Mutagenic analysis of Thr‐232 in rhodanese from Azotobacter vinelandii highlighted the differences of this prokaryotic enzyme from the known sulfurtransferases

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Abstract
Azotobacter vinelandii RhdA uses thiosulfate as the only sulfur donor in vitro, and this apparent selectivity seems to be a unique property among the characterized sulfurtransferases. To investigate the basis of substrate recognition in RhdA, we replaced Thr‐232 with either Ala or Lys. Thr‐232 was the target of this study since the corresponding Lys‐249 in bovine rhodanese has been identified as necessary for catalytic sulfur transfer, and replacement of Lys‐249 with Ala fully inactivates bovine rhodanese. Both T232K and T232A mutants of RhdA showed significant increase in thiosulfate‐cyanide sulfurtransferase activity, and no detectable activity in the presence of 3‐mercaptopyruvate as the sulfur donor substrate. Fluorescence measurements showed that wild‐type and mutant RhdAs were overexpressed in the persulfurated form, thus conferring to this enzyme the potential of a persulfide sulfur donor compound. RhdA contains a unique sequence stretch around the catalytic cysteine, and the data here presented suggest a possible divergent physiological function of A. vinelandii sulfurtransferase.