Blue copper proteins trigonal structure

Blue copper proteins in their oxidized form contain a Cu2+ ion in the active site. The copper atom has a rather unusual tetra-hedral/trigonal pyramidal coordination formed by two histidine residues, a cysteine and a methionine residue. One of the models of plastocyanin used in our computational studies (160) is pictured in Fig. 7. Among the four proteins, the active sites differ in the distance of the sulfur atoms from the Cu center and the distortion from an approximately trigonal pyramidal to a more tetrahedral structure in the order azurin, plastocyanin, and NiR. This unusual geometrical arrangement of the active site leads to it having a number of novel electronic properties (26). 

Why does a Cu(II) ion with the ligands in the blue copper proteins assume a trigonal structure, whereas most inorganic cupric conplexes are tetragonal (square planar, square pyramidal, or distorted octahedral) [63,64] We have faced this question by optimising the geometry of a number of models of the type... 

Blue copper proteins (also known as type 1 Cu proteins or cupredox-ins), involved in biological electron transport, share a common structural motif, in which a Cu " ion is trigonally coordinated by a pair of histidine side chains and a cysteine thiolate ligand (Fig. 14). A methionine S atom is found along the trigonal axis at a long distance, 2.6-3.1... 

The crystal structures of blue copper proteins show that the T1 site is in a trigonally distorted tetrahedral environment (Fig. la) [34—40]. The trigonal plane has a very short ( 2.1 A) Cu-S(Cys) and two typical ( 2.0 A) Cu-N His bonds. An additional axial thioether ligand is usually present with a long Cu-S(Met) bond ( 2.8 A), although the Met residue is replaced by a non-ligating one, such as Lys or Phe, in some tree and fungal laccases [41-44]. 

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... 

In the blue, Type I copper proteins plastocyanin and azurin, the active-site structure comprises the trigonal array [CuN2S] of two histidine ligands and one cysteine ligand about the copper,... 

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