Structural domains comprising this superfamily share the structure of the VKOR (Vitamin K epoxide reductase) domain, found naturally in complex with its naturally fused redox partner, a thioredoxin-like domain. VKOR generates vitamin K hydroquinone, which is required to sustain gamma-carboxylation of many blood coagulation factors. The catalytic core of VKOR is a four transmembrane helix bundle that surrounds a quinone, connected through an additional transmembrane segment with the periplasmic thioredoxin-like domain PMID:24477003,PMID:20110994.