Which proteins have more S-S bridges
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Covalent bonds between two sulphur atoms of Cys residues, so called
called S-S bridges, are found in many proteins and are thought to
increase protein stability.

Questions:

- how many different proteins described in the PDB archive contain S-S
  bridges? What is the distribution of this number, what are its
  parameters? What can we conclude from this distribution?

- how many S-S bridges are there on average per one (or per 100 ;)
  protein residues? Does this size correlate with other proteins
  parameters? Which ones? Why? What a statistic of this size
  distribution, what are its parameters? What can we say from them?

- Are there protein classes for which the number of S-S bridges is
  unusual? If there are, what are they? If such proteins exist, what
  is the biological significance of the S-S bridge number in them?

* (harder additional task) How the number of S-S bridges correlates
  with protein stability? Can you estimate that correlation for at
  least a few for proteins or classes of proteins?

Program:

This research requires a program that reads PDB archive files and
determines and outputs the number of S-S bridges, and maybe the bridge
forming residues. It is necessary not to rely on annotations, but to
discover S-S bridges by atomic coordinates.