Nucleoside deaminases include adenosine, guanine and cytosine deaminases. These enzymes are Zn dependent and catalyze the deamination of nucleosides. The zinc ion in the active site plays a central role in the proposed catalytic mechanism, activating a water molecule to form a hydroxide ion that performs a nucleophilic attack on the substrate. The functional enzyme is a homodimer. Cytosine deaminase catalyzes the deamination of cytosine to uracil and ammonia and is a member of the pyrimidine salvage pathway. Cytosine deaminase is found in bacteria and fungi but is not present in mammals; for this reason, the enzyme is currently of interest for antimicrobial drug design and gene therapy applications against tumors. Some members of this family are tRNA-specific adenosine deaminases that generate inosine at the first position of their anticodon (position 34) of specific tRNAs; this modification is thought to enlarge the codon recognition capacity during protein synthesis. Other members of the family are guanine deaminases which deaminate guanine to xanthine as part of the utilization of guanine as a nitrogen source.
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