The chemical reactions and pathways involving any member of a large group of derivatives or analogs of porphyrin. Porphyrins consists of a ring of four pyrrole nuclei linked each to the next at their alpha positions through a methine group. [ GOC:jl http://en.wikipedia.org/wiki/Porphyrin#Natural_formation https://www.worldcat.org/search?q=bn%3A0198506732 ]