This is a flavin adenine dinucleotide (FAD) binding domain found in Quiescin sulfhydryl oxidases (QSOX) (EC:1.8.3.2). QSOX is a multi-domain disulfide catalyst that is localized primarily to the Golgi apparatus and secreted fluids and has attracted attention due to its over-production in tumors. Structural studies indicate that the closure of the Trx1 domain over the FAD-binding site may enhance the active-site chemistry for disulfide formation [1 -3].

1: QSOX contains a pseudo-dimer of functional and degenerate sulfhydryl oxidase domains. Alon A, Heckler EJ, Thorpe C, Fass D; FEBS Lett. 2010;584:1521-1525. PMID:20211621

2: The dynamic disulphide relay of quiescin sulphydryl oxidase. Alon A, Grossman I, Gat Y, Kodali VK, DiMaio F, Mehlman T, Haran G, Baker D, Thorpe C, Fass D; Nature. 2012;488:414-418. PMID:22801504

3: Enzyme structure captures four cysteines aligned for disulfide relay. Gat Y, Vardi-Kilshtain A, Grossman I, Major DT, Fass D; Protein Sci. 2014;23:1102-1112. PMID:24888638