Fig. 1.

Catalytic activity and molecular architecture of human STEAP4. a Ferric-reductase activity of detergent-purified STEAP4_EM with varying Fe³⁺-NTA concentrations. STEAP4_EM exhibits a K_M of 13.7 ± 0.8 µM for Fe³⁺-NTA and a k_cat of 49.4 ± 0.8 min⁻¹. Error bars represent the standard deviation between triplicate measurements. b Sharpened density map of the cofactor-bound dataset at 3.8-Å resolution colored by chain. c Sharpened density map of the cofactor/substrate-bound dataset at 3.1-Å resolution colored by chain. The grey densities do not exhibit protein features but likely represent the sterol moiety of digitonin. d–f Trimeric arrangement of STEAP4, viewed parallel to the membrane as a sideview (d) and perpendicular to the membrane from the cytoplasm (e), and extracellular milieu (f). For the cytoplasmic and extracellular views, the domain in the background is shown as surface