D-alanyl-D-alanine terminus of cell wall precursor unit

The D-alanyl-D-alanine terminus of cell wall precursor units is a critical structural motif found at the end of the peptide side chains in bacterial peptidoglycan precursors. This dipeptide sequence forms part of the pentapeptide stem attached to N-acetylmuramic acid residues within peptidoglycan, which provides mechanical strength and rigidity to bacterial cell walls. The synthesis involves enzymes such as D-Ala–D-Ala ligase (ddlA) that catalyze formation of this dipeptide from two molecules of D-alanine. The terminal D-Ala–D-Ala serves as a substrate for transpeptidases during cross-linking reactions that stabilize the mature cell wall. This site is also an important therapeutic target because several classes of antibiotics exploit its unique presence in bacteria. Glycopeptides like vancomycin bind directly to this moiety, preventing proper cross-linking by sterically hindering access by transglycosylases and transpeptidases. β-Lactam antibiotics act indirectly by mimicking this structure, thereby inhibiting penicillin-binding proteins involved in cross-linking. Resistance can arise when bacteria replace one or both terminal alanines with other amino acids such as lactate or serine, reducing drug binding affinity. This molecular feature does not exist in mammalian cells—making it an ideal selective antibacterial target with minimal risk for direct toxicity to human tissues.

Other names

D-Ala-D-Ala terminusD-alanyl-D-alanine dipeptidePeptidoglycan pentapeptide C-terminal dipeptide

Glycopeptide antibiotics (e.g., vancomycin) bind directly to the D-alanyl-D-alanine terminus, blocking transpeptidation and transglycosylation steps required for cross-linking peptidoglycan strands.