Ferrochelatase, the Cornerstone Sensor Between Porphyrin Synthesis and Iron Metabolism

The terminal step of heme biosynthesis links the distinct, but synchronized, pathways of porphyrin synthesis and iron transport in nearly all living organisms. Ferrochelatase (or protoporphyrin IX ferrochelatase; Enzyme Commission number 4.9.9.1.1) catalyzes this step, the incorporation of ferrous iron into protoporphyrin IX to yield heme. How ferrochelatase, which, can also catalyze the insertion of other divalent metal ions into protoporphyrin IX, albeit at lower rates, ensures that only ferrous iron is used as the physiological substrate and prevents the potentially damaging oxidative toxicity of ferrous iron remains an unsolved question. Similarly, the ferrochelatase-catalyzed heme synthesis according to the cellular requirements thwarts the accumulation of protoporphyrin IX, a photoreactive and toxic molecule. To define the path the metal ion takes to the catalytic site, we examined the kinetic mechanism of ferrochelatase and ferrochelatase variants, in which the postulated ferrous iron-binding amino acids were replaced with either non-ferrous iron ligands or potential alternative iron-binding amino acids, using pre-steady state kinetics. In addition, the significance of frataxin, a mitochondrial iron chaperone, as the ferrous iron donor of ferrochelatase, was assessed by studying the kinetic mechanism of the ferrochelatase-catalyzed reaction when the metal ion substrate was supplied in the form of “iron-bound frataxin”.  Results from this study demonstrate that ferrochelatase is the ultimate cellular sensor of ferrous iron and protoporphyrin IX, which are directly used in heme synthesis and thus bypass their cellular accumulation and consequent toxicities.

 

 Supported by grant 13GRNT16970019 from the American Heart Association, Greater Southeast Affiliate.