This is a thioredoxin-like domain found at the C-terminal of aromatic-ring hydroxylases [1-5], such as PgaE from Bacillus subtilis and Rifampicin monooxygenase from Streptomyces venezuelae. The function of this domain is not clear. It may be involved in folding and stability [6] as it lacks the catalytic cysteine residues of thioredoxin, therefore, is not likely to have a redox function [1].

1: Crystal structures of two aromatic hydroxylases involved in the early tailoring steps of angucycline biosynthesis. Koskiniemi H, Metsa-Ketela M, Dobritzsch D, Kallio P, Korhonen H, Mantsala P, Schneider G, Niemi J; J Mol Biol. 2007;372:633-648. PMID:17669423

2: Crystallographic trapping in the rebeccamycin biosynthetic enzyme RebC. Ryan KS, Howard-Jones AR, Hamill MJ, Elliott SJ, Walsh CT, Drennan CL; Proc Natl Acad Sci U S A. 2007;104:15311-15316. PMID:17873060

3: Crystal structure of Baeyer-Villiger monooxygenase MtmOIV, the key enzyme of the mithramycin biosynthetic pathway . Beam MP, Bosserman MA, Noinaj N, Wehenkel M, Rohr J; Biochemistry. 2009;48:4476-4487. PMID:19364090

4: Structural basis for substrate recognition and specificity in aklavinone-11-hydroxylase from rhodomycin biosynthesis. Lindqvist Y, Koskiniemi H, Jansson A, Sandalova T, Schnell R, Liu Z, Mantsala P, Niemi J, Schneider G; J Mol Biol. 2009;393:966-977. PMID:19744497

5: Structures of the Apo and FAD-bound forms of 2-hydroxybiphenyl 3-monooxygenase (HbpA) locate activity hotspots identified by using directed evolution. Jensen CN, Mielke T, Farrugia JE, Frank A, Man H, Hart S, Turkenburg JP, Grogan G; Chembiochem. 2015;16:968-976. PMID:25737306

6: The Structure of the Antibiotic Deactivating, N-hydroxylating Rifampicin Monooxygenase. Liu LK, Abdelwahab H, Martin Del Campo JS, Mehra-Chaudhary R, Sobrado P, Tanner JJ; J Biol Chem. 2016;291:21553-21562. PMID:27557658