Marjorie Harding, at the University of Edinburgh, has a resource that can be helpful in learning about coordination sites in metalloproteins that have been deposited within the PDB. In addition to coordination number, there are also statistics related to geometry, distances, model resolution and donor group preferences (such as Asp usually donates to Mg instead of His).
This work has resulted in a number of publications over the years:
1999 The geometry of metal-ligand interactions relevant to proteins
2000 The geometry of metal-ligand interactions relevant to proteins. II.
2001 Geometry of metal-ligand interactions in proteins
2002 Metal-ligand geometry relevant to proteins and in proteins: sodium and potassium
2004 The architecture of metal coordination groups in proteins
2006 Small revisions to predicted distances around metal sites in proteins
In addition, Dr. Harding has been involved in the development of a database dedicated to ‘Metal Sites in Proteins’ — MESPEUS database. This database is current as of January 2007 and allows you to search by PDB code, specific metals, coordination groups and numbers. Regular updates are expected in the future. The citation for the database can be found at the bottom of this page.