Structure and function of Escherichiαcoli NifS homologs with selenocysteine lyase activity

Abstract

We have purified and characterized three NifS homologs from Escherichia coli, CSD, CsdB, and IscS, which appear to be involved in iron-sulfur cluster formation and/or biosynthesis of selenophosphate. All of them catalyze eliminations of Se and S from L-selenocysteine and L-cysteine, respectively, to form L-alanine. We substituted Ala for each of Cys358 of CSD, Cys364 of CsdB, and Cys328 of IscS, which correspond to catalytically essential Cys325 of Azotobacter vinelandii NifS. The enzyme activity toward L-cysteine was almost completely abolished by the mutations, whereas the activity toward L-selenocysteine was much less affected. This indicates that the reaction mechanism of L-cysteine desulfurization is different from that of the decomposition of L-selenocysteine and that the conserved cysteine residues play a critical role only for L-cysteine desulfurization.