Comparison of co-crystal structures of 8g (PDB-ID: 6FT8; upper row) and 16 (PDB-ID: 6FT9; lower row) in complex with CLK1, respectively; red spheres: water molecules; black dashed lines: hydrogen bonds.

<p>A: Top view of <b>8g.</b> B: Side view of <b>8g</b>. C: Top view of <b>16</b>. D: Side view of <b>16</b>. A, C: While the lactam motives of the ligands perform the canonical hydrogen bonds to the hinge region, the indole nitrogen atoms are connected to a conserved water molecule. B, D: An area near the Lys191 and Glu206 side chains, unoccupied by both <b>7g</b> and <b>16</b>, is filled by three water molecules. The opening to the entrance of the binding pocket is delimited by Asp250. Compared to the shape of the pocket with bound <b>8g</b> (B), the entrance of the binding pocket is widened to accommodate the 2-bromo substituent of <b>16</b> (D).