STRUCTURE AND MECHANISM OF SLT70, A BACTERIAL MURAMIDASE Bauke W. Dijkstra, Andy-Mark W. H. Thunnissen, Erik van Asselt BIOSON Research Institute and Laboratory of Biophysical Chemistry Groningen University, Nijenborgh 4, 9747 AG Groningen, the Netherlands
The main component of the bacterial cell wall is murein or peptidoglycan, a polymer built up of linear glycan strands consisting of alternating N-acetylglucosamine (GlcNAc) and N-acetyl muramic acid (MurNAc) residues, which are cross-linked by short peptides. Inhibition of the enzymes involved in murein-synthesis and degradation by e.g. antibiotics is often lethal to the bacterium1.
The 70 kDa soluble lytic transglycosylase from E. coli (SLT70) catalyzes the cleavage of the beta-1,4-glycosidic bond between MurNAc and GlcNAc. Its 3D-structure has been elucidated at 1.65 Å resolution. The unusually doughnut-shaped SLT70 molecule is built up of three distinct domains, which are all very rich in a-helices. The C-terminal domain (residues 451 to 618) contains the active site; this domain resembles the fold of lysozyme2,3. To confirm a lysozyme-like reaction mechanism crystallographic binding studies were carried out with bulgecin A4, (GlcNAc)3, and a natural 1,6-anhydro-muropeptide product. All compounds bind in the active site of SLT. Bulgecin A is a glucosamine containing molecule that enhances the bulging activity of many beta-lactam inhibitors. Its glucosaminyl part is bound at subsite C, close to the position of the GlcNAc residue in the active sites of lysozymes. The (GlcNAc)3 is also bound in the active site of SLT, and extends over subsites B, C and D. The two saccharide moieties (GlcNAc-beta(1->4)-1,6-anhydro-MurNAc) of the muropeptide product are bound near subsites E and F in the SLT70 active site groove. Combination of the results obtained with these three substrate analogues allows a detailed picture of how a natural peptidoglycan polymer binds in the active site.
1Thunnissen, A.-M.W.H. & Dijkstra, B.W. Nature Struct. Biol. 3, 218-221 (1996)
2Thunnissen, A.-M.W.H. et al. Nature 367, 750-753 (1994)
3Thunnissen, A.-M.W.H. et al. PROTEINS: Struct. Funct. Genet. 22, 245-258 (1995)
4Thunnissen, A.-M.W.H. et al. Biochemistry 34, 12729-12737 (1995)