Copper proteins Axial 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." ... 

The azurin structural gene has been cloned and expressed in large amounts in E. coli (Karlson et al., 1989). The copper site in azurin is distorted-planar with two additional weakly interacting groups in axial positions. Site directed mutagenesis has been used to exchange His-46 for Met, Cys-112 for His and Met-121 for all other amino acids, in order to study the relationship between structure and function and to determine the prerequisites for the blue copper site. The Met-121 mutant proteins were characterised by their absorption and ESR spectra (Karlson et al., 1991). At low pH, all mutants exhibit the characteristics of the blue (Type I) copper protein, indicating... 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

The overall picture describing the electron delocalization in Cua resembles that found in Type I sites most of the delocalized unpaired spin density is found on the Cys ligands. The electron spin density on each P-CH2 Cys proton is about half of that observed on the equivalent protons in blue copper proteins (Bertini etal., 1996, 1999). The unpaired electron is distributed over the two copper ions and the two Cys ligands. The observed values for the hyperfine shifts are consistent with the fact that the hyperfine couplings found in the P-CH2 Cys protons in Type I sites are twice as large as those observed in Cua centers. However, as already discussed, the Cu(H)-Cys covalency in Type I sites can be severely altered by the strength of the Cu(H)-axial ligand interaction. This... 

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... 

In nature there are only two major types of electron-carrier sites in addition to the blue copper proteins and the Cua site, viz. cytochromes and iron-sulphur clusters [161,162]. The cytochromes consist of an iron ion bound to a porphyrin ring. Two axial ligands coii5)lete the octahedral coordination sphere. During electron transfer, iron alternates between Fe(II) and Fe(III). Several types of cytochromes exist in biological systems, depending on the substituents on the por-... 

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