STRUCTURE NOTE Crystal structure of tflp: A ferredoxin-like metallo-b-lactamase superfamily protein from Thermoanaerobacter tengcongensis

Ferredoxins are a group of proteins containing nonheme iron and acid-labile sulfur. They are ubiquitous electron transfer proteins participating in a wide variety of redox reactions. Many organisms express multiple ferredoxins.1 Three types of [Fe-S] clusters have been described so far in ferredoxins: [2Fe-2S], [3Fe-4S], and [4Fe-4S]. The structures of these [Fe-S] clusters have substantial inherent stability in anaerobic solution, but oxygen can convert the exposed [Fe-S] clusters to unstable forms that decompose rapidly. Because of the high affinity of iron for thiolate residue, cysteine residue is the most common ligand of the Fe atom, but some other amino acid residues including histidine, aspartate, serine, and even backbone amide have also been found to coordinate the iron.2 Despite the limited [Fe-S] cluster types, ferredoxins present largely diverse structural architectures.3 A group of metallo-b-lactamase superfamily (MBLs) proteins were found to be ferredoxins bearing [2Fe-2S] cluster, but their crystal structures have not been reported.4 Here we report the crystal structure of a ferredoxin-like protein from Thermoanaerobacter tengcongensis (Tflp). Although Tflp is considered as a member of MBLs based on its threedimensional structural similarity, its crystal structure represents some novel structural features. A novel structural motif comprising four antiparallel b strands is found and a di-Fe center is located in the conserved active site. Tflp is one of the 15 proteins in T. tengcongensis considered to be unique to thermophiles.5 The gene encoding Tflp clusters with another six genes encoding ferredoxins. The ultraviolet/visible (UV/Vis) and electronic paramagnetic resonance (EPR) spectra reveal the existence of a [Fe-S] cluster in Tflp reconstituted with sulfur and iron under dithionite-reduced condition. These results characterize Tflp as a kind of ferredoxin.