Many proteins or enzymes contain metal ions within their active sites.
These metals are often directly involved in the reaction catalysed bu the enzyme. In certain cases the function is well known and understood, including the identity of the metal and the changes in oxidation state throughout the reaction.
I wonder how the metal and oxidation state is determined a priori when a new structure is obtained. For examples if I solved a structure of a previously unknown enzyme by crystallography I may be able to determine that it contains Zinc (iron a better example), but knowing the density map tells me nothing about the oxidation state which may be important to function.
I think it must be possible to determine this, however I do not find much reliable information (inorganic complexes typically have other techniques in addition to X-ray). The situation is made harder as many protin structures in the pdb appear to be assigned without any real study.