Crystal Structure of Murein-Tripeptide Amidase MpaA from Escherichia coli O157 at 2.6 Å Resolution

Peptidoglycan (PG) is an essential component of the cell wall, and undergoes reconstruction by various PG hydrolases during cell growth, development and division. The murein- tripeptide (Mtp) amidase MpaA belongs to PG hydrolase family and is responsible for cleaving the γ-D-Glumeso- Dap amide bond in the Mtp released during PG turnover. The current paper reports the crystal structure of MpaA from Escherichia coli (E. coli) O157 at 2.6 Å resolution. The asymmetric unit consists of two protein molecules and each monomer represents the common α/β fold of metallocarboxypeptidases (MCP). The Tyr133-Asp143 loop appears to mediate the entrance and binding of the substrate into the active groove. A structural comparison of MpaA with its homologue from Vibrio harveyi showed that MpaA has narrower active pocket entrance with a smaller surface opening, which is determined by the Val204-Thr211 loop. The reported structure provides a starting point for the molecular mechanism of MpaA in a significant human pathogen.