Expanding the Scope of Polyoxometalates as Artificial Proteases towards Hydrolysis of Insoluble Proteins

Despite the enormous importance of insoluble proteins in biological processes, their structural investigation remains a challenging task. Development of artificial enzymatic catalysts would greatly facilitate elucidation of their structure as currently used enzymes in proteomics largely lose activity in the presence of surfactants, which are necessary to solubilize insoluble proteins. In this study the hydrolysis of a fully insoluble protein by a polyoxometalate complex as an artificial protease in surfactant solutions is reported for the first time. The hydrolysis of zein as a model protein was investigated in the presence of Zr(IV)-substituted Keggin-type polyoxometalate (POM), (Et2NH2)10[Zr(α-PW11O39)2], and different concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). The selective hydrolysis of the protein upon incubation with the catalyst was observed, and the results indicate that hydrolytic selectivity and activity of the POM catalysts strongly depends on the concentration of surfactant. The molecular interactions between the POM catalyst and zein in the presence of SDS were explored using a combination of spectroscopic techniques which indicated competitive binding between POM and SDS towards the protein. The formation of micellar superstructures in tertiary POM/surfactant/protein solutions has been confirmed by electrical conductivity and Dynamic Light Scattering.