Forms hydrogen bonds with both the side chain hydroxyl of Ser470 and the hydrogen NH of Gly429 in PARP2, while one of the hydrogens on the primary amide forms a hydrogen bond with the main chain oxygen of Gly429 in PARP2. In addition, the imidazole of ABT-888 stacks with the side chain of Tyr472 of PARP2. Recently, tankyrases have gained increased attention as potential drug targets. They were first discovered as factors that regulate telomere homeostasis by modifying the negative regulator of telomere length, TRF1. Tankyrases also mark axin, the concentration-limiting component of the b-catenin destruction complex, for degradation, and tankyrase inhibition antagonizes the Wnt signal transduction pathway by stabilizing axin and promoting b-catenin degradation. In this TNKS2 structure, XAV939 cyclic amide behaves as an isosteres for ABT-888��s primary amide. There is also a Zelboraf stacking interaction between the pyrimidinone of XAV939 and the Tyr1071 side chain of TNKS2. IWR compounds, however, do not share these features for anchoring in the nicotinamide pocket. It is not clear how these IWR compounds bind to tankyrases and thus the structure-activity relationship for these compounds has been difficult to interpret. Herein, we report a high-resolution crystal structure of the Human TNKS1 catalytic domain in complex with IWR2 and describe the structural basis for its potency and selectivity over PARP1 and PARP2. Our structure reveals a novel binding mode for a tankyrase inhibitor and provides a clear explanation for the reported structure-activity relationship of the IWRs, and important clues for the further optimization of these compounds. The crystals of the TNKS1/2 complex diffracted to 1.9 A ? with -Pugnac-chemical-structure.gif)
synchrotron radiation. There are two crystallographically independent TNKS1/2 complexes in the crystal structure, highly similar to each other. The TNKS1/2 complex structure reveals that 2 does not bind to the nicotinamide pocket but instead occupies a different pocket, which is not present in either apo or XAV939 bound tankyrase structures. It only becomes available upon the binding of 2 and we thus refer to it as the induced pocket. This induced pocket is created by the movement of Phe1188 of the a3 helix and the D-loop, part of which is disordered in the present crystal structure, away from one BI-D1870 another. The binding of 2 to the induced pocket of TNKS1 suggests that IWR compounds are likely non-competitive inhibitors of tankyrases. In the crystal structure, 2 adopts a conformation in which the central phenyl is almost perpendicular to the norbornyl group and rotated by about 60u away from the plane of the amide group. There are three hydrogen bonds between 2 and TNKS1. One of the two carbonyl oxygens of the pyrrolidine dione group is hydrogen bonded to the main chain NH of Tyr1213 and the carbonyl oxygen of the amide group is hydrogen bonded to the main chain NH of Asp1198. The CH at the 6-position of the quinoline is also involved in a CH��O=C hydrogen bonding interaction with the main chain carbonyl oxygen of Gly1196. Moreover, the quinoline group in 2 engages in hydrophobic interaction with the side chain of Phe1188 and stacking interaction with the side chain of His1201 of the D-loop. The quinoline group is co-planar to the amide group as a result of the intra-molecular hydrogen bond between the quinoline nitrogen and the amide NH. Structure-activity relationship studies carried out previously with some of the analogs of 2 in a cellular luciferase-based reporter assay can now be interpreted with the hydrogen bonding and hydrophobic interactions identified from the TNKS1/2 crystal structure.