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Figure 3.
FIG. 3. The active site. A, stereo view of the 2F[o] - F[c]
electron density map of the active site contoured at 1.5  level
and shown in cyan and the difference anomalous map for the zinc
ion contoured at 30 level and shown in
purple. The densities of the inhibitor molecule are highlighted
in green. The active-site residues and the inhibitor
(3,4-dihydrouracil (DHU)) are shown as ball-and-stick
representations, and the zinc ion is shown as a magenta sphere.
B, stereo view of the interaction networks in the active site.
There are six direct hydrogen bonds between the protein molecule
and the inhibitor (see The Active Site Architecture and
Substrate Binding). C, stereo view of a comparison of the active
sites of the yeast (magenta) and E. coli cytosine deaminase
(Protein Data Bank code 1K70 [PDB]
, green) based on superposition of the inhibitor
3,4-dihydrouracil. The residue numbering is labeled in the same
color for each protein. The bacterial enzymes utilize three
histidines and one aspartate for iron ligation (His61, His63,
His214, and Asp313). Asp313 also forms a hydrogen bond with the
attacking water molecule. Glu217 interacts with the N3 atom of
the pyrimidine ring, and Gln156 interacts with the O2 and N1
atoms.
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