Structural Insights into MltC from <i>Acinetobacter baumannii</i> : Conservation of the Catalytic Residue and Flexibility in Substrate Recognition

Lytic transglycosylases (LTs) are key enzymes involved in bacterial peptidoglycan remodeling. Here, we present the crystal structure of MltC from <i>Acinetobacter baumannii</i> (AbMltC), representing the second reported MltC structure after that of <i>Escherichia coli</i> (EcMltC). The AbMltC structure reveals a conserved catalytic residue, E224, equivalent to E217 of EcMltC, which directly participates in glycosidic bond cleavage. Notably, the substrate-binding residue R234, corresponding to R227 of EcMltC, is conserved in sequence but exhibits multiple conformations in AbMltC. This conformational heterogeneity suggests structural flexibility in substrate recognition and provides the structural insights consistent with prior hypothesis that R234 (R227 in EcMltC) functions as a molecular ratchet, facilitating processive cleavage.