5.B.6 The Transmembrane Epithelial Antigen Protein-3 Ferric Reductase (STEAP) Family
The daily production of 200 billion erythrocytes requires 20 mg of iron, accounting for nearly 80% of the iron demand in humans. Thus, erythroid precursor cells possess an efficient mechanism for iron uptake in which iron loaded transferrin (Tf) binds to the transferrin receptor (TfR) at the cell surface. The Tf:TfR complex then enters the endosome via receptor-mediated endocytosis. Upon endosomal acidification, iron is released from Tf, reduced to Fe2+ by Steap3 (also called TSAP6), and transported across the endosomal membrane by divalent metal iron transporter 1. Steap3, the major ferrireductase in erythrocyte endosomes, is a member of a unique family of reductases. Steap3 is comprised of an N-terminal cytosolic oxidoreductase domain and a C-terminal heme-containing transmembrane domain (YedZ). Cytosolic NADPH and a flavin are predicted cofactors, but the NADPH/flavin binding domain differs from those in other eukaryotic reductases. Instead, Steap3 shows limited homology to FNO, an archaeal oxidoreductase. Sendamarai et al. (2008) have determined the crystal structure of the human Steap3 oxidoreductase domain in the absence and presence of NADPH. The structure reveals an FNO-like domain with an unexpected dimer interface and substrate binding sites that are positioned to direct electron transfer from the cytosol to a heme moiety predicted to be fixed within the transmembrane domain.
Six-transmembrane epithelial antigen of the prostate 3 (Steap3) is the major ferric reductase in developing erythrocytes. Steap family proteins are defined by a shared transmembrane domain that in Steap3 has been shown to function as a transmembrane electron shuttle, moving cytoplasmic electrons derived from NADPH across the lipid bilayer to the extracellular face where they are used to reduce Fe3+ to Fe2+ and potentially Cu2+ to Cu1+ (Kleven et al. 2015). The cytoplasmic N-terminal oxidoreductase domains of Steap3 and Steap4 are relatively well characterized. High affinity FAD, iron and b-type heme binding sites are in the Steap3 transmembrane domain and Steap3 is functional as a homodimer. It utilizes an intrasubunit electron transfer pathway through the single heme moiety rather than an intersubunit electron pathway through a potential domain-swapped dimer. The sequence motifs in the transmembrane domain that are associated with the FAD and metal binding sites are not only present in Steap2 and Steap4 but also in Steap1 which lacks the N-terminal oxidoreductase domain, suggesting that Steap1 harbors latent oxidoreductase activity (Kleven et al. 2015).