TtuA and TudS, Two [4Fe‐4S]‐Dependent Enzymes Catalyzing Nonredox Sulfuration or Desulfuration Reactions

Abstract U54‐tRNA sulfurtransferase TtuA and thiouracil desulfidase (TudS) catalyze two opposite reactions that necessitate a [4Fe‐4S] cluster: nonredox sulfuration or desulfuration reactions. On the one hand, TtuA is an ATP‐dependent sulfurtransferase (thiouridine synthase) that transfers a sulfur atom, originating from free l ‐cysteine, to uridine at position 54 within the T‐loop of tRNAs in thermophilic organisms. In some cases, thiouridine biosynthesis involves a sulfur transfer between a thiocarboxylate formed at the C‐terminal glycine of a TtuB protein and the [4Fe‐4S] cluster of TtuA, within a TtuA/TtuB complex. On the other hand, TudS catalyzes sulfur abstraction from its 2‐thiouracil or 4‐thiouracil substrate. Interestingly, in both reactions, the [4Fe‐4S] cluster, bound to three cysteines only, is presumably used as a cofactor to bind and activate a sulfur atom, coming from a sulfur donor for TtuA or from the thiouracil substrate for TudS. Therefore, catalysis most probably occurs via the formation of a [4Fe‐4S] cluster‐bound sulfide intermediate, as illustrated by the [4Fe‐5S] cluster state that was trapped by soaking crystals of TudS with the 4‐thiouracil substrate.