Identification of a Rhodobacter capsulatus l-Cysteine Desulfurase That Sulfurates the Molybdenum Cofactor When Bound to XdhC and before Its Insertion into Xanthine Dehydrogenase†

The molybdenum cofactor (Moco) containing enzymes aldehyde oxidase and xanthine dehydrogenase (XDH) require for activity a sulfuration step that inserts a terminal sulfur ligand into Moco. XdhC was shown to be essential for the production of active XDH in Rhodobacter capsulatus but is itself not a subunit of the purified enzyme. XdhC binds stoichiometric amounts of Moco and is further able to transfer its bound Moco to XDH. Previous work suggested that XdhC particularly stabilizes the sulfurated form of Moco before the insertion into XDH. In this work, we identify an R. capsulatus l-cysteine desulfurase, NifS4, which is involved in the formation of the MoS ligand of Moco. We show that NifS4 interacts with XdhC and not with XDH. NifS4 mobilizes sulfur from l-cysteine by formation of a protein-bound persulfide intermediate and transfers this sulfur further to Moco. This reaction was shown to be more effective than the chemical sulfuration of Moco using sulfide as sulfur source. Further studies clearly showe...