Cytidine deaminase is a cytosolic enzyme of the pyrimidine salvage pathways, which catalyzes the hydrolytic deamination of cytidine and 2’deoxycytidine to uridine and deoxyuridine. The cytidine deaminase functional unit can be either a homotetramer, found in human and Bacillus subtilis, or a homodimer, found in Escherichia coli and flowering plants. The first cytidine deaminase crystal structure was solved from the homodimeric E. coli enzyme evidencing, in each active site, the presence of an essential zinc ion tetrahedrally coordinated by one histidine and two cysteine residues. From the crystal structure of Bacillus subtilis cytidine deaminase and from site-directed mutagenesis, molecular modelling studies on human enzyme it was established that the homotetrameric cytidine deaminase is composed of four identical monomers of 14.9 kDa, each containing a zinc ion coordinated with three negatively charged cysteine residues. The active site of one monomer is formed by the contribution of residues coming from the other three subunits, therefore the monomer alone is completely inactive. The AID/APOBEC family is another branch of this family able to deaminate cytidine in DNA or RNA, exhibiting different physiological functions: AID and APOBEC-3 deaminate DNA to trigger pathways in adaptive and innate immunity, APOBEC1 mediates apolipoprotein B RNA editing. In this review will be considered the structure and the enzymatic properties of the cytidine deaminase family, in order to give a contribution on the understanding the mechanism of action of these enzymes.
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