8, 9]
Rhizobial exopolysaccharides were used for the first time for the modification of C. unicolor laccase properties, especially storage stability
Summary
Rhizobiaceae are a very diverse family of symbiotic Gramnegative bacteria occurring in the soil as free-living microorganisms or existing in special structures inside the rootsDespite the high diversity of bacterial EPSs and their physicochemical properties, only a few of them have been1 3 Vol.:(0123456789)Bioprocess and Biosystems Engineering (2018) 41:973–989 really used till in industry, e.g., xanthan from Xanthomonas campestris, gellan gum from Pseudomonas elodea, and alginate from Pseudomonas aeruginosa used as a thickener and stabilizer in food industry [5]. Bacterial EPSs are used as drug carriers, bioflocculants, biosorbents, and heavy metal-eliminating factors [6]. Their capacity to bind heavy metals has been successfully applied in the bioremediation process, in the metal and mining industry [7]. Suitable metal ion content in the reaction medium may significantly modulate the activity of biocatalysts. It can eliminate the diminishing effect of some metal ions and increase their activating effect. This is one of the reasons why the presence of exopolysaccharides can modulate the native properties of enzymes and yield high stability of proteins under extreme stress conditions. EPSs of this type were used, e.g., to preserve lipase from deactivation by chemicals produced during the enzymecatalyzed reaction [10]
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