Linking physiological functions of iron.

Virtually all aerobic organisms synthesize heme, an iron protoporphyrin complex that functions as an oxygen transporter and electron carrier in multiple enzymes and mitochondrial respiratory chain complexes. Heme is synthesized by a highly conserved multistep process, beginning with the condensation of glycine and succinyl-CoA by ALA synthase (ALAS) to form 5-aminolevulinic acid, and continuing through seven successive enzymatic steps that culminate with insertion of iron into the porphyrin ring to form heme. It is advantageous to inhibit heme biosynthesis in iron-deprived cells at the first step of the pathway, because the final step of heme synthesis is catalyzed by ferrochelatase, an iron-sulfur protein that inserts iron into protoporphyrin IX. If cells are iron deficient and cannot complete the final step of synthesis, later heme biosynthetic intermediates such as uroporphyrin and protoporphyrin IX can accumulate and cause toxicity1. Thus, it is of great interest that Wingert et al., in the 18 July issue of Nature, have discovered a functional connection between initiation of heme biosynthesis in red cells and an enzyme important for activity of proteins that contain iron-sulfur clusters2. Shiraz mutants are zebrafish mutants that have a heritable disorder of hemoglobin synthesis. Using positional cloning to identify the gene responsible for shiraz anemia in zebrafish, the authors established that the anemia was due to loss of glutaredoxin 5 function, a gene first identified as important for the activity of iron-sulfur cluster proteins in yeast3. Iron-sulfur clusters are prosthetic groups composed of inorganic sulfur and iron that are of fundamental importance in respiration, photosynthesis and nitrogen fixation4. Thus, this result suggested a previously unrecognized connection between two fundamental biochemical processes, the iron-sulfur cluster assembly and the heme biosynthetic pathways of higher eukaryotes. To clarify the mechanistic connection between glutaredoxin 5 deficiency and anemia, the authors extrapolated from insights gained over the past decade about regulation of mammalian erythropoiesis. Mammalian red blood cells contain a distinct form of ALA synthase (eALAS) in which the 5′ untranslated region (UTR) of the transcript contains an RNA stem-loop element known as an iron-responsive element (IRE)5. The IRE is a binding site for iron-sensing proteins known as iron regulatory proteins (IRPs)6, which repress heme synthesis in iron-depleted cells by binding to the IRE of eALAS, thereby preventing initiation of translation. Mammalian cells contain two IRPs, each of which can bind to IREs found in mRNAs of important iron-metabolism proteins, including the ironstorage protein, ferritin and the transferrin receptor (TfR), an iron-uptake protein. IRP1 is a bifunctional protein that functions as a cytosolic aconitase in iron-replete cells, or as an RNA-binding protein that inhibits translation of transcripts such as ferritin with IREs at the 5′ end of the transcript, while it stabilizes the TfR transcript by binding to IREs in the 3′ UTR in iron-depleted cells. The function of IRP1 is determined by the presence or absence of a cubane iron-sulfur cluster ligated to the enzymatic active site7 (Fig. 1). Much has been learned about bacterial iron-sulfur cluster assembly enzymes4, and the process of ironsulfur cluster assembly seems to be highly conserved in mammals8. In the zebrafish genome, Wingert et al. discovered that, as in mammalian cells, there are two IRPs, and there are also two forms of ALAS, one of which, ALAS2, contains an IRE at the 5′ end and is expressed uniquely in erythroid cells9. Recognizing that glutaredoxin 5 is required for