Biochemical characterization of iron-sulfur cluster assembly in the scaffold IscU of Escherichia coli.

Iron-sulfur cluster is one of the most common prosthetic groups, and it functions in numerous biological processes. However, little is currently known about the mechanisms of iron-sulfur cluster biosynthesis. In this study, we cloned and purified iron-sulfur cluster assembly proteins from Escherichia coli and assembled the cluster in vitro. The results showed that the assembly of iron-sulfur cluster is completed in about 20 min. Although iron or sulfur binds with IscU equivalently, 2-fold amount of iron or cysteine compared with that of IscU is better for the cluster formation, while high concentrations of IscS (IscS/IscU > 1 : 10) do not facilitate the cluster formation. Environmental pH plays an important role in iron-sulfur cluster assembly; the cluster was well assembled at pH 7.6-8.0, but was inhibited at pH less than 7.4. On supply of a catalytic amount of IscS (1/50 of IscU) and excess of other substrates, with increasing each of IscU, iron, or cysteine concentration, the iron-sulfur cluster assembly process developed from first order reaction, mixed order reaction to zero order reaction, and up to 64% of apo-IscU was converted to the [2Fe-2S] cluster-bound IscU under the optimal laboratory conditions.