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Figure 3.
Fig. 3. Increased copper accessibility in zinc-deficient SOD.
(a) Access to the active-site copper (orange) of wild-type Cu,Zn
SOD (green) is limited by the electrostatic and zinc-binding
loops. The red dashes illustrate the dimensions of the
active-site channel, which narrows from  vert,
similar 12 Å to vert,
similar 4 Å. (b) The active site of zinc-deficient SOD
(blue) is more open and accessible due to the disorder of the
electrostatic and zinc-binding loops. Although the disordered
residues in these loops may transiently restrict the active-site
channel, their flexibility allows small molecules greater access
to the copper.
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