Catalyzes the ferrous insertion into protoporphyrin IX. 2 H(+) + heme b = Fe(2+) + protoporphyrin IX Binds 1 [2Fe-2S] cluster. Inhibited by nitric oxide (NO). The 2Fe-2S cluster could act as a NO sensor (By similarity). Porphyrin-containing compound metabolism; protoheme biosynthesis; protoheme from protoporphyrin-IX: step 1/1. Homodimer (By similarity). Interaction with PGRMC1; the interaction results in decreased FECH activity (PubMed:27599036). Interacts with ABCB10 and SLC25A37; this interaction forms an oligomeric complex (PubMed:20427704). Forms a complex with ABCB7 and ABCB10, where a dimeric FECH bridges ABCB7 and ABCB10 homodimers; this complex may be required for cellular iron homeostasis, mitochondrial function and heme biosynthesis (PubMed:30765471). Interacts with ABCB7 and ABCB10 (PubMed:30765471). Erythroid and hepatic cells. During erythroid differentiation. Defects in Fech are the cause of a viable autosomal recessive mutation (named Fechm1Pas or Fch) that causes jaundice and anemia. Belongs to the ferrochelatase family. Extended N-terminus.