Cys Sg, and two His Nd-s form a plane. The Cu is just above this plane, and the Met Sd is almost straight above the Cu. However, another arrangement, with one of the Histidines as the apex instead of the Methionine is not a much worse solution...

There is something else striking. This is often seen for proteins that operate near the membrane. It must have to do with the layers of counter-ions that float just outside the negatively charged membrane surface. I don't know any fine details about this. But you see circles of charged residues:

Plastocyanin has two rings of negative charges. Some proteins also have positive rings (e.g. Annexin V), but plastocyanin doesn't.

The protein binds Cu2+ The word cyan (blue) might suggest Cu binding to the students or they can look it up. Zn ions give a grey solution, Ni a green solution.

After this they can speculate about amino acids capable of Cu and they might decide to colour the His and Cys residues.

It is a protein in a redoxchain. There are 2 His, 1 Met and 1 Cys involved in metal binding. Cd and Pb will bind to this protein and this is one of the reasons that these ions are poisonous.

How can you know that this is not a protease? There is no dent in the protein (for the substrate) and the Cu is located inside, far from the surface.

All beta. 2 x 3 strands. There are 2 kinds of loops. Some of them are within one sheet and some of them are crossing over, they are called arches. But the loops never cross each other!!! This is because of the way the protein is folded. It can be folded by 3 times taking the protein and folding it double. Cross-over loops are very rare, so are knots. At this moment (2003) 1 knot is known (and in 2011 it is a little over 10).

half a barrel would be a better name, because if you look at the strands only, they do not form a closed structure