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Copper protein

Copper proteins are proteins that contain one or more copper ions as prosthetic groups. The metal centres in the copper proteins can be classified into several types:[1]

  • Type I copper centres (T1Cu) are characterized by a single copper atom coordinated by two histidine residues and a cysteine residue in a trigonal planar structure, and a variable axial ligand. In class I T1Cu proteins (e.g. amicyanin, plastocyanin and pseudoazurin) the axial ligand is the sulfur of methionine, whereas aminoacids other than methionine (e.g. glutamine) give rise to class II T1Cu copper proteins. Azurins contain the third type of T1Cu centres: besides a methionine in one axial position, they contain a second axial ligand (a carbonyl group of a glycine residue). T1Cu-containing proteins are usually called "cupredoxins", and show similar three-dimensional structures, relatively high reduction potentials (> 250 mV), and strong absorption near 600 nm (due to SCu charge transfer), which usually gives rise to a blue colour. Cupredoxins are therefore often called "blue copper proteins". This may be misleading, since some T1Cu centres also absorb around 460 nm and are therefore green. When studied by EPR spectroscopy, T1Cu centres show small hyperfine splittings in the parallel region of the spectrum (compared to common copper coordination compounds).
  • Type II copper centres (T2Cu) exhibit a square planar coordination by N or N/O ligands. They exhibit an axial EPR spectrum with copper hyperfine splitting in the parallel region similar to that observed in regular copper coordination compounds. Since no sulfur ligation is present, the optical spectra of these centres lack distinctive features. T2Cu centres occur in enzymes, where they assist in oxidations or oxygenations.[2]
  • Type III copper centres (T3Cu) consist of a pair of copper centres, each coordinated by three histidine residues. These proteins exhibit no EPR signal due to strong antiferromagnetic coupling (i.e. spin pairing) between the two S = 1/2 metal ions due to their covalent overlap with a bridging ligand. These centres are present in some oxidases and oxygen-transporting proteins (e.g. hemocyanin and tyrosinase).[3]
  • Binuclear Copper A centres (CuA) are found in cytochrome c oxidase and nitrous-oxide reductase (EC 1.7.99.6). The two copper atoms are coordinated by two histidines, one methionine, a protein backbone carbonyl oxygen, and two bridging cysteine residues.[4]
  • Copper B centres (CuB) are found in cytochrome c oxidase. The copper atom is coordinated by three histidines in trigonal pyramidal geometry.
  • Tetranuclear Copper Z centre (CuZ) is found in nitrous-oxide reductase. The four copper atoms are coordinated by seven histidine residues and bridged by a sulfur atom.

Blue Copper Protein

Figure 1: Blue copper protein Type-I
Figure 2: Blue copper protein Type-II

The blue copper protein owes their name to the intense blue coloration of corresponding Cu(II) ions. The blue copper protein often called as “moonlighting protein” which means a protein can perform more than one function. Copper is an essential metal in living organisms. The most important role of copper protein in living systems is as an outer sphere electron transfer agent. In this role, both Cu(I) and Cu(II) play major role. The Cu2+ in the oxidized copper protein accepts one electron to form Cu1+ in the reduced protein. The Cu2+ in the blue copper protein has major impact on protein geometry. The Geometric distortion has an effect on the d-orbital splitting pattern and energies. The geometry distortion happens because the ligand positions the donors in a distorted geometry. The blue copper protein geometry has no Jahn-Teller(JT) distortion of the Cu2+ d9 ion. Since these large structural changes associated with redox states are due to a JT distorting force present in typical which is ordinary Cu complexes with monodentate ligands oxidized Cu(d9) centers, leading to a tetragonal (i.e., square planar) geometry. The JT distortion occurs in the ordinary Td and C3v geometries for the Cu2+ d9 ion but not in the blue copper

protein Cs geometry. The JT distortion is prominent in the Td and C3v geometries because they both have degenerated d-orbital levels with an odd number of electrons that can be split by JT distortion where the new lower electron level has more electrons than the new higher electron level. But the Cs geometry of blue copper proteins does not have a degenerated d-orbital levels, all five d-orbitals are singly degenerate, so a JT geometry distortion will not results in any stabilization and therefor does not occur in Cs geometry.

