Structural Evidence for a [4Fe‐5S] Intermediate in the Non‐Redox Desulfuration of Thiouracil

We recently discovered a [Fe‐S]‐containing protein with in vivo thiouracil desulfidase activity, dubbed TudS. The crystal structure of TudS refined at 1.5 Å resolution is reported; it harbors a [4Fe‐4S] cluster bound by three cysteines only. Incubation of TudS crystals with 4‐thiouracil trapped the cluster with a hydrosulfide ligand bound to the fourth non‐protein‐bonded iron, as established by the sulfur anomalous signal. This indicates that a [4Fe‐5S] state of the cluster is a catalytic intermediate in the desulfuration reaction. Structural data and site‐directed mutagenesis indicate that a water molecule is located next to the hydrosulfide ligand and to two catalytically important residues, Ser101 and Glu45. This information, together with modeling studies allow us to propose a mechanism for the unprecedented non‐redox enzymatic desulfuration of thiouracil, in which a [4Fe‐4S] cluster binds and activates the sulfur atom of the substrate. [4Fe‐5S] clusters are emerging as possible key catalytic intermediates in various sulfuration and desulfuration reactions. A structurally characterized [4Fe‐5S] intermediate, resulting from the transfer of a sulfur atom from thiouracil to the [4Fe‐4S] cluster of an enzyme, suggests that [4Fe‐4S] clusters can function as sulfur transfer agents.