5.B.6.1.3
STEAP4 metaloreductase of 459 aas.  It is also called Stamp2, TNFAIP9. It is an integral membrane protein that functions as an NADPH-dependent ferric-chelate reductase, using NADPH from one side of the membrane to reduce a Fe³⁺ chelate that is bound on the other side of the membrane. STAMP2 mediates sequential transmembrane electron transfer from NADPH to FAD and onto heme, and finally to the Fe³⁺ chelate (Oosterheert et al. 2018) and can also reduce Cu²⁺ to Cu¹⁺. It plays a role in systemic metabolic homeostasis, integrating inflammatory and metabolic responses and is associated with obesity and insulin-resistance (Zhang et al. 2008, Arner et al. 2008). It is also involved in inflammatory arthritis, through the regulation of inflammatory cytokines (Inoue et al. 2009). It inhibits anchorage-independent cell proliferation (Tamura and Chiba 2009). The crystal structure is known (Gauss et al. 2013). Hepatic STAMP2 mediates recombinant FGF21-induced improvement of hepatic iron overload in nonalcoholic fatty liver disease (Kim et al. 2020).