I understand that Porphyrin is defined as 4 pyrole rings linked by methenyl bridges. When this porphyrin ring is associated with a Ferrous ion (Fe 2+) we get the Heme molecule. However my book mentions that Heme is a complex of Protoporhyrin IX and Ferrous ion. So what is the difference between a Porphyrin ring and a Protoporphyrin ring?
Protoporphyrin is a derivative of porphyrin. The key difference between porphyrin and protoporphyrin is that porphyrin is a group of aromatic chemicals which has four modified pyrrole subunits interconnected to each other, whereas protoporphyrin is a derivative of porphyrin which has propionic acid groups.
Protoporphyrine contains porphine as core which is the simplest of the porphyrin chain. Picture from Wikipedia: