Structure of a bulgecin-inhibited g-type lysozyme from the egg white of the Australian black swan. A comparison of the binding of bulgecin to three muramidases

Bulgecin A, a bacterial metabolite, has been shown to bind in the active-site groove of the chicken-type lysozyme from the rainbow trout (RBTL) and in the lysozyme-like C-terminal domain, of a soluble lytic transglycosylase (C-SLT) from Escherichia coli. These enzymes are muramidases that cleave the glycosidic bonds in the glycan strands of the murein polymer. Here we report the crystal structure of a complex between the goose-type lysozyme from the egg white of the Australian black swan (SEWL) and bulgecin A at 2.45 Å resolution. As is the case for the C-SLT/bulgecin and RBTL/bulgecin complexes, the ligand binds with the N-acetylglucosamine ring in subsite C and the proline moiety in site D where it interacts with the catalytic glutamic acid. The taurine residue interacts with the β-sheet region. Comparisons of the three buigecin complexes show that the inhibitor has the same binding mode to the muramidases with similar protein-ligand interactions, particularly for SEWL and RBTL. From our results, it seems likely that bulgecin, in general, inhibits enzymes with lysozyme-like domains and thus might represent a novel class of natural antibiotics that act on murein-degrading rather than murein-synthesizing enzymes.