Copper proteins Rhombic type

Copper sites in proteins have traditionally been classified into three classes blue type 1 sites (present in the blue copper proteins), normal type 2 sites (tetragonal mononuclear copper sites), and type 3 (spin-coupled pairs of copper ions). The type 1 sites have been further classified as axial or rhombic depending on their EPR (and other spectroscopic) characteristics." Plastocyanin (axial) and nitrite reductase (rhombic) are typical examples of type 1 proteins. Proteins with properties intermediate between those of type 1 and type 2 sites have been termed type 1.5." ... 

On the basis of the electronic, resonance Raman, and EPR spectra, the cysteine-containing copper proteins have been divided into four groups axial type 1 (e.g. plastocyanin), rhombic type 1 (e.g. nitrite reductase and stellacyanin), type 1.5, and type 2 (mutant) copper proteins [81]. We have studied the spectra of members of each group with the CASPT2 method [33,34,36,38]. 

Even if the calculations were performed on a simple model, the results presented in Figure 8 nicely reflect the structure-electronic spectroscopy relationship between the various types of copper-cysteinate proteins. The copper coordination geometry of axial type 1 proteins is close to trigonal, and their spectroscopic characteristics are reflected by the results obtained for (p > 80°. Rhombic type 1 proteins like pseudoazurin and cucumber basic protein, on the other hand, have (p angles between 70° and 80°. As can be seen from Figure 8, even at such a small... 

The EPR spectrum due to Cu in the native Zn site in the Ag CuSOD derivative indicates that Cu 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 Ay (97 G), supporting the conclusion based on the visible spectrum 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 Cu EPR signal seen when either laccase or cytochrome c oxidase is partially reduced (see Figure 5.21). Partial... 

Calculations of EPR parameters were also performed on some of the complexes. Experimental EPR spectra are either axial (gx = gy-, axial type 1 copper proteins) or rhombic (other blue copper proteins). The results indicate that the geometry is more important than the electronic structure for the rhom-bicity of the spectrum the optimized trigonal structure of Cu(imidazole)2(SCH3)(S(CH3)2) and the crystal structure of plastocyanin both give an axial spectrum, while both the crystal structure of nitrite reductase and the other optimized model of Cu(imidazole)2(SCH3)(S(CH3)2)" give a rhombic spectrum, although the latter structure is mainly n bonded with... 

The only exception from this scheme is stellacyanin, which has a (p angle (82-86°) well within the range of the axial type 1 proteins but with spectroscopic characteristics typical for a rhombic type 1 protein. The reason is probably that this protein has another axial ligand, a glutamine residue that makes a much shorter bond to copper than the normal methionine ligand. Recent theoretical calculations show that the structure is mainly trigonal, but the short bond to the axial ligand (around 224 pm) leads to a Cu-Scys bond that contains a mixture of n and a interactions. 

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