Expression, purification and spectroscopic analysis of an HdrC: An iron-sulfur cluster-containing protein from Acidithiobacillus ferrooxidans

Iron-sulfur cluster-containing proteins are present in all living organisms and are considered to be very ancient due to their ubiquity on the three domains of life, their importance in anaerobic metabolic pathways and energy conservation mechanisms. To date, there is little information regarding these proteins in Acidithiobacillus ferrooxidans, a bacterium involved in bioleaching processes. In this study are described the cloning, expression, purification and spectroscopic characterization of a new Fe-S cluster-containing protein, a putative HdrC homologue, from A. ferrooxidans strain LR. The oligomeric state of the protein was assessed by both dynamic light scattering and size-exclusion chromatography, demonstrating that it is present as a monomer in solution. Far-UV CD measurements revealed that this protein is extremely stable at acidic pHs, in which it assumes a different structure that exhibits a predominance of α-helixes. In contrast, at basic pHs, from 9 to 12, the protein showed a spectral profile which is characteristic for proteins with high content of beta structure and random coil. Thermal unfolding studies demonstrated that this protein tolerates high temperatures at acidic conditions, with a melting temperature of 95 °C. Furthermore, bioinformatics analysis suggested a heterodisulfide reductase function for the analyzed protein and for the other identified subunits, HdrA and HdrB, which could be related to hydrogen and/or formate metabolism in A. ferrooxidans.