RCSB - every PDB will have a paper with it. RCSB lists the protein's ID with a publication. If you are lucky you will find the publication which will give you all you need to know or will point you in the right direction.
You can compliment the previous search with NCBI or start from there e.g.
http://www.ncbi.nlm.nih.gov/
NCBI links across multiple databases .. so at this stop you can find EVERYTHING you need by 1 search.
e.g. I just searched atherosclerosis in it and found 345 structures.
Try Reactome (a list of proteins, genes, metabolites, pathways can be found out upon searching the term!) by searching the disease term:http://www.reactome.org/content/query?q=atherosclerosis&species=Homo+sapiens&types=Reaction&types=Complex&cluster=true
Alternatively, try Poly Search from Professor David Wishart:http://wishart.biology.ualberta.ca/polysearch/cgi-bin/DiseaseFind.cgi?type=1 and search the terms to get the list of genes/ proteins as may be the case.
The above two would be a good start than going to IPA which is NOT free, commercial,; but DAVID would be easy too! PDB would be structural information and little on functional relatedness/ interaction. Yes, but NCBI is a good resource to sort proteins/ genes in addition to UniProt.
If you are interested in knowing about a gene (not necessarily the protein) and its relationship with a disease, try DisGeNET. (http://www.disgenet.org/web/DisGeNET/menu;jsessionid=15gl6du1cmyrv77xqzmqt2b1a)
It links articles which may have suggested association or functional aspects of a given gene with the disease.