Enzyme - Cytosine deaminase

Alternative Name(s)
  • Cytosine aminohydrolase.

Catalytic Activity

ytosine + H(+) + H2O = NH4(+) + uracil


There are no Cofactors for this Enzyme

Reaction Mechanisms

    Cytosine deaminase catalyses the hydrolytic deamination of cytosine to uracil. It will also catalyse the deamination of the prodrug 5-fluorocytosine. The is present in bacterial and fungal cells, where it plays an important role in pyrimidine salvage, but not in mammalian cells which use cytidine deaminase instead. The bacterial and yeast cytosine deaminases are have dissimilar folds and catalytic site architectures, and have evolved independently. Yeast cytosine deaminase is however structurally related to the extensively studied bacterial cytidine deaminase. It has a very similar catalytic apparatus and is thought to use the same mechanism.

    The key catalytic residues are Glu 64 and a zinc ion. Glu 64 first deprotonates the zinc-bound water molecule and protonates N3 of cytosine, thus activating both nucleophile and electrophile. Attack by the water molecule on C4 then generates a tetrahedral intermediate. Generation of the tetrahedral intermediate is thought to occur in this stepwise fashion since calculations suggest that a concerted mechanism would have a much higher energy barrier. Collapse of the tetrahedral intermediate involves deprotonation of the zinc-bound C4 hydroxyl followed by cleavage of the C4-N bond with concerted protonation of the departing amino group by Glu 64.
    The produced uracil is still coordinated to the zinc by O4. Calculations suggest that the energy barrier for cleavage of this bond is surprisingly high, so it is proposed that freeing of the uracil from the active site involves a gem-diol intermediate and oxygen-exchange. Formation of the gem-diol intermediate involves attack by a water molecule on C4, with concomitant deprotonation of this water molecule by Glu 64. Protonation of the the C4-O-Zn oxygen by Glu 64 is followed by cleavage of the C4-OHZn bond with simulataneous deprotonation of C4-OH by Glu 64.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Cys Q12178 94 1uaq 94
    His Q12178 62 1uaq 62
    Cys Q12178 91 1uaq 91
    Cys Q12178 91 1uaq 91
    Glu Q12178 64 1uaq 64
    Ser Q12178 89 1uaq 89
    Step Components

    cofactor used, intermediate formation, native state of cofactor regenerated, bimolecular nucleophilic addition, rate-determining step, proton transfer, native state of enzyme regenerated, overall reactant used, unimolecular elimination by the conjugate base, intermediate collapse, overall product formed

    Step 1.

    Glu64 deprotonates a zinc bound water.

    Step 2.

    N3 of cytosine accepts a proton from Glu64.

    Step 3.

    The deprotonated water can now nucleophilicaklly attack C4 of cytosine.

    Step 4.

    Glu64 deprotonates the hydroxyl.

    Step 5.

    The deprotonation of the hydroxyl results in the initiation of an elimination from the oxyanion. This results in the cleavage of the C3 amino group which accepts a proton from Glu64 to produce ammonia.

    Step 6.

    Glu64 deprotonates a water so that it can attack the C3 carbon of the zinc bound uracil.

    Step 7.

    Glu64 protonates the oxygen bound zinc to form the gemdiol intermediate.

    Step 8.

    Glu64 accepts a proton from the hydroxyl which isn't coordinated to zinc on C3 which results in the cleavage of the zinc bound hydroxyl from uracil.

    Step 9.

    The zinc bound hydroxyl deprotonates Glu64 which regenerates the cofactor to its native state and the active site.


    The products of the reaction.

Reaction Parameters

  • Kinetic Parameters
    Organism KM Value [mM] Substrate Comment
    Escherichia coli 0.004 cytosine mutant D313N, pH 8.5, 30°C
    Saccharomyces cerevisiae 0.0000014 5-fluorocytosine mutant E64A, pH 7.5, temperature not specified in the publication
  • Temperature
    Organism Temperature Range Comment
    Serratia marcescens 30 - 70
  • pH
    Organism pH Range Comment
    Salmonella enterica subsp. enterica serovar Typhimurium 6.5 - 9

Associated Proteins

Protein name Organism
Probable cytosine deaminase Fission yeast
Cytosine deaminase Escherichia coli (strain K12)
Hydroxydechloroatrazine ethylaminohydrolase protein Herbaspirillum seropedicae (strain SmR1)
Cytosine deaminase and related metal-dependent hydrolases Corynebacterium variabile
Strongly similar to fungal cytosine deaminase Kuenenia stuttgartiensis