3T36: Crystal structure of lytic transglycosylase MltE from Eschericha coli

Citation:
Abstract
The lytic transglycosylase MltE from Escherichia coli is a periplasmic, outer membrane-attached enzyme that cleaves the beta-1,4-glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine residues in the cell wall peptidoglycan, producing 1,6-anhydromuropeptides. Here we report three crystal structures of MltE: in a substrate-free state, in a binary complex with chitopentaose, and in a ternary complex with the glycopeptide inhibitor bulgecin A and the murodipeptide N-acetylglucosaminyl-N-acetylmuramyl-l-Ala-d-Glu. The substrate-bound structures allowed a detailed analysis of the saccharide-binding interactions in six subsites of the peptidoglycan-binding groove (subsites -4 to +2) and, combined with site-directed mutagenesis analysis, confirmed the role of Glu64 as catalytic acid/base. The structures permitted the precise modeling of a short glycan strand of eight saccharide residues, providing evidence for two additional subsites (+3 and +4) and revealing the productive conformational state of the substrate at subsites -1 and +1, where the glycosidic bond is cleaved. Full accessibility of the peptidoglycan-binding groove and preferential binding of an N-acetylmuramic acid residue in a (4)C(1) chair conformation at subsite +2 explain why MltE shows only endo- and no exo-specific activity toward glycan strands. The results further indicate that catalysis of glycosidic bond cleavage by MltE proceeds via distortion toward a sofa-like conformation of the N-acetylmuramic acid sugar ring at subsite -1 and by anchimeric assistance of the sugar's N-acetyl group, as shown previously for the lytic transglycosylases Slt70 and MltB.
PDB ID: 3T36Download
MMDB ID: 101416
PDB Deposition Date: 2011/7/24
Updated in MMDB: 2012/12
Experimental Method:
x-ray diffraction
Resolution: 2.25  Å
Source Organism:
Similar Structures:
Biological Unit for 3T36: monomeric; determined by author and by software (PISA)
Molecular Components in 3T36
Label Count Molecule
Protein (1 molecule)
1
Endo-type Membrane-bound Lytic Murein Transglycosylase a(Gene symbol: emtA)
Molecule annotation
Chemical (1 molecule)
1
1
* Click molecule labels to explore molecular sequence information.

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