The x-ray crystallographic structure of selenomethionyl cytosine-5-monophosphate-acylneuraminate synthetase (CMP-NeuAc synthetase) from Neisseria meningitidis has been determined at 2.0-Å resolution using multiple-wavelength anomalous dispersion phasing, and a second structure, in the presence of the substrate analogue CDP, has been determined at 2.2-Å resolution by molecular replacement. This work identifies the active site residues for this class of enzyme for the first time. The detailed interactions between the enzyme and CDP within the mononucleotide-binding pocket are directly observed, and the acylneuraminate-binding pocket has also been identified. A model of acylneuraminate bound to CMP-NeuAc synthetase has been constructed and provides a structural basis for understanding the mechanism of production of "activated" sialic acids. Sialic acids are key saccharide components on the surface of mammalian cells and can be virulence factors in a variety of bacterial species (e.g. Neisseria, Haemophilus, group B streptococci, etc.). As such, the identification of the bacterial CMP-NeuAc synthetase active site can serve as a starting point for rational drug design strategies.Cytosine-5Ј-monophosphate-acylneuraminate synthetase (cytosine-5Ј-monophosphate-N-acetyl neuraminic acid synthetase, CMP-NeuAc synthetase) 1 catalyzes the penultimate step in the addition of sialic acids to the oligosaccharide component of glycoconjugates and is a required component of sialylation pathways. CMP-NeuAc synthetase-deficient mutants do not express sialylated glycoconjugates and can be complemented with functional CMP-NeuAc synthetase in both mammalian (1) and bacterial systems (2). Sialic acids participate in a myriad of signaling, recognition, and cell-cell adhesion phenomena in mammalian cells (3) and are overexpressed in some highly malignant tumors (4). Genetic disorders that lead to altered physiological levels of sialic acids have many consequences in humans and can be fatal (5). In bacterial systems, sialic acids are less common and frequently have roles as virulence factors (6). In Neisseria gonorrhoeae, sialylated glycoconjugates protect the organism from phagocytosis and increase serum resistance (7). In Haemophilus ducreyi the presence of 2-keto-3-deoxy-manno-octulosonate-containing glycoconjugates correlate with the organism's pathogenicity (8). Likewise, the sialylated capsule of Neisseria meningitidis, Escherichia coli K1, and group B streptococci are virulence factors (9-11). It has been suggested that bacterial species produce sialylated glycoconjugates to mimic the host and escape detection by the immune system (12). Clearly, the mechanism of production of sialic acid-containing glycoconjugates is of clinical interest. Although bacterial and eucaryotic CMPNeuAc synthetase enzymes share many catalytic properties, several important differences have been reported, including substrate specificity, tertiary structure, inhibitor sensitivity, and cellular localization (13). As a result, bacterial CMP-NeuAc synthetase en...