Choline kinase catalyzes the phosphorylation of choline by ATP, the first committed step in the CDP-choline pathway for phosphatidylcholine biosynthesis. To begin to elucidate the mechanism of catalysis by this enzyme, choline kinase A-2 from Caenorhabditis elegans was analyzed by systematic mutagenesis of highly conserved residues followed by analysis of kinetic and structural parameters. Specifically, mutants were analyzed with respect to K m and k cat values for each substrate and Mg 2؉ , inhibitory constants for Mg 2؉ and Ca 2؉ , secondary structure as monitored by circular dichroism, and sensitivity to unfolding in guanidinium hydrochloride. The most severe impairment of catalysis occurred with the modification of Asp-255 and Asn-260, which are located in the conserved Brenner's phosphotransferase motif, and Asp-301 and Glu-303, in the signature choline kinase motif. For example, mutation of Asp-255 or Asp-301 to Ala eliminated detectable catalytic activity, and mutation of Asn-260 and Glu-303 to Ala decreased k cat by 300-and 10-fold, respectively. Additionally, the K m for Mg 2؉ for mutants N260A and E303A was approximately 30-fold higher than that of wild type. Several other residues (Ser-86, Arg-111, Glu-125, and Trp-387) were identified as being important: Catalytic efficiencies (k cat /K m ) for the enzymes in which these residues were mutated to Ala were reduced to 2-25% of wild type. The high degree of structural similarity among choline kinase A-2, aminoglycoside phosphotransferases, and protein kinases, together with the results from this mutational analysis, indicates it is likely that these conserved residues are located at the catalytic core of choline kinase.