SOLID NANOSTRUCTURED HYDROPHOBIC SURFACES ACT AS A CHAPERONE FOR RUBREDOXIN REFOLDING

Rubredoxins (Rds) are small (~54-residue) electron transfer proteins, with a tetrahedral Fe(SCys)4 site surrounded by a pair of iron-ligating CXXC loops. Rds are ideal for folding studies as they are unable to incorporate metals when in their (folded) apoprotein form, but undergo quantitative refolding when metals are added to the chaotrope-unfolded apoprotein. Specific spectroscopic signals allow to discriminate among individual steps in the uptake/refolding process. In this work we investigate the folding of Rd when adding iron to a folded, but iron-uptake incompetent Rd apoprotein in the presence catalytic amounts of polystyrene nanoparticles (NP) as “denaturing” agents. Our rationale was that hydrophobic contacts with the NP's surface would expose protein regions (including ligand cysteines) making them capable of capturing added metals, triggering refolding to the native protein structure and detachment from the NP of the folded Rd. Rate and yield of Rd refolding increased significantly in the presence of NP, were influenced by the NP concentration, were independent of ionic strength, and depended on the oxidation state of the added iron, as observed for chaotrope-unfolded Rd. Analysis of the optimal NP/apoprotein ratio and of spectroscopic data indicate that specific binding of an iron-bound unfolded intermediate is critical for NP-catalyzed refolding.