Endo-β-N-acetylmuramidases, commonly known as lysozymes, are well-characterized antimicrobial enzymes that catalyze an endo-lytic cleavage of peptidoglycan; i.e., they hydrolyze
the β-1,4-glycosidic bonds connecting N-acetylmuramic acid
(MurNAc) and N-acetylglucosamine (GlcNAc). In contrast, little
is known about exo-β-N-acetylmuramidases, which catalyze
an exo-lytic cleavage of β-1,4-MurNAc entities from the
non-reducing ends of peptidoglycan chains. Such an enzyme
was identified earlier in the bacterium Bacillus subtilis, but the
corresponding gene has remained unknown so far. We now
report that ybbC of B. subtilis, renamed namZ, encodes the
reported exo-β-N-acetylmuramidase. A ΔnamZ mutant accumulated specific cell wall fragments and showed growth defects
under starvation conditions, indicating a role of NamZ in cell
wall turnover and recycling. Recombinant NamZ protein
specifically hydrolyzed the artificial substrate para-nitrophenyl
β-MurNAc and the peptidoglycan-derived disaccharide MurNAc-β-1,4-GlcNAc. Together with the exo-β-N-acetylglucosaminidase NagZ and the exo-muramoyl-L-alanine amidase AmiE,
NamZ degraded intact peptidoglycan by sequential hydrolysis
from the non-reducing ends. A structure model of NamZ, built
on the basis of two crystal structures of putative orthologs from
Bacteroides fragilis, revealed a two-domain structure including a
Rossmann-fold-like domain that constitutes a unique glycosidase fold. Thus, NamZ, a member of the DUF1343 protein
family of unknown function, is now classified as the founding
member of a new family of glycosidases (CAZy GH171; www.
cazy.org/GH171.html). NamZ-like peptidoglycan hexosaminidases are mainly present in the phylum Bacteroidetes and less
frequently found in individual genomes within Firmicutes
(Bacilli, Clostridia), Actinobacteria, and γ-proteobacteria