Comparative modelling of BBPs. A) Three-dimensional structure of the avidin subunit with bound ligand determined by X-ray crystallography (Livnah et al., 1993). B) The modelled structure of the BBP-A subunit and C) BBP-B subunit. Secondary structures are shown as cartoons: α-helix is red and β-sheets are grey. Cysteine bridges are shown as yellow sticks D) View of helix-β-barrel contact in RBP structure determined by X-ray crystallography (Cowan et al., 1990). The side-chains forming the contacts are shown as sticks. In E) is an enlargement of the modelled BBP-B helix-β-barrel contact. The conservation of the inner part and changes in the outer part of the modelled barrels of BBP-A (F) and BBP-B (G) are shown when compared to avidin. Conserved side-chains are shown in green and non-conserved are shown in red. The loop regions are omitted from the figure as well as the C-terminal predicted α-helix of BBP-B. H) Avidin tetramer with bound biotins, subunits are numbered according to Livnah et al., 1993. I) Tetrameric model of BBP-B. Proposed α-helices are oriented outwards from the possible tetramer.