Figure 2.

Stabilizing contacts with the Trp2 double conformation inside the hydrophobic acceptor pocket. Two hydrogen bonds are formed between the N atom of the indole in its double conformation and the carbonyl O atoms of Asp90 and Lys25. The stacking arrangement of electron-rich residues involving Glu89, Trp2 and Met92 (shown in the two insets at a 90° rotation from each other) further contributes to the overall stabilization of the system.