Blue copper proteins
The blue copper proteins owe their name to their intense blue coloration. Specifically, λmax = 16,000 cm-1, molar extinction coefficient (ε) ∼5000 M-1cm-1. They serve as electron transfer agents, with the active site shuttling between Cu(I) and Cu(II). The Cu2+ in the oxidized state can accept one electron to form Cu1+ in the reduced protein. The geometry of the Cu center has a major impact on its redox properties. The absence of large reorganizational changes enhances the rate of their electron transfer. The active site of a type-I blue copper protein: two 2-histidines, 1 methionine and 1 cysteine present in the coordination sphere. and a single Type I copper ion coordinated by two histidine N-donors, a cysteine thiolate S-donor and a methionine thioether S-donor. In the oxidized state, the Cu+2 ion will form either a trigonal bipyramidal or tetrahedral coordination. The Type 1 copper proteins are identified as blue copper proteins due to the ligand to metal charge transfer an intense band at 600 nm that gives the characteristic of a deep blue colour present in the electron absorption spectrum. Example for Type-I blue copper protein are plastocyanine, azurin, and nitrite reductase, haemocyanin and tyrosinase.
The strong bond between the copper ion and the cysteinyl thiolate.
Structure and bonding of the type I copper centers
The Blue Copper Proteins, a class of Type 1 copper proteins, are small proteins containing a cupredoxin fold.
The protein structure of a Type 1 blue copper protein, amicyanin, is built from polypeptide folds that are commonly found in blue copper proteins β sandwich structure. The structure is very similar to plastocyanin and azurin as they also identify as Type 1 copper proteins.
Inner and outer sphere metal coordination
In azurin, the cysteine112 thiolate accepts the hydrogen bonds from the amide backbone of asparagine47, and phenylalanine114, and histidine46 donates a hydrogen bond to the carbonyl backbone of asparagine10. The Cysteine84 thiolate of plastocyanin accepts a hydrogen bond from a amide backbone, Asparagine38, and Histidine37 interacts strongly with the carbonyl backbone of Alanine33 and more weakly with the carbonyl backbone of Leucine5, Glycine34, and the amide backbone of Phenylalanine35.
Blue Copper Protein "Entatic State"
Cu2+ complexes often have relatively slow transfer rates. An example is the Cu2+/+ aquo complex, which is 5 × 10−7 M−1 s−1 compared to the blue copper protein which is between 1 ms and 0.1 μs. Upon electron transfer the Cu2+ state at the blue copper protein active site will be minimized because the Jahn-Teller effect is minimized. The distorted geometry prevents Jahn-Teller distortion. The orbital degeneracy is removed due to the asymmetric ligand field. The asymmetric ligand field is influenced by the strong equatorial cysteine ligand and the weak axial methionine ligand. In Figure 2, an energy level diagram shows three different relevant geometries and their d-orbital splitting and the Jahn-Teller effect is shown in blue. (i) shows the tetrahedral geometry energy level diagram with a that is degenerate. The tetrahedral structure can undergo Jahn-Teller distortion because of the degenerate orbitals. (ii) shows the C3v symmetric geometry energy level splitting diagram with an 2E ground state that is degenerate. The C3v geometry was formed by the elongated methionine thioether bond at the reduced site. The unpaired electrons leads to the Jahn-Teller effect. (iii) shows the ground state energy level splitting diagram of the Cs geometry with a longer thioester bond and a subsequently shorter thiolate bond. This is the proper geometry of the blue copper protein. This shows that there is no presence of the Jahn-Teller effect. The energy diagram shows that the asymmetry of the short Cu-S(Cys) bond and the highly distorted Cu-L bond angles causes the degeneracy of the orbitals to be removed and thereby removing the Jahn-Teller effect, which is due to the weak donor at an Cu-S(Met) and strong donor at Cu-S(Met).