Poly-γ-glutamic acid coupled Pseudomonas putida cells surface-displaying metallothioneins: composited copper(ii) biosorption and inducible flocculation in aqueous solution

Abstract

Poly-γ-glutamic acid (γ-PGA) is a biodegradable macromolecular polymer that is capable of adsorbing heavy metal ions through its side chains. In this study, a new biocomposite biosorbent that incorporates Cu²⁺ adsorption and recovery from aqueous solutions was developed through immobilization of a glutamine-binding protein GlnBP and cyanobacterial metallothionein SmtA onto the surface of Pseudomonas putida cells using an InaQ-N protein anchor, followed by coupling the cells with side chain-activated γ-PGA. The surface localization of the fusion protein (InaQ)₂–GlnBP–(SmtA)₂ was confirmed by SDS-PAGE/Western blot, immunofluorescence microscopy and fluorescence-activated cell sorting assays. The engineered biocomposite MB546-PGA was capable of Cu²⁺ adsorption across a pH range of 3.5–5.5 and a relatively broad temperature range of 20–60 °C, with a maximum adsorption capacity for Cu²⁺ of 145.99 mg g⁻¹. The pseudo-second-order equation was applicable to the sorption data. Moreover, MB546-PGA exhibited an inducible flocculation capability following Cu²⁺ adsorption. The infrared spectroscopy data showed that the hydroxyl and amino groups of MB546-PGA were involved in Cu²⁺ adsorption, and γ-PGA likely bound to the cells through its carbonyl and amino groups. Given the features of the high capacity of Cu²⁺ adsorption and the easily inducible flocculation, the developed biosorbent shows potential for use in the remediation and purification of polluted or waste waters.