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Figure 2.
Fig. 2. he four observed crystal forms of ligand-free (top) and biotin-
bound (bottom) streptavidin are depicted schematically to illustrate the
behavior f [he loop (residues 45 S2) relative lo the biotin
binding site. The circles represent the streptavidin tetramer subunits with
the binding sites (missing ectors). I and 2 . and 3 and 4. respec-
tively, build the dimer pairs. Subunit 1 and 4. and 2 and 3, respectively,
donate rp 120 to each others binding site. The curved lines over the
binding sites trace the loop with dotted lines representing
disordered conformations. Triangles in the binding sites symbolize biotin.
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