PDBsum entry 2vq3

Oxidoreductase

PDB id: 2vq3

Contents

Protein chains

181 a.a.

Ligands

NAP ×2

CIT

Waters ×125

References listed in PDB file

Key reference

Title

Structure of the membrane proximal oxidoreductase domain of human steap3, The dominant ferrireductase of the erythroid transferrin cycle.

Authors

A.K.Sendamarai, R.S.Ohgami, M.D.Fleming, C.M.Lawrence.

Ref.

Proc Natl Acad Sci U S A, 2008, 105, 7410-7415. [DOI no: 10.1073/pnas.0801318105]

PubMed id

18495927

Abstract

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 Fe(2+) by Steap3, 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. Cytosolic NADPH and a flavin are predicted cofactors, but the NADPH/flavin binding domain differs significantly from those in other eukaryotic reductases. Instead, Steap3 shows remarkable, although limited homology to FNO, an archaeal oxidoreductase. We 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 well positioned to direct electron transfer from the cytosol to a heme moiety predicted to be fixed within the transmembrane domain. Here, we discuss possible gating mechanisms for electron transfer across the endosomal membrane.

Figure 1.
Steap3 Structure. (A) The structure of the oxidoreductase dimer is depicted with α-helices in red, β-strands in blue, and connecting loops in green. The twofold axis runs vertically within the plane of the paper (double headed arrow). The truncated C termini, which must connect to the C terminal transmembrane domain, are in green at the top of the structure. NADPH (C, yellow; N, blue; O, red; and P, orange) runs up the front side of the right subunit (back side of the left subunit) with the adenine-ribose-2′phosphate moieties near the bottom and the nicotinamide ring near the top. (B) Stereo figure of the Steap3 subunit with labeled secondary structural elements. A color gradient runs from the N terminus (blue) to the C terminus (red). Note the proximity of the NADPH binding site to the dimer interface. (C) Stereo figure depicting the superposition of FNO on the Steap3 protomer. The Steap3 C[α] trace is in blue, and FNO in red. The approximate location and extent of the FNO dimer interface is indicated by black arrows along the top of FNO. In contrast, the Steap3 interface is formed by α7, α1, and the C-terminal end of α2, which is significantly shorter in Steap3. Relocation of the dimer interface, combined with the shorter β5 and α9 elements, allow the Steap3 NADPH binding site to approach the membrane.

Figure 3.
The electrostatic potential mapped to the surface of the Steap3 dimer. Positive potentials are in blue, negative potentials in red (±25 kT/e). Steap3-NADPH and the docked FMN from BVR-B are also shown. The orientation of the dimer and colors for NADPH are as in Fig. 1. FMN is similarly colored, but carbons are in cyan. Note the cleft running up the front side and across the top of the dimer interface. Because of the symmetry of the dimer, the cleft continues along the interface down the back side of the dimer.

Secondary reference #1

Title

Identification of a ferrireductase required for efficient transferrin-Dependent iron uptake in erythroid cells.

Authors

R.S.Ohgami, D.R.Campagna, E.L.Greer, B.Antiochos, A.Mcdonald, J.Chen, J.J.Sharp, Y.Fujiwara, J.E.Barker, M.D.Fleming.

Ref.

Nat Genet, 2005, 37, 1264-1269. [DOI no: 10.1038/ng1658]

PubMed id

16227996

Figure 2.
Figure 2. Steap3 mRNA expression. (a) In situ hybridization of homozygous mutant (nm/nm) and wild-type mouse embryos at embryonic day 15.5 showing high-level fetal liver (open arrow) and labyrinthine placental (closed arrow) expression. (b) Quantitative real-time PCR of STEAP3 in human tissues. Relative RNA abundance is normalized to spleen, which was defined as a ratio of 1.0. Error bars represent one standard deviation.

Figure 3.
Figure 3. Steap3 subcellular localization. (a–i) Colocalization of epitope-tagged Steap3 with endogenous Tf and Tfr1 and epitope-tagged DMT1. (a) Steap3. (b) Tf. (c) Tf and Steap3 merged. (d) Steap3. (e) Tfr1. (f) Tfr1 and Steap3 merged. (g) Steap3. (h) Dmt1. (i) Dmt1 and Steap3 merged.

The above figures are reproduced from the cited reference which is an Open Access publication published by Macmillan Publishers Ltd

Secondary reference #2

Title

Nm1054: a spontaneous, Recessive, Hypochromic, Microcytic anemia mutation in the mouse.

Authors

R.S.Ohgami, D.R.Campagna, B.Antiochos, E.B.Wood, J.J.Sharp, J.E.Barker, M.D.Fleming.

Ref.

Blood, 2005, 106, 3625-3631.

PubMed id

15994289