Big_5 domain of LdtcMt2. (a) Topology diagrams of the Big_5 domain (His150–Gly250) of LdtMt2Δ130 (inset) and of four distinct subtypes of the Ig-like fold (modified from Bork et al., 1994): c-type (constant), v-type (variable), s-type (switched) and h-type (hybrid). The four-stranded structural core (strands b, c, e, and f; β2, β3, β5 and β6 of LdtMt2Δ130) common to all Ig-like domains (orange) is surrounded by structurally more variable strands (green). (b, c) Ribbon diagrams of the Big_5 domain (His150–Gly250) in the apo model of LdtMt2Δ130 (b) and meropenem-complexed LdtMt2Δ130 (c), and enlarged views of the β6–β7 loop (insets) coloured as in (a). The bound calcium ion and the residues around it (Asp232–Met237) are shown as a purple ball and as stick models, respectively, with a 2mFo − DF electron-density map (contoured at 1.5σ).