Curatorial Notes for `mdo` (`B9U5L8`, `VPARA_05600`) in Variovorax paradoxus TBEA6

1. Gene & Protein Identity

UniProt Accession: B9U5L8 (B9U5L8_VARPD)
Gene Symbol: mdo
Locus Tag / EMBL CDS: VPARA_05600 / KLN58445.1 on contig JZWI01000003 (KEGG:ag:ACB72254, MetaCyc:MONOMER-18546, BRENDA:1.13.11.91, RefSeq:WP_047783194.1)
Protein Length: 195 aa (21,607 Da); 3-mercaptopropionate (3MP) dioxygenase Mdo (EC 1.13.11.66 / BRENDA 1.13.11.91).
Domain Architecture & Catalytic Triad:
Belongs to the RmlC-like cupin / cysteine dioxygenase type-I family (CDD:cd10548 cupin_CDO, Pfam:PF05995 CDO_I, InterPro:IPR010300 CDO_1, InterPro:IPR011051 RmlC_Cupin_sf, PANTHER:PTHR12918:SF1).
Coordinates a catalytic non-heme ferrous iron ($\text{Fe}^{2+}$) cofactor via the conserved 3-His cupin triad at His90, His92 (within 89-IHNHTVW-95), and His143 (FT BINDING 90, 92, 143 /ligand="Fe cation" /ligand_note="catalytic", PIRSR:PIRSR610300-51).

PMID:19001372 (Bruland, Wübbeler & Steinbüchel 2009 J Biol Chem):
Isolated Variovorax paradoxus strain TBEA6 for its ability to grow on 3,3'-thiodipropionic acid (TDP) as sole carbon and energy source.
Tn5::mob transposon mutagenesis of V. paradoxus TBEA6 identified two TDP-negative mutants disrupted in mdo (VPARA_05600, ACB72254.1 / KLN58445.1).
Heterologous expression, purification, and in vivo / in vitro enzyme assays of V. paradoxus TBEA6 Mdo demonstrated that it catalyzes the dioxygenation of 3-mercaptopropionic acid (3MP) with $\text{O}_2$ to 3-sulfinopropionate (3SP), and does not use L-cysteine or cysteamine as substrates.
PMID:24317404 (Schürmann et al. 2014 J Bacteriol): Confirms the downstream pathway in V. paradoxus TBEA6 in which 3SP produced by Mdo is activated to 3SP-CoA by Act (VPARA_05450) and desulfinated to propionyl-CoA and sulfite by Acd (VPARA_05440).

GO:0005506 (iron ion binding, IEA GO_REF:0000002, enables, InterPro:IPR010300): TOO_GENERAL — parent of GO:0008198 (ferrous iron binding) already accepted in this review.
GO:0008198 (ferrous iron binding, IEA GO_REF:0000118, enables, PANTHER:PTN002293406): ACCEPT — core cofactor-binding activity at the 3-His cupin triad (His90, His92, His143).
GO:0016702 (oxidoreductase activity, acting on single donors with incorporation of molecular oxygen, incorporation of two atoms of oxygen, IEA GO_REF:0000120, enables): ACCEPT — most specific existing GO molecular function for 3-mercaptopropionate dioxygenase (EC 1.13.11.66 / RHEA:53808); propose 3-mercaptopropionate dioxygenase activity in proposed_new_terms.
GO:0044273 (sulfur compound catabolic process, IMP from PMID:19001372, involved_in): NEW — directly demonstrated by Tn5::mob mutagenesis and enzyme assays in V. paradoxus TBEA6 (PMID:19001372).
GO:0005829 (cytosol, IC from file:VARPD/mdo/mdo-uniprot.txt, located_in): NEW — soluble 195-aa cytoplasmic enzyme purified from the soluble fraction (PMID:19001372).