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. 2020 Mar 21;8(3):447. doi: 10.3390/microorganisms8030447

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© 2020 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Determination of M4Lys cleavage sites. Analysis of E. coli MG1655 peptidoglycan digested by (A) mutanolysin and (B) mutanolysin with purified M4LysΔTMD. Soluble muropeptides were reduced and analyzed by RP-HPLC coupled to MS. Peaks corresponding to m/z values matching previously identified muropeptides are numbered. The fragmentation pattern (peaks 1, 2, and 3) is typical of Tri (L-Ala-D-Glu-m-DAP), Tetra (L-Ala-D-Glu-m-DAP-D-Ala), and Tetra-Tetra muropeptides, respectively. The fragmentation event leading to the loss of a nonreduced GlcNAc residue (203.078, theoretical mass; peaks 4 and 5) indicates the N-acetylmuramidase activity of M4LysΔTMD. Cleavage of D-Glu-mDAP crosslink (peak 6) shows the endopeptidase activity of M4LysΔTMD; (C) Inferred structures, theoretical monoisotopic masses, and theoretical and observed m/z values of individual peaks are tabulated. M^R, reduced MurNAc; G, GlcNAc; Di, m-DAP (meso-diaminopimelic acid)-D-Ala; Tri, L-Ala-D-Glu-m-DAP; and Tetra, L-Ala-D-Glu-m-DAP-D-Ala.