title: Pseudomonas aeruginosa LD-carboxypeptidase, a serine peptidase with a Ser-His-Glu triad and a nucleophilic elbow.
abstract: LD-Carboxypeptidases (EC 3.4.17.13) are named for their ability to cleave amide bonds between l- and d-amino acids, which occur naturally in bacterial peptidoglycan. They are specific for the link between meso-diaminopimelic acid and d-alanine and therefore degrade GlcNAc-MurNAc tetrapeptides to the corresponding tripeptides. As only the tripeptides can be reused as peptidoglycan building blocks, ld-carboxypeptidases are thought to play a role in peptidoglycan recycling. Despite the pharmaceutical interest in peptidoglycan biosynthesis, the fold and catalytic type of ld-carboxypeptidases are unknown. Here, we show that a previously uncharacterized open reading frame in Pseudomonas aeruginosa has ld-carboxypeptidase activity and present the crystal structure of this enzyme. The structure shows that the enzyme consists of an N-terminal beta-sheet and a C-terminal beta-barrel domain. At the interface of the two domains, Ser(115) adopts a highly strained conformation in the context of a strand-turn-helix motif that is similar to the "nucleophilic elbow" in alphabeta-hydrolases. Ser(115) is hydrogen-bonded to a histidine residue, which is oriented by a glutamate residue. All three residues, which occur in the order Ser-Glu-His in the amino acid sequence, are strictly conserved in naturally occurring ld-carboxypeptidases and cannot be mutated to alanines without loss of activity. We conclude that ld-carboxypeptidases are serine peptidases with Ser-His-Glu catalytic triads.
authors.affiliation: International Institute of Molecular and Cell Biology, Ulica Trojdena 4, 02-109 Warsaw, Poland.
authors.forename: Henryk J
authors.initials: HJ
authors.lastname: Korza
authors.forename: Matthias
authors.initials: M
authors.lastname: Bochtler
mesh_terms: D000409:Alanine / Q000378:metabolism; D000595:Amino Acid Sequence; D001665:Binding Sites; D002268:Carboxypeptidases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017124:Conserved Sequence; D003460:Crystallization; D019281:Dimerization; D004926:Escherichia coli / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D016366:Open Reading Frames; D010446:Peptide Fragments / Q000737:chemistry; D017433:Protein Structure, Secondary; D011550:Pseudomonas aeruginosa / Q000201:enzymology*; D011994:Recombinant Proteins; D012996:Solutions; D013379:Substrate Specificity
chemical_list: D010446:Peptide Fragments; D011994:Recombinant Proteins; D012996:Solutions; D002268:Carboxypeptidases; C021505:LD-carboxypeptidase; D000409:Alanine