D-alanyl-D-alanine moiety of peptidoglycan precursor (D-Ala-D-Ala)

The **D-alanyl-D-alanine moiety of peptidoglycan precursor** is a dipeptide structure found at the terminal end of the pentapeptide stem in peptidoglycan precursors of most Gram-positive and Gram-negative bacteria. It serves as the critical substrate for the transpeptidase enzymes (also known as penicillin-binding proteins), which cross-link the peptide stems and form the rigid, mesh-like cell wall. This moiety is the specific binding site for glycopeptide antibiotics such as vancomycin, which inhibit cell wall biosynthesis by forming stable complexes with the D-Ala-D-Ala dipeptide, thereby blocking access of transpeptidases and preventing proper cell wall cross-linking. Modification of this moiety to D-Ala-D-Lac or D-Ala-D-Ser is a known mechanism of resistance in clinical pathogens, dramatically reducing antibiotic binding affinity. The D-Ala-D-Ala motif is critical for bacterial survival, cell shape maintenance, and rigidity, and its biosynthesis involves dedicated ligases that join two D-alanine residues before attachment to the growing peptidoglycan chain. The presence or replacement of D-Ala-D-Ala is a key biomarker for antibiotic susceptibility and resistance in bacterial infections.

Other names

D-Ala-D-Ala dipeptideD-alanyl-D-alanine terminusD-Ala-D-Ala endD-alanine-D-alanine moiety

Inhibition of peptidoglycan cross-linking by direct binding to D-Ala-D-Ala, blocking transpeptidase activity (vancomycin and similar glycopeptides)\nCompetitive inhibition of D-Ala-D-Ala ligase by structural mimicry (β-lactams and D-cycloserine)