Copper zinc superoxide dismutase 309 typical of

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VII. COPPER-ZINC SUPEROXIDE DISMUTASE 309 typical of square-planar Cu II complexes with four nitrogen donor ligands and the lower All observed in blue copper proteins (see Chapter 6). The large line- width seen in the g -l region indicates that the copper ion is in a rhombic (i.e., distorted) environment. Thus, the EPR spectrum is entirely consistent with the distorted square-pyramidal geometry observed in the x-ray structure. Removal of zinc from the native protein to give copper-only SOD results in a perturbed EPR spectrum, with a narrower g 1- resonance and a larger All value (142 G) more nearly typical of CUll in an axial N 4 environment (Figure 5.20B). Apparently the removal of zinc relaxes some constraints imposed on the geom- etry of the active-site ligands, allowing the copper to adopt to a geometry closer to its preferred tetragonal arrangement. The EPR spectrum due to Cu II in the native Zn II site in the Ag ICuSOD derivative indicates that Cu II is in a very different environment than when it is in the native copper site (Figure 5.20C). The spectrum is strongly rhombic, with a low value of All (97 G), supporting the conclusion based on the visible spec- trum that copper is bound in a tetrahedral or five-coordinate environment. This type of site is unusual either for copper coordination complexes or for copper proteins in general, but does resemble the CUll EPR signal seen when either laccase or cytochrome c oxidase is partially reduced (see Figure 5.21). Partial non-blue, Type 2, and copper-substituted proteins ,/ -0 1 I \ 0 \ "- "- 2.20 00 o 0 0 "- 0 0 "- 0 "- 0 "- 0 0 <9 0 \ 0 0 \ , 0 \ I dO I a O o / cO 0 fO e " b O C{) 0 "- 0 "- 0 0\ \ I I I 2.25 2.30 gil ~ o o o o o o I Type 1 /,/ 10 I I I I I I \ "-~ - 20 - 5- 15 - I E () M I o :::-10- «:- Figure 5.21 Relationship between gil and All for naturally occurring copper proteins and copper-substituted metalloproteins. Points labeled a-f are for laccases (a and b), cytochrome c oxidase (c), Cu- ZnSOD (d), Ag1CuSOD (e), and CUICuIISOD (f). See text for discussion. (From M. W. Panto- liana, L. A. Nafie, and J. S. Valentine, J. Am. Chern. Soc. 104 (1982),6310-6317.)
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310 5 / DIOXYGEN REACTIONS reduction disrupts the magnetic coupling between these Cu II centers that makes them EPR-silent in the fully oxidized protein. The EPR spectrum of CuCuSOD is very different from that of any of the other copper-containing derivatives (Figure 5.22) because the unpaired spins on I I I I I I I I 1,400 1,800 2,200 2,600 3,000 3,400 3,800 4,200 magnetic field (gauss) Figure 5.22 Frozen-solution EPR spectrum of CuCuSOD. Note the very different appearance of this spec- trum from those shown in Figure 5.20. These differences are due to the fact that the two Cu 1I centers are magnetically coupled across the imidazolate bridge. 100 the two copper centers interact and magnetically couple across the imidazolate bridge, resulting in a triplet EPR spectrum. This spectrum is virtually identical with that of model imidazolate-bridged binuclear copper complexes.
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