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Plastocyanin.

This page shows some interactive JSmol views of the type1 ("blue") copper proteins plastocyanin and azurin.

In the inital view, [Cu(II) Pc (pH 6) 1], the protein is shown in the conventional orientation and in cartoon form. It is shaped like a barrel, with the sides made up of sections of β-sheet.
The copper(II) ion (bright green) is not buried inside the protein but sits at the "top" end of the protein, coordinated to two histidine residues, one methionine and one cysteine [Cu(II) Pc (pH 6) 2]. The geometry at the metal ion is approx. tetrahedral (although the Cu-Met bond is unusually long):

• Cu - His37 1.94 Å
• Cu - His87 1.99 Å
• Cu - Cys84 2.16 Å
• Cu - Met 92 2.78 Å

The active site is separated from the outside only by His87. This is the likely route by which electrons enter and leave.
To see this more clearly, return to the first view, right-click in the graphics box, click "Select", then "Protein", then "All". Right-click again, choose "Surfaces", then "van der Waals". Now rotate the molecule to see that His87 is at the surface.
Click on [apoPc] to see the structure of the apoprotein. Note that the metal-binding site is already assembled - i.e. the protein is imposing some geometry on the metal ion.

The reduced, Cu(I) form of the protein has almost identical structure to the oxidised form at neutral pH but at lower pH the geometry at copper changes to approx trigonal planar [Cu(I) Pc (pH 4) 1] and [Cu(I) Pc (pH 4) 1]. The Cu-Met bond length is more normal and the Cu - His87 bond breaks as the His becomes protonated. (This group is actually disordered over two overlapping positions - both are shown.)

• Cu - His37 1.88 Å
• Cu - His87 3.47 Å
• Cu - Cys84 2.10 Å
• Cu - Met92 2.39 Å

The last pair of views [Cu(I) Az 1] and [Cu(I) Az 2] show azurin, a second type 1 (or blue) copper protein.

The metalloprotein as a whole is fairly similar to plastocyanin, with the copper site at the top end of a barrel-shaped protein. However, the copper site is different in that the Cu appears to be 5-coordinate:

• Cu - His46 2.08 Å
• Cu - His117 2.01 Å
• Cu - Cys112 2.12 Å
• Cu - Met129 3.12 Å
• Cu - Gly45 3.16 Å

Comparisons such as this give rise to the idea that the type 1 site should be regarded as having three main donors and one or two weaker "interactions". So the plastocyanin ligation could be described as 3+1, whereas azurin would be described as 3+2.

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