Polysulfide Sulfur Transferases

Figure 2 Composition of the electron transport chain catalyzing polysulHde sulfur respiration with H2 or formate in W. succinogenes. FdhA/B/C, formate dehydrogenase PsrA/B/C, polysulHde reductase HydA/B/C, hydrogenase Ni, nickel-iron center Fe/S, iron-sulfur centers Mo, molybdenum ion bound to MGD the dark squares designate heme b groups Sud, polysulfide sulfur transferase.

The polysulfide sulfur transferase (Sud) of W. succinogenes consists of two identical subunits (14.3 kDa) and does not contain prosthetic groups or heavy metal ions. The protein was originally isolated as a sulfide dehydrogenase [54]. Sud catalyzes the transfer of sulfur from polysulfide sulfur to cyanide [Reaction (15)] at a high... 

Sud contains a single cysteine residue which is essential for polysulfide sulfur transferase activity [Reaction (15)], for sulfur binding, and for the decrease of Am for polysulfide sulfur in polysulfide sulfiu reduction [53]. Sulfur appears to be covalently bound to the cysteine residue. When subjected to MALDI mass spectrometry, Sud incubated with polysulfide sulfur was found to carry one or two sulfur atoms per monomer. No sulfur was bound to the monomer after treatment of Sud with cyanide. A Sud variant carrying a serine instead of the cysteine residue did not carry sulfur even upon incubation with polysulfide sulfur. To explain the effect of Sud on the apparent Am for polysulfide sulfur in polysulfide sulfur reduction, it is assumed that sulfur is transferred from Sud to the active site of polysulfide reductase. From the concentration of Sud required for Am decrease and from its concentration in the bacterial periplasm it is likely that Sud is bound to polysulfide reductase during polysulfide sulfur transfer. The two enzymes occur in about equimolar amounts in cells of W. succinogenes grown with polysulfide sulfur. 

This chapter has focused only on biological dissolubility of elemental sulfur by sulfur transferases and on biological reduction of sulfur and polysulfide sulfur by terminale reductases. Other sulfur containing species (sulfite, sulfate, and others) which play an important role in the biogeochemical cycle of sulfur compounds have been reviewed previously [2,7,60-63]. 

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