D-glutamate is an essential building block of the peptidoglycan layer in bacterial cell walls and can be synthesized from L-glutamate by glutamate racemase (RacE). The structure of a complex of B. subtilis RacE with D-glutamate reveals that the glutamate is buried in a deep pocket, whose formation at the interface of the enzyme's two domains involves a large-scale conformational rearrangement. These domains are related by pseudo-2-fold symmetry, which superimposes the two catalytic cysteine residues, which are located at equivalent positions on either side of the alpha carbon of the substrate. The structural similarity of these two domains suggests that the racemase activity of RacE arose as a result of gene duplication. The structure of the complex is dramatically different from that proposed previously and provides new insights into the RacE mechanism and an explanation for the potency of a family of RacE inhibitors, which have been developed as novel antibiotics.

Glutamate racemase (MurI, RacE; E.C.5.1.1.3) catalyses the cofactorindependent conversion of l-glutamate to d-glutamate, an essential step in the synthesis of components of the bacterial cell wall. The gene for RacE from Bacillus subtilis has been cloned and the protein expressed in Escherichia coli, puri®ed and crystallized in the presence of l-glutamate using the hanging-drop method of vapour diffusion with diammonium tartrate as the precipitant. The crystals belong to the monoclinic space group C2, with approximate unit-cell parameters a = 133.6, b = 60.1, c = 126.2 A Ê , = 117.6 . Consideration of the possible values of V M suggests that the asymmetric unit contains either two (V M = 3.75 A Ê 3 Da À1 ) or three (V M = 2.5 A Ê 3 Da À1 ) subunits. The crystals diffract X-rays to at least 2.1 A Ê resolution on a synchrotron-radiation source and are suitable for structural studies. Determination of the structure may provide insight into the molecular basis of substrate recognition and catalysis by this enzyme.

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