Cytidine Deaminases Catalyze the Conversion of N(S,O)4-substituted Pyrimidine Nucleosides.

Cytidine deaminases (CDAs) catalyze the hydrolytic deamination of cytidine and 2′-deoxycytidine to uridine and 2′-deoxyuridine. Here, we report that prokaryotic homo-tetrameric CDAs catalyze the nucleophilic substitution at the fourth position of N 4 -acyl-cytidines, N 4 -alkyl-cytidines, and N 4 -alkyloxycarbonyl-cytidines, and S 4 -alkylthio-uridines and O 4 -alkyl-uridines, converting them to uridine and corresponding amide, amine, carbamate, thiol, or alcohol as leaving groups. The x-ray structure of a metagenomic CDA_F14 and the molecular modeling of the CDAs used in this study show a relationship between the bulkiness of a leaving group and the volume of the binding pocket, which is partly determined by the flexible β3α3 loop of CDAs. We propose that CDAs that are active toward a wide range of substrates participate in salvage and/or catabolism of variously modified pyrimidine nucleosides. This identified promiscuity of CDAs expands the knowledge about the cellular turnover of cytidine derivatives, including the pharmacokinetics of pyrimidine-based prodrugs.