Tryptophan 2,3-dioxygenase is a heme-containing cytosolic enzyme encoded by gene TDO2. Crystallographic studies of xcTDO (Xanthomonas campestris TDO) and rmTDO (Ralstonia metallidurans TDO) have revealed that the crystal structures of xcTDO and rmTDO are essentially identical and are intimately associated homotetrameric enzymes. They are best described as a dimer of dimers because the N terminal residues of each monomer form part of the substrate binding site in an adjacent monomer. The proteins are completely helical, and a flexible loop, involved in L-tryptophan binding, is observed just outside the active-site pocket. Interestingly, this loop appears to be substrate-binding induced, as it is observed only in crystals grown in the presence of L-tryptophan.
The only structure available with substrate bound at the active site in the catalytically active ferrous state is xcTDO. In the structure of xcTDO, the carboxy group of L-tryptophan interacts with Arg117, Tyr113 and Thr254. Amino acid residues equivalent to Arg117 and Tyr113 are found in nearly all TDO and IDO proteins. This carboxy-binding motif appears to be essential for substrate binding; arginine reorients in the presence of substrate, co-ordinating the carboxy group of L-tryptophan. The substrate ammonium group is hydrogen-bonded to the side-chain hydroxyl group of Thr254, the 7-propionate group of the heme, and a water molecule.
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