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 the α-family of pyridoxal-5′-phoshate dependent enzymes. They are proposed to donate elementary sulphur, generated from cysteine, via a cysteinepersulphide intermediate during iron sulphur cluster biosynthesis, an important albeit not well understood process. Here, we report on the crystal structure of a NifS-like protein from the hyperthermophilic bacterium Thermotoga maritima (tmNifS) at 2.0 Å resolution. The tmNifS is structured into two domains, the larger bearing the pyridoxal-5′-phosphate-binding active site, 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 the protein. Based on the three-dimensional structures of the native as well as the substrate complexed form and on spectrophotometric results, a mechanism of sulphur activation is proposed. The His99, which stacks on top of the pyridoxal-5′-phosphate co-factor, is assigned a crucial role during the catalytic cycle by acting as an acid-base catalyst and is believed to have a p K a value depending on the co-factor redox state.