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PDBsum entry 1ux1
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Structural, Kinetic, And mutational studies of the zinc ion environment in tetrameric cytidine deaminase.
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Authors
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E.Johansson,
J.Neuhard,
M.Willemoës,
S.Larsen.
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Ref.
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Biochemistry, 2004,
43,
6020-6029.
[DOI no: ]
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PubMed id
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Abstract
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The zinc-containing cytidine deaminase (CDA, EC 3.5.4.5) is a pyrimidine salvage
enzyme catalyzing the hydrolytic deamination of cytidine and 2'-deoxycytidine
forming uridine and 2'-deoxyuridine, respectively. Homodimeric CDA (D-CDA) and
homotetrameric CDA (T-CDA) both contain one zinc ion per subunit coordinated to
the catalytic water molecule. The zinc ligands in D-CDA are one histidine and
two cysteine residues, whereas in T-CDA zinc is coordinated to three cysteines.
Two of the zinc coordinating cysteines in T-CDA form hydrogen bonds to the
conserved residue Arg56, and this residue together with the dipole moments from
two alpha-helices partially neutralizes the additional negative charge in the
active site, leading to a catalytic activity similar to D-CDA. Arg56 has been
substituted by a glutamine (R56Q), the corresponding residue in D-CDA, an
alanine (R56A), and an aspartate (R56D). Moreover, one of the zinc-liganding
cysteines has been substituted by histidine to mimic D-CDA, alone (C53H) and in
combination with R56Q (C53H/R56Q). R56A, R56Q, and C53H/R56Q contain the same
amount of zinc as the wild-type enzyme. The zinc-binding capacity of R56D is
reduced. Only R56A, R56Q, and C53H/R56Q yielded measurable CDA activity, R56A
and R56Q with similar K(m) but decreased V(max) values compared to wild-type
enzyme. Because of dissociation into its inactive subunits, it was impossible to
determine the kinetic parameters for C53H/R56Q. R56A and C53H/R56Q display
increased apparent pK(a) values compared to the wild-type enzyme and R56Q. On
the basis of the structures of R56A, R56Q, and C53H/R56Q an explanation is
provided of kinetic results and the apparent instability of C53H/R56Q.
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