Ferrochelatase mammalian

Within the past few years, there has been considerable progress in understanding the role played by the mitochondria in the cellular homeostasis of iron. Thus, erythroid cells devoid of mitochondria do not accumulate iron (7, 8), and inhibitors of the mitochondrial respiratory chain completely inhibit iron uptake (8) and heme biosynthesis (9) by reticulocytes. Furthermore, the enzyme ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) which catalyzes the insertion of Fe(II) into porphyrins, appears to be mainly a mitochondrial enzyme (10,11,12,13, 14) confined to the inner membrane (15, 16, 17). Finally, the importance of mitochondria in the intracellular metabolism of iron is also evident from the fact that in disorders with deranged heme biosynthesis, the mitochondria are heavily loaded with iron (see Mitochondrial Iron Pool, below). It would therefore be expected that mitochondria, of all mammalian cells, should be able to accumulate iron from the cytosol. From the permeability characteristics of the mitochondrial inner membrane (18) a specialized transport system analogous to that of the other multivalent cations (for review, see Ref. 19) may be expected. The relatively slow development of this field of study, however, mainly reflects the difficulties in studying the chemistry of iron. 

Dailey, H.A., Sellers, V.M., Dailey, T.A. Mammalian ferrochelatase expression and characterization of normal and two human protoporphyria ferrochelatases. J. Biol. Chem. 1994 269 390—395... 

Figure 2-1 Schematic representation of the heme biosynthetic pathway in mammalian cells. ALAS, S-aminolevulinate synthase PBGS, porphobiUnogen synthase PBGD, porphobilinogen deaminase Uro III synthase, uroporphyrinogen III synthase Uro III decarboxylase, uroporphyrinogen III decarboxylase CPO, coproporphyrinogen oxidase PPO, protoporphyrinogen oxidase FC, ferrochelatase.

Hie expression of recombinant mammalian liver ferrochelatases in Escherichia coli [4, 5], allowed the production of increased amounts of the enzyme and led to the unexpected discovery of a [2Fe-2S] cluster in the mammalian enzymes (mouse [6] and human [7]). The large amoimts of protein made available using heterologous over-producing systems allowed detailed spectroscopic studies on the cluster-containing ferrochelatases. Yeast Saccharomyces cerevisiae) ferrochelatase, which is devoid of an iron-sulfur cluster, has also been extensively studied and several site-directed mutants have been produced [8], making it a powerful model system to study the mechanism of this enzyme. 

In this chapter we will review the present knowledge of ferrochelatase in terms of its structure and function, with special emphasis on the mammalian enzymes and their intriguing [2Fe-2S] cluster. 

The function of a [2Fe-2S] cluster in mammalian ferrochelatases remains an open... 

The combined use of spectroscopy and protein engineering techniques provides a unique approach for the study of the mechanism of ferrochelatase and in the elucidation of the role of its intriguing [2Fe-2S] cluster. Mossbauer studies of kinetic intermediates and fluorimetric and resonance Raman studies of both mammalian and yeast (i.e. devoid of the [2Fe-2S] cluster) ferrochelatases, reacted with porphyrin or porphyrin analogs, should be undertaken. Determination of the three-dimensional structures of the complexes of ferrochelatase with substrate or inhibitors will also be of crucial importance. 

A [2Fe-2S] cluster is also present in mammalian ferrochelatase, the terminal enzyme of the heme biosynthetic pathway, and it appears to be the determinant for catalysis as reviewed by Franco et al. (Chapter 3). Nevertheless, the specific role of this cluster in the enzyme s stability and function remains to be established. The search for the function of this cluster becomes even more challenging as bacterial, yeast and plant ferrochelatases do not possess it, although they perform the same catalytic task. 

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