Comparison of the crystal structure of the α2 A-domain either (a) free or (b) complexed with a collagenous peptide [41, 47]. Secondary structure elements are shown as red α-helices or cyan β-strands/ribbons. Spheres represent the divalent cation coordinated by the metal ion-dependent adhesion site (MIDAS) motif. The α7 helix is shown in pink. Note the difference in position of α7 in the two structures and the fact that the construct used in (b) contained a truncated α7 helix. The collagen glutamate residue that coordinates the MIDAS cation is shown in green. The MIDAS cation is shown as a blue circle.