Figure 3

Ligand binding to xenavidin. Simplified presentation of the ligand-binding sites: A, subunit A of unliganded xenavidin; B, subunit D of unliganded xenavidin; C, subunit A of the xenavidin-BTN complex; and D, subunit D of the xenavidin-BTN complex. Amino acid residues are numbered according to xenavidin and avidin (in brackets). Colouring scheme for the stick models: red, oxygen atoms; blue, nitrogen atoms; green, carbon atoms. Water molecules are shown as red spheres. ACT, acetate ion; BTN, D-biotin. Difference Fo-Fc electron density map (blue) contoured at 3σ around the bound ligands is depicted; the maps were calculated in the absence of ligands. An asterisk indicates the conserved water molecule found at the entrance of the water channel, at site 2 [33], and an arrow pinpoints the water molecule at site 1 [33] that was present only in the unliganded xenavidin structure (see text for details).