The active site of the Ldt domain and the substrate-binding sites. (a) Active-site superposition of the apo (orange), mercury-derivatized (magenta) and meropenem-complexed (chain B, cyan) LdtMt2Δ130. Dotted lines denote interactions: hydrogen bonds to the catalytic triad (red) and the oxyanion hole (green), Ser351 with the oxyanion hole (black), His336 with Asn356 (black) and His352 with Cys354 Sγ and the main-chain carbonyl O atom of His352 in the mercury-derivatized model (purple). The covalently bound meropenem adduct in the meropenem complex is shown as a stick model. (b) Active-site superposition of meropenem-complexed LdtMt2 (chain B, cyan), LdtBs (green) and Ldtfm (grey) in the same view as in (a). (c, d) Electrostatic surface representations of the predicted binding sites for the donor substrate in the open conformation of mercury-derivatized LdtMt2Δ130 (c) and the acceptor substrate in the closed conformation of meropenem-complexed LdtMt2Δ130 (d) coloured as in Fig. 1(d). The surfaces of the binding site are represented with constituent residues as stick models. The movement of the imidazole ring of His352 in the apo form (orange) and the mercury-derivatized form (magenta) is presented in stick models with dotted surfaces in (c). The peptide bond of the donor substrate (meso-DAP3-D-Ala4) is schematically modelled into the active site in the open conformation. The terminus of the acceptor substrate (meso-DAP3) is schematically modelled into the active site in the closed conformation, with red dotted lines depicting a plausible site for recognizing the terminal amine of meso-DAP3.