Does one regard the metal atom in a metalloprotein as being part of the protein?
If so, a shared metal could occupy a special position in a dimer for example. In Acta Cryst. (2008). D64, 257-263 "Metals in proteins: correlation between the metal-ion type, coordination number and the amino-acid residues involved in the coordination" I. Dokmanic, M. Sikic and S. Tomic ( http://scripts.iucr.org/cgi-bin/paper?S090744490706595X ) it says there are 25 cases of metal atoms in special positions. Also Acta Cryst. (2002). D58, 29-38 "The 2.6 Å resolution structure of Rhodobacter capsulatus bacterioferritin with metal-free dinuclear site and heme iron in a crystallographic `special position' "D. Cobessi, L.-S. Huang, M. Ban, N. G. Pon, F. Daldal and E. A. Berry ( http://scripts.iucr.org/cgi-bin/paper?S0907444901017267 ) though the 'special position' is justifiably in quotation marks in this example as disorder is present. Colin
