Salt bridges and hydrogen bonding

I have been playing with PISA tool as it offered insight into size and nature of the interface between the two subunits in NHases. I stuck in the PDB data for the most distinct cobalt-centred NHases- 1UGP, 1V29 and 3HHT, and PISA suggested that these have 61, 67 and 65 hydrogen bonds (respectively) between the alpha and beta subunits, and between 26 and 29 salt bridges across this interface. Compare these numbers with that for 2QDY (the iron-centred NHase from AJ270)... 51 hydrogen bonds and 8 salt bridges. That looks quite a difference... sadly things might not be so clear-cut in grouping these enzymes by metal centre- the newest PDB for 3QXE gives an average of 53 hydrogen bonds and 15 salt bridges, and that is cobalt-centred. Furthermore I do wonder, with such a small set of structures, whether we are seeing a distinction based on the fact that the first three cobalt-centred PDBs are of enzymes specifically tagged as "thermophilic", and AJ270 and Pseudomonas putida aren't. [I also looked at 2DPP and got 62 hydrogen bonds and 27 salt bridges].