Structural and functional insights into Bcp4: component of the antioxidant system of Sulfolobus solfataricus

The detoxification from peroxides in Sulfolobus solfataricus is performed by the Bacterioferritin comigratory proteins (Bcps), Bcp1, Bcp2, Bcp3 and Bcp4, antioxidant enzymes belonging to one of the subfamilies of the Peroxiredoxins [1]. A detailed functional and structural analysis of Bcp4 has been performed. Bcp4, characterized by the CXXXXC motif in the active site, presents two conserved cysteine residues (Cys45 and Cys50) and unlike Bcp1, was recently shown to be dimeric [2]. Bcp4 represents the first dimeric Bcp so far investigated. In order to verify whether the quaternary structure could play a role also in the catalytic mechanism of Bcp4, the characterization of the catalytic activity of this enzyme and the crystal structure of its C45S/C50S double mutant are reported. Biochemical studies showed that the protein has a non-covalent dimeric structure and adopts an atypical 2-Cys catalytic mechanism. The X-ray structure of the double mutant C45S/C50S, representative of the fully reduced enzyme state, provided a detailed description of the protein dimeric arrangement and clear identification of residues responsible for dimerization. Finally, concurrent availability of the crystallographic structure of the monomeric Bcp1 allowed comparative analysis of the interaction with the Protein Disulfide Oxidoreductase SsPDO (Sso0192) [3], the first component of the reducing cascade, involved in the reduction of both Bcp1 and Bcp4, through a protein-protein docking approach.