What is the relationship between protein-protein interaction networks and metabolic networks?
In short, proteins in Protein-protein interaction networks perform a function by directly interacting with one another.  They may for instance bind to one another forming permanent or momentary complexes (e.g. insulin binding to insulin receptor or the pieces of F0-F1 ATPase combining), or they may chemically modify one another (e.g. protein kinases adding a phosphate to the residue of another protein). 



Proteins in a Metabolic network might never be in physical contact one another. They effect one another by producing chemical compounds which are utilized by other enzymes, forming a network of chemical transformations.  

Of course an enzyme in a metabolic network might also have its activity modulated through interacting with other proteins in a protein-protein network.  The two terms are of course not mutually exclusive, but describe different sorts of relationships between proteins.