Protected Sulfur Transfer Reactions by the <i>Escherichia coli</i> Suf System

The first step in sulfur mobilization for the biosynthesis of Fe–S clusters under oxidative stress and iron starvation in <i>Escherichia coli</i> involves a cysteine desulfurase SufS. Its catalytic reactivity is dependent on the presence of a sulfur acceptor protein, SufE, which acts as the preferred substrate for this enzyme. Kinetic analysis of the cysteine:SufE sulfurtransferase reaction of the <i>E. coli</i> SufS that is partially protected from reducing agents, such as dithiothreitol and glutathione, was conducted. Under these conditions, the reaction displays a biphasic profile in which the first phase involves a fast sulfur transfer reaction from SufS to SufE. The accumulation of persulfurated/polysulfurated forms of SufE accounts for a second phase of the slow catalytic turnover rate. The presence of the SufBCD complex enhances the activity associated with the second phase, while modestly inhibiting the activity associated with the initial sulfur transfer from SufS to SufE. Thus, the rate of sulfur transfer from SufS to the final proposed SufBCD Fe–S cluster scaffold appears to be dependent on the availability of the final sulfur acceptor. The use of a stronger reducing agent [tris­(2-carboxyethyl)­phosphine hydrochloride] elicited the maximal activity of the SufS–SufE reaction and surpassed the stimulatory effect of SufBCD. This concerted sulfur trafficking path involving sequential transfer from SufS to SufE to SufBCD guarantees the protection of intermediates at a controlled flux to meet cellular demands encountered under conditions detrimental to thiol chemistry and Fe–S cluster metabolism