Alanine racemase and D-alanine—D-alanine ligase (Alr and Ddl)

Alanine racemase is an essential bacterial enzyme that catalyzes the reversible conversion of L-alanine to D-alanine, a necessary precursor for the synthesis of peptidoglycan, which forms the structural backbone of most bacterial cell walls[1][2][4][8]. D-alanine—D-alanine ligase catalyzes the ATP-dependent formation of the D-alanyl-D-alanine dipeptide, an indispensable precursor incorporated into the peptidoglycan pentapeptide side chain during cell wall biosynthesis[5]. These enzymes are universally conserved among both Gram-positive and Gram-negative bacteria, but notably absent from higher eukaryotes, making them highly attractive and selective targets for antibacterial drug development[2][8]. D-cycloserine, an antibiotic used primarily to treat multidrug-resistant tuberculosis, inhibits both alanine racemase and D-alanine—D-alanine ligase, blocking cell wall synthesis and leading to bacterial cell death[3][7]. Given their unique presence in bacteria and vital roles in maintaining cell viability, these enzymes are the focus of antibiotic research, though the clinical application of inhibitors like D-cycloserine is limited by central nervous system side effects[7].

Other names

L-alanine racemaseD-alanine:D-alanine ligaseAlanylalanine synthetaseAlanine:alanine ligaseAlrDdl

Competitive inhibition of alanine racemase and D-alanine—D-alanine ligase by D-cycloserine, which is a structural analog of native substrates, leading to inhibition of peptidoglycan synthesis and cell death in bacteria[7][3][6][8] Irreversible inactivation of alanine racemase through adduct formation with pyridoxal phosphate cofactor (by D-cycloserine)[6] Formation of phosphorylated inhibitor on D-alanine—D-alanine ligase by D-cycloserine, disrupting D-Ala-D-Ala peptide bond synthesis[7]