HdrC2 from Acidithiobacillus ferrooxidans owns two iron-sulfur binding motifs but binds only one variable cluster between [4Fe-4S] and [3Fe-4S].

The heterodisulfide reductase complex HdrABC from Acidithiobacillus ferrooxidans was suggested to own novel features that act in reverse to convert the sulfane sulfur of GS( n )H species (n > 1) into sulfite in sulfur oxidation. The HdrC subunit is potentially encoded by two different highly upregulated genes sharing only 29 % identity in A. ferrooxidans grown in sulfur-containing medium, which were named as HdrC1 and HdrC2, respectively and had been confirmed to contain iron-sulfur cluster by expression and characterization, especially the HdrC1 which had been showed to bind only one [4Fe-4S] cluster by mutations. However, the mutations of the HdrC2 remain to be done and the detailed binding information of it is still unclear. Here, we report the expression, mutations, and molecular modeling of the HdrC2 from A. ferrooxidans. This HdrC2 had two identical motifs (Cx(2)Cx(2)Cx(3)C) containing total of eight cysteine residues potentially for iron-sulfur cluster binding. This purified HdrC2 was exhibited to contain one variable cluster converted between [4Fe-4S] and [3Fe-4S] according to different conditions by the UV-scanning and EPR spectra. The site-directed mutagenesis results of these eight residues further confirmed that the HdrC2 in reduction with Fe(2+) condition loaded only one [4Fe-4S](+) with spin S = 1/2 ligated by the residues of Cys73, Cys109, Cys112, and Cys115; the HdrC2 in natural aeration condition lost the Fe atom ligated by the residue of Cys73 and loaded only one [3Fe-4S](0) with spin S = 0; the HdrC2 in oxidation condition loaded only one [3Fe-4S](+) with spin S = 1/2. Molecular modeling results were also in line with the experiment results.