DcsB, one of the enzymes encoded in the d-cycloserine (d-CS) biosynthetic gene cluster, displays a high sequence homology to arginase, which contains two manganese ions in the active site. However, DcsB hydrolyzes N ! -hydroxy-larginine, but not l-arginine, to supply hydroxyurea for the biosynthesis of d-CS.Here, the crystal structure of DcsB was determined at a resolution of 1.5 Å using anomalous scattering from the manganese ions. In the crystal structure, DscB generates an artificial dimer created by the open and closed forms. Gel-filtration analysis demonstrated that DcsB is a monomeric protein, unlike arginase, which forms a trimeric structure. The active center containing the binuclear manganese cluster differs between DcsB and arginase. In DcsB, one of the ligands of the Mn A ion is a cysteine, while the corresponding residue in arginase is a histidine. In addition, DcsB has no counterpart to the histidine residue that acts as a general acid/base during the catalytic reaction of arginase. The present study demonstrates that DcsB has a unique active site that differs from that of arginase.
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