This family is the delta subunit of the nitrate reductase enzyme, The delta subunit is not part of the nitrate reductase enzyme but is most likely needed for assembly of the multi-subunit enzyme complex [1]. In the absence of the delta subunit the core alpha beta enzyme complex is unstable [1]. The delta subunit is essential for enzyme activity in vivo and in vitro [1]. The nitrate reductase enzyme, EC:1.7.99.4 catalyse the conversion of nitrite to nitrate via the reduction of an acceptor. The nitrate reductase enzyme is composed of three subunits [1]. Nitrate is the most widely used alternative electron acceptor after oxygen [1]. This family also now contains the family TorD, a family of cytoplasmic chaperone proteins; like many prokaryotic molybdoenzymes, the TMAO reductase (TorA) of Escherichia coli requires the insertion of a bis(molybdopterin guanine dinucleotide) molybdenum (bis(MGD)Mo) cofactor in its catalytic site to be active and translocated to the periplasm. The TorD chaperone increases apoTorA activation up to four-fold, allowing maturation of most of the apoprotein. Therefore TorD is involved in the first step of TorA maturation to make it competent to receive the cofactor [2].

1: Identification and characterization of the Staphylococcus carnosus nitrate reductase operon. Pantel I, Lindgren PE, Neubauer H, Gotz F; Mol Gen Genet 1998;259:105-114. PMID:9738886

2: Involvement of a mate chaperone (TorD) in the maturation pathway of molybdoenzyme TorA. Ilbert M, Mejean V, Giudici-Orticoni MT, Samama JP, Iobbi-Nivol C; J Biol Chem 2003;23:1-2. PMID:12766163