The big hole in the middle has just the right size for a fully stretched peptide.
The number of contacting secondary structure elements depends very much on how one
counts. Strands and helices only: 0. Counting loops too, than any answer around 6 should
be considered OK. The idea is that the contact surface is extensive. When WHAT_TEACH
brings up the molecule (get that view back with the RSET button), you see a hole in
the middle. The protein that is to be cleaved goes through that hole, from your eye
into the screen.
The enzyme is only active when you have 2 subunits
It is a symmetric dimer
2 small helices, many loops
This molecule represents a victory of bioinformatics: The X-ray structure of the
dimer could not be solved properly, the loops of the subunits were connected in
the X-ray model. With this structure they could not explain the mechanism of action.
Bioinformatics solved this problem.