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D-Alanine-D-alanine ligase (Ddl) is one of the key enzymes in peptidoglycan biosynthesis and is an important target for drug discovery. The enzyme catalyzes the condensation of two D-­Ala molecules using ATP to produce D-Ala-D-Ala, which is the terminal peptide of a peptidoglycan monomer. The structures of five forms of the enzyme from Thermus thermophilus HB8 (TtDdl) were determined: unliganded TtDdl (2.3 Å resolution), TtDdl-adenylyl imidodiphosphate (2.6 Å), TtDdl-ADP (2.2 Å), TtDdl-ADP-D-Ala (1.9 Å) and TtDdl-ATP-D-Ala-D-Ala (2.3 Å). The central domain rotates as a rigid body towards the active site in a cumulative manner in concert with the local conformational change of three flexible loops depending upon substrate or product binding, resulting in an overall structural change from the open to the closed form through semi-open and semi-closed forms. Reaction-intermediate models were simulated using TtDdl-complex structures and other Ddl structures previously determined by X-ray methods. The catalytic process accompanied by the cumulative conformational change has been elucidated based on the intermediate models in order to provide new insights regarding the details of the catalytic mechanism.

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