Peptidoglycan recognition proteins (PGRPs) are pattern recognition receptors that bind, and in certain cases, hydrolyze peptidoglycans (PGNs) of bacterial cell walls. PGRPs have been divided into three classes: short PGRPs (PGRP-S), that are small (20 kDa) extracellular proteins; intermediate PGRPs (PGRP-I) that are 40-45 kDa and are predicted to be transmembrane proteins; and long PGRPs (PGRP-L), up to 90 kDa, which may be either intracellular or transmembrane. Several structures of PGRPs are known in insects and mammals, some bound with substrates like Muramyl Tripeptide (MTP) or Tracheal Cytotoxin (TCT). The substrate binding site is conserved in PGRP-LCx, PGRP-LE, and PGRP-Ialpha proteins. This family includes Zn-dependent N-Acetylmuramoyl-L-alanine Amidase, EC:18.104.22.168. This enzyme cleaves the amide bond between N-acetylmuramoyl and L-amino acids, preferentially D-lactyl-L-Ala, in bacterial cell walls. The structure for the bacteriophage T7 lysozyme shows that two of the conserved histidines and a cysteine are zinc binding residues. Site-directed mutagenesis of T7 lysozyme indicates that two conserved residues, a Tyr and a Lys, are important for amidase activity.