Iron protoporphyrin IX group

In many of the haemoproteins we shall be discussing, the protoporphyrin IX group is held to the polypeptide chain only by hydrogen bonding, Van der Wools forces and iron-protein bonds. In several other cases, notably in cytochrome-c and its related compounds, the haem is covalently linked to the protein via substituents at the pyrrole carbon atoms. Cyto-chrome-c can be regarded as an iron protoporphyrin IX group with the addition of a protein cysteine side-chain across the vinyl double bonds giving two thio-ether links (Fig. 3). 

---