ABSTRACT
N-terminal dinucleotide binding motif (DBM, GXGXXG) reduces the ability of the enzyme to bind FAD and modify tRNA.46 GidA behaves as a homodimer in solution and physically interacts with MnmE, suggesting that MnmE and GidA form an a2p2 heterotetrameric complex.46 The three crystal structures of GidA (E. coli, Chlorobium an&Aquifex) revealed that the noncovalently, tightly-bound FAD is a genuine cofactor (ref. 79 and Bessho et al unpublished data) (Fig. 4). The overall fold of GidA is consistent with a global structure encompassing three domains. The main domain belongs to an FAD-binding domain with the classical Rossmann fold, which is charac teristic of a dinucleotide-binding fold. The second a/p domain is inserted between two strands of the Rossmann fold (Fig. 2C). The C-terminal domain is organized as an all-helical domain. A large-scale sequence and structural analysis classified the FAD-containing proteins into four different FAD-family folds, exemplified by glutathione reductase (GR), ferredoxin reductase (FR), p-cresol methylhydroxylase (PCMH) and pyruvate oxidase (PO).80 Among them, the FAD domain of GidA can be categorized in the GR family (especially, the GR2 subfamily), which is characterized as proteins with the DBM mainly at the N-terminus. Since the insertion domain, in the Rossmann fold of the FAD domain, shows some similarities to the presumed NADH-binding domain of phenol hydroxylase, also a member of the GR2 subfamily, it was proposed that this domain in GidA is an NADH-binding domain.79 GidA actually binds the NADH cofactor with high specificity, suggesting that it works as an initial donor of electrons.79 These features of GidA suggest that this protein catalyzes an oxidation-reduction reaction.
