(a) Domain arrangement in the extramembrane segment of the L,D-transpeptidase LdtMt2 from M. tuberculosis. TM, transmembrane helix. (b) Stereo cartoon of the structure of the BC module of LdtMt2. The catalytic domain is shown in grey and the B-domain in blue. The C-terminal extension that folds over the B domain is shown in green. The three tryptophan residues within this segment are shown as black sticks. (c) Stereo cartoon of the structure of the AB module of LdtMt2. The two immunoglobulin-like domains are shown in blue (B domain) and red (A domain).