HIF-asparaginyl hydroxylase (FIH-1) normally couples O₂-activation to hydroxylation of Asn⁸⁰³ on the α-subunit of the hypoxia-inducible factor (HIFα), a key step in pO₂ sensing; in the absence of HIFα, O₂-activation becomes uncoupled, leading to self-hydroxylation at Trp²⁹⁶ and a purple Fe(III)–O–Trp chromophore—this alternative reactivity may affect human hypoxia sensing.