The first step in
sulfur mobilization for the biosynthesis of Fe-S clusters under oxidative stress and
iron starvation in Escherichia coli 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 E. coli 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.