D-alanine racemase, D-alanine–D-alanine ligase (Alr, Ddl)

D-alanine racemase is an enzyme found in most bacteria that catalyzes the reversible conversion of L-alanine to D-alanine, a key precursor for bacterial cell wall (peptidoglycan) biosynthesis; it is a pyridoxal phosphate-dependent enzyme and is essential for bacterial viability[1][5][9]. D-alanine–D-alanine ligase (Ddl) catalyzes the ATP-dependent condensation of two molecules of D-alanine to produce the D-alanyl–D-alanine dipeptide, which is subsequently incorporated into the peptidoglycan pentapeptide and is also essential for cell wall cross-linking[2][4][7]. Both enzymes are critical for the integrity and survival of prokaryotes and are prominent targets for antibacterial drug development, particularly as they are absent in humans—making them attractive for selective therapeutic intervention[3][5][8][9]. The antibiotic D-cycloserine inhibits both enzymes, thereby disrupting bacterial cell wall biosynthesis and resulting in cell death[3][8]. Their essentiality in bacterial physiology and lack in higher eukaryotes underpins their value as validated antibacterial targets.

Other names

Alanine racemase (Alr)D-alanine:D-alanine ligase (Ddl)L-alanine racemaseAlanylalanine synthetaseAlanine:alanine ligaseD-alanyl-D-alanine synthetase

Enzyme inhibition (competitive, irreversible, and allosteric inhibition). D-cycloserine binds and inactivates both Alr and Ddl, blocking cell wall precursor synthesis and peptidoglycan cross-linking[3][8]. Some inhibitors form a stable covalent adduct with the enzyme cofactor in Alr (external aldimine/PLP adduct)[3].