Structure comparison of wt bradavidin, wt streptavidin [PDB: 2BC3] and chicken avidin [PDB: 1AVD].

<p>(<b>A</b>) Cartoon models of tetrameric proteins. Subunits are shown in different colors as follows: I (blue), II (cyan), III (magenta) and IV (yellow). The key biotin-binding pocket occupying residues (wt bradavidin and wt streptavidin) and biotin (chicken avidin) are shown as spheres. The N- and C-termini are indicated by letters. (<b>B</b>) Superimposition of the Cα traces of subunits I of wt bradavidin (blue), wt streptavidin (orange) and chicken avidin (cyan). The Cα trace for C-terminal residues starting from Lys127 of subunit III of wt bradavidin is shown, too. The ligand-binding site occupying residues Glu131-Leu133 of wt bradavidin and Gly151-Pro153 of wt streptavidin, and the biotin ligand of avidin, are shown as sticks. The left and right arrows pinpoint the N- and C-terminal sites, respectively, were major differences are seen between the proteins. Trp5 (left arrow) of wt bradavidin is shown as sticks. The N- and C-termini are indicated and loops are numbered. (<b>C</b>) Monomeric cartoon models. The N-terminus and C-terminus of each protein are indicated in red and blue, respectively, the colouring starting from equal positions in all proteins. The key residues occupying the biotin-binding site are shown as sticks. (<b>D</b>) Loop design. Colouring of subunits are as in (A) and representation of the key residues as in (C). The arrow pinpoints the varying beginnings of the L7,8-loops in the three structures. The L3,4-loop of the wt streptavidin structure is not fully visible (dashed line).