The single cysteine residue of the Sud protein is required for its function as a polysulfide‐sulfur transferase in Wolinella succinogenes

The periplasmic Sud protein which is induced in Wolinella succinogenes growing by polysulfide respiration, has been previously proposed to serve as a polysulfide binding protein and to transfer polysulfide‐sulfur to the active site of polysulfide reductase [Klimmek, O, Kreis, V., Klein, C., Simon, J., Wittershagen, A. & Kröger, A. (1998) Eur. J. Biochem. 253 , 263–269.]. The results presented in this communication suggest that polysulfide‐sulfur is covalently bound to the single cysteine residue (Cys109) of the Sud monomer, and that Cys109 is required for tight binding of polysulfide‐sulfur and for sulfur transfer. A modified Sud protein [(C109S)Sud‐His 6 ] in which the cysteine residue was replaced by serine, did not catalyze sulfur transfer from polysulfide to cyanide and did not stimulate electron transport to polysulfide, in contrast to Sud‐His 6 . The polysulfide‐sulfur bound to (C109S)Sud‐His 6 was fully removed upon dialysis against sulfide. After this treatment, Sud‐His 6 retained one sulfur atom per monomer; thiocyanate was formed upon addition of cyanide to the preparation. After incubation of Sud‐His 6 with polysulfide, a proportion of the Sud‐His 6 monomers carried one or two sulfur atoms, as shown by matrix‐assisted laser desorption ionization mass spectrometry. The sulfur atoms were absent from monomers derived from Sud‐His 6 treated with cyanide and from (C109S)Sud‐His 6 incubated with polysulfide.