A Tour of Blue Copper Proteins

Please be patient while the structures in the left frame load. In order to display all of the structures in the tour properly, press 'View' buttons below in order (from 1 to the end).

Blue copper proteins are relatively small copper-containing proteins. The two most studied are Azurin and Plastocyanin, which are extracted from bacteria and Poplar leaves, respectively. The protein shown here is Plastocyanin. These proteins are blue because of a sulfur-to-Cu2+charge- transfer transition (LMCT). All atoms, including hydrogen atoms, are shown in this display. In order to omit the hydrogens, press the right mouse button, click on "Style" and then "Atoms" and uncheck "Show Hydrogens"

The protein structure of Plastocyanin is of the beta-barrel type. These proteins carry out reversible electron-transfer

Cu2+ + e- arrows (1K) Cu+

When Cu2+ is reduced to Cu+ the electronic configuration changes from d9 to d10, and the colour of the complex changes from blue to colourless.

The copper ion (green sphere) is near the surface of the protein because it must be accessible for electron transfer. This type of reaction would be too slow if the Cu2+ was buried deep inside the protein.

The coordination environment of the Cu is comprised of two nitrogens from His, a thiolate sulfur from one Cys and a thioether sulfur from Met.

The active site of the oxidized form (Cu2+) of Plastocyanin is shown here. The two nitrogens and the thiolate sulfur define an approximately trigonal planar coordination about the copper while the thioether sulfur lies above this triangle with a long Cu-S distance of 2.82 Å. This distance is very long, suggesting a very weak Cu-S interaction. The Cu-S(Cys) bond length is 2.07 Å which is short and therefore this bond is strong.

This is the structure of the active site of the reduced form (Cu+) of the protein. There is very little change upon reduction. The Cu-S(Met) distance changes from 2.82 to 2.87 Å while the Cu-S(Cys) distance goes from 2.07 to 2.17 Å. Clearly the Cu is held in an entatic state by the protein in order to minimize structural and electronic change during the reaction.

In this display, distances between the donor atoms of the ligands in the oxidized site are displayed. The corresponding distances are given for the reduced and apo-protein sites in the next displays. Then a table will be displayed in order to compare these distances.

This shows the distances between donor atoms in the reduced protein, which are all slightly longer than those of the oxidized form.

This shows the distances between the potential donor atoms in the protein without copper (the apo-protein). In this crystal structure one histidine sidechain has moved slightly from the position found in the copper complexes.

This table shows that in the oxidized form all the distances are shorter, in comparison with the reduced form. This is because Cu2+ is smaller and forms stronger electrostatic bonds than Cu+. Therefore Cu2+ will pull the ligands inward. In the apo form there are three distances which are very large. This is due to His 87 which re-orientates itself when the copper ion is not bonded.

This representation of the apo-protein of plastocyanin shows that there is very little structural change of the protein when copper is not present. This clearly shows the copper binding site assembles spontaneously when the protein folds and that the entatic state is enforced by the protein.

This is the oxidized form of plastocyanin. Feel free to play around with it and all the other molecules displayed in the previous pages.

The crystal structure of the copper blue protein was reported by P.M.Colman et al. in Nature 1978, 272, 319.

Copyright Robert H. Morris, Adrian Lee and Alen Hadzovic, 1998, 2006, 2011.