Crystal structure of a NifS-like protein from Thermotoga maritima: Implications for iron sulphur cluster assembly

NifS-like proteins are ubiquitous, homodimeric, proteins which belong to th e alpha-family of pyridoxal-5'-phoshate dependent enzymes. They are propose d to donate elementary sulphur, generated from cysteine, via a cysteinepers ulphide intermediate during iron sulphur cluster biosynthesis, an important albeit not well understood process. Here, we report On the crystal structu re of a NifS-like protein from the hyperthermophilic bacterium Thermotoga m aritima (tmNifS) at 2.0 Angstrom resolution. The tmNifS is structured into two domains, the larger bearing the pyridoxal-5'-phosphate-binding active s ite, the smaller hosting the active site cysteine in the middle of a highly flexible loop, 12 amino acid residues in length. Once charged with sulphur the loop could possibly deliver S-0 directly to regions far remote from th e protein. Based on the three-dimensional structures of the native as well as the substrate complexed form and on spectrophotometric results, a mechan ism of sulphur activation is proposed. The His99, which stacks on top of th e pyridoxal-5'-phosphate co-factor, is assigned a crucial role during the c atalytic cycle by acting as an acid-base catalyst and is believed to have a pK(a) value depending on the co-factor redox state. (C) 2000 Academic Pres s.