Blue copper protein types structure

The figure 1 shows the structure of blue copper protein Type-I. This structure is strongly distorted coordination shape. There are 2-histidines, 1 methionine and 1 cysteine present in this type-I structure. Example for Type-I blue copper protein are plastocyanine , azurin and nitrite tedactase. The figure 2 shows the structure of Type-II blue copper protein which is non-blue copper protein. This structure is essentially planar coordination shape. There are 3 histidine , 1 water or substrate molecule in this type-II blue copper protein. Examples for Type-II bluer copper protein are galactose oxidase, amine oxidase and doparmine monooxidase. The figure 3 is showing the structure of Type-III blue copper protein. In this structure oxygen bridged dimer with a Cu-Cu distance of ca 360 PM. Example for type-III blue copper protein are haemocyanin and tyrosinase .

Electronic Structure of D4h(CuCl4)-2

Figure 3: Blue copper protein Type-III

The Jahn –Teller (JT) distortion basically happens in copper ion compounds is at tetragonal elongation. The figure A explains geometric structure that the bigger JT effect bring it to square planar D4h(CuCl4)-2 structure. Based on the LF theory, the energy splitting of the d-orbitals is sensitive to the geometry of the metal center. The energy splitting was given in figure B. The 3d(x2-y2 ) orbital is at highest energy. Since the cupper ion has four chloride ligands in the equatorial plane and have largest repulsive interaction. The square planar splitting of the d-orbitals leads to a ground state of D4h(CuCl4)-2. The figure C complex has been analyzed to different types of spectroscopic methods. It was accurately quantitated the ground state having 39% of chloride with anti-sigma bonding characters mixed with 3d(x2-y2 ) orbitals due to the covalency. The figure D is showing the EPR spectrum is plotted as the first derivative of the microwave absorption to increase sensitivity. Powder or frozen solution of D4h(CuCl4)-2 was used to plot this figure. Copper had a nuclear spin which was coupled to the electron spin and produced a hyperfine splitting of the EPR signals.

Spectral changes with temperature:

Lowering the temperature may change the transitions.The intense absorbance at about 16000 cm-1 was characterized the absorptions feature of blue copper. There was a second lower energy feature band with moderate absorption intensity. Polarized signal-crystal absorption data on plasto-cyanin showed that both bands have the same polarization ratio that associated with Cu(II)-S(Cys) bond. This is explained that the normal cupric complex has high energy intense sigma and low energy weak π bonds. However, in the blue copper protein case have low energy intense sigma and high energy weak π bonds because CT intensity reflects overlap of the donor and acceptor orbitals in the CT process. This required that the 3d(x2-y2 ) orbital of the blue copper site be oriented such that its lobes bisect the Cu-S(Cys) bond giving dominant π overlap with sulfur directly. Finally, the nature of the ground state wave function of the blue copper protein is rich in electron absorption spectrum.


See also

References

  1. ^ Holm, Richard H.; Kennepohl, Pierre; Solomon, Edward I. (1996), "Structural and Functional Aspects of Metal Sites in Biology", Chemical Reviews, 96 (7): 2239–2314, doi:10.1021/cr9500390
  2. ^ Klinman, Judith P. (1996), "Mechanisms Whereby Mononuclear Copper Proteins Functionalize Organic Substrates", Chemical Reviews, 96 (7): 2541–2562, doi:10.1021/cr950047g.
  3. ^ Lewis, E. A. and Tolman, W. B., "Reactivity of Dioxygen-Copper Systems", Chemical Reviews 2004, 104, 1047-1076. doi:10.1021/cr020633r.
  4. ^ Solomon, Edward I.; Sundaram, Uma M.; Machonkin, Timothy E. (1996), "Multicopper Oxidases and Oxygenases", Chemical Reviews, 96 (7): 2563–2606, doi:10.1021/cr950046o