FIGURE 1.

l,d-CPase activity of Csd6. A and B, mass spectra and structural formulas of the synthesized muramyl tetrapeptide (A) and muramyl pentapeptide (B). The observed peaks correspond to a series of sodium adduct ions of the peptides. C–E, mass spectra of the peptide samples upon incubation with Ldt_Mt2 and Csd6. In the control reaction catalyzed by Ldt_Mt2, the dimeric cross-linked tetra-tripeptide species is produced from the muramyl tetrapeptide (C). In contrast, in the reaction catalyzed by Csd6, the muramyl tetrapeptide is converted to the muramyl tripeptide but not to dimeric cross-linked tetra-tripeptide species (D). Upon incubation with Csd6, the muramyl pentapeptide undergoes no change (E). The observed m/z values for the charged species with bound sodium ions (up to four: [M + Na]⁺¹, [M + 2Na-H]⁺¹, [M + 3Na-2H]⁺¹, and [M + 4Na-3H]⁺¹) agree with the calculated m/z values for muramyl tri-, muramyl tetra-, muramyl penta-, or dimeric cross-linked tetra-tripeptides (average relative mass of neutral species = 679.67, 750.75, 821.83, or 1412.41, respectively).