The 2.0 Å structure of human ferrochelatase, the terminal enzyme of heme biosynthesis

Human ferrochelatase (E.C. 4.99.1.1) is a homodimeric (86 kDa) mitochondrial membrane-associated enzyme that catalyzes the insertion of ferrous iron into protoporphyrin to form heme. We have determined the 2.0 Angstrom structure from the single wavelength iron anomalous scattering signal. The enzyme contains two NO-sensitive and uniquely coordinated [2Fe-2S] clusters. Its membrane association is mediated in part by a 12-residue hydrophobic Lip that also forms the entrance to the active site pocket. The positioning of highly conserved residues in the active site in conjunction with previous biochemical studies support a catalytic model that may have significance in explaining the enzymatic defects that lead to the human inherited disease erythropoietic protoporphyria.