Mining and Expression of a Metagenome-Derived Keratinase Responsible for Biosynthesis of Silver Nanoparticles

A keratinase gene kerBv was mined from soil metagenomes. The open reading frame consisted of 1149 bp and potentially encoded a protein of 382 amino acid residues. It shared the same active site with several reported typical keratinases via analysis of the amino acid sequence. The keratinase was successfully expressed in B. subtilis WB600 with pMA5 expression vector. The maximum activity of 164.8 U/mL in the fermentation supernatant was observed after incubating for 30 h in Terrifc Broth (TB) medium. The keratinase exhibited outstanding resistance to metal ions and was surfactant-stable. Additionally, the enzyme displayed broad substrate specificity especially toward insoluble substrate feather meal because of its disulfide bond-reducing activity. Furthermore, the reducing power of the recombinant keratinase was investigated. It showed that the protein exhibited a relatively high reducing power, which was subsequently used in the biosynthesis of silver nanoparticles (AgNPs). The biosynthesized AgNPs were characterized by ultraviolet-visible (UV-vis) spectroscopy, dynamic light scattering (DLS), transmission electron microscope (TEM), as well as Fourier transform infrared spectroscopy (FTIR) and displayed obvious antibacterial activities toward Escherichia coli.