Biotin-binding sites of avidin, AVR2 and AVR4. Stereo images of the biotin-binding sites of (A) AVR2-b, (B) AVR4 [PDB:1Y52]  and (C) avidin [PDB:1AVD]  are shown. The cavities (transparent) around the bound biotin molecules of subunit A are shown. The water molecules within the cavities are shown as red spheres. Hydrogen bonds between the atoms of D-biotin and the surrounding proteins are shown with dashed lines. Carbon atoms of residues from subunit 1 are coloured white; those from a neighbouring subunit 2 are indicated in cyan: residues 108 and 109 (110 and 111 in avidin).