Crystal structure of the apo form of D‐alanine: D‐alanine ligase (Ddl) from Thermus caldophilus: A basis for the substrate‐induced conformational changes

Introduction. D-alanine:D-alanine ligase (Ddl) catalyses the dimerization of D-alanine before its incorporation in peptidoglycan precursors. The synthesis of D-alanine:Dalanine begins with an attack on the first D-alanine by the -phosphate of adenosine triphosphate (ATP) to yield an acylphosphate. That is followed by attack by the amino group of the second D-alanine, which eliminates the phosphate and produces the D-alanine: D-alanine dipeptide. Peptidoglycan biosynthesis has long been an attractive target for antibacterial drugs, such as D-cycloserine, glycopeptide antibiotics (vancomycin and teicoplanins), and -lactams (penicillin and cephalosporins). Vancomycintype antibiotics, for example, bind directly to the D-alanine: D-alanine terminus, thereby inhibiting crosslinking by the transpeptidase. Notably, bacteria that show vancomycin resistance, which develops after prolonged clinical treatment with vancomycin, possess an inactive Ddl and rely on another ligase, D-alanine:D-lactate ligase (Van), which produces D-alanine:D-lactate rather than D-alanine:Dalanine for cell wall synthesis. The switch from D-alanine: D-alanine peptidoglycan termini to D-alanine:D-lactate results in the loss of crucial hydrogen bonding interactions that causes a 1000-fold reduction in vancomycin binding affinity. X-ray crystallographic studies of Ddl and Van have contributed significantly to our understanding of the ligand specificity these two enzymes and suggest that a His residue in Van plays a critical role. A positive charge on the side chain imidazole nitrogen of His would attract the negatively charged lactate to the second substrate binding site at pH values less than 7, but at higher pH values Van would predominantly synthesize D-alanine:D-alanine. In Ddl, a Tyr residue [Tyr216 in Escherichia coli (Eco) DdlB, Tyr232 in Thermus caldophilus (Tca) Ddl] occupies the same spatial position as the His residue, and the hydroxyl group of the Tyr interacts with the COOH-terminal of the second D-alanine substrate. The structure of Eco DdlB complexed with ADP/ phosphorylated phosphinate (PDB ID: 2DLN) or with ADP/phosphorylated phosphonate (PDB ID: 1IOV) has been determined, as have the structures of Leuconostoc mesenteroides (Lme) D-Alanine:D-Lactate ligase complexed with ADP and phosphinophosphate (PDB ID:1EHI) and Enterococcus faecium (Efa) VanA complexed with ADP and phosphinophosphate (PDB ID:1E4E). However, to analyze the reaction mechanisms of these enzymes and their associated conformational changes, it is necessary to know the structures of both the substrate-bound and substrate-free forms of these enzymes. Our aim in the present study, therefore, was to grow crystals of Ddl that diffracted to high resolution in the absence of substrates. Here we report the X-ray structure of TcaDdl resolved to a resolution of 1.9 A and describe the conformational differences of the apo structure, comparing it with the structures of the previously described transition state analogue complex.