Mechanistic studies of the SufS-SufE cysteine desulfurase: evidence for sulfur transfer from SufS to SufE

SufS is a cysteine desulfurase of the suf operon shown to be involved in iron-sulfur cluster biosynthesis under iron limitation and oxidative stress conditions. The enzyme catalyzes the conversion of L-cysteine to L-alanine and sulfide through the intermediate formation of a protein-bound cysteine persulfide in the active site. SufE, another component of the suf operon, has been previously shown to bind tightly to SufS and to drastically stimulate its cysteine desulfurase activity. Working with Escherichia coli proteins, we here demonstrate that a conserved cysteine residue in SufE at position 51 is essential for the SufS/SufE cysteine desulfurase activity. Mass spectrometry has been used to demonstrate (i). the ability of SufE to bind sulfur atoms on its cysteine 51 and (ii). the direct transfer of the sulfur atom from the cysteine persulfide of SufS to SufE. A reaction mechanism is proposed for this novel two-component cysteine desulfurase.