Heme B group of hemoglobin. An iron (Fe) atom in the middle is shown in red, complexed to four interior nitrogen atoms shown in blue. Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent porphyrin is porphin, and substituted porphines are called porphyrins.