Fig. 4.
Crystal structures of divalent streptavidins. (a) Electron density (2Fo − Fc contoured at 1 rmsd) at residues 23, 27, and 45 in the 1,3 trans-divalent structure shown for chains b/c (SAe, green) and chains a/d (D, blue), with the side chain found indicated below. (b) Crystal structure of the 1,3 trans-divalent streptavidin, with green showing the biotin-binding SAe subunits and blue showing the non-biotin-binding D subunits. (c) Electron density in the cis-divalent structure, with chains b/d (SA, green) and chains a/c (Dd, blue). (d) Crystal structure of cis-divalent streptavidin, with green showing the biotin-binding SA subunits and blue showing the non-biotin-binding Dd subunits.