There's a very interesting rule of thumb that is used in the protein folding field. If 30% of the amino acids in two different proteins are identical, then you can be very certain that they will have identical (or nearly identical) 3D shapes. However, it is entirely possible that two proteins have no sequence in common, yet still fold to the same shape. A friend actually stumbled upon just such a case when I was still doing protein folding work, and it was amazing to look at the sequence of these two different proteins and see nothing in common, then flip to the 3D structure and see that they line up almost perfectly.
Proteins are immensely cool.