Crystal Structure of MltA from Escherichia coli Reveals a Unique Lytic Transglycosylase Fold

Lytic transglycosylases are bacterial enzymes involved in the maintenance and growth of the bacterial cell-wall peptidoglycan. They cleave the β-(1,4)-glycosidic bonds in peptidoglycan forming non-reducing 1,6-anhydromuropeptides. The crystal structure of the lytic transglycosylase MltA from Escherichia coli without a membrane anchor was solved at 2.0 Å resolution. The enzyme has a fold completely different from those of the other known lytic transglycosylases. It contains two domains, the largest of which has a double-psi β-barrel fold, similar to that of endoglucanase V from Humicola insolens. The smaller domain also has a β-barrel fold topology, which is weakly related to that of the RNA-binding domain of ribosomal proteins L25 and TL5. A large groove separates the two domains, which can accommodate a glycan strand, as shown by molecular modelling. Several conserved residues, one of which is in a position equivalent to that of the catalytic acid of the H. insolens endoglucanase, flank this putative substrate-binding groove. Mutation of this residue, Asp308, abolished all activity of the enzyme, supporting the direct participation of this residue in catalysis.