Examples of tyrosine hot spots. A detail of the interaction between colicin E9 immunity protein (in red) and colicin E9 DNase (in grey, surface representation) is shown (pdb code 1BXI). The side chain of the hot spots Tyr54 (ΔΔG = 4.83 kcal/mol) and Tyr55 (ΔΔG = 4.63 kcal/mol) are highlighted by a stick representation and are coloured in orange and magenta respectively. They fit into a groove of the E9 DNase surface. The residue Phe86 of E9 DNase is also displayed by sticks (dark grey with transparent surface representation). The hydroxyl group of Tyr54 is hydrogen bonded with the main-chain oxygen of Phe86 (green line); the aromatic ring of Tyr55 forms a stacking interaction with the aromatic ring of Phe86.