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Title
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Three-dimensional structure of human erythrocytic purine nucleoside phosphorylase at 3.2 A resolution.
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Authors
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S.E.Ealick,
S.A.Rule,
D.C.Carter,
T.J.Greenhough,
Y.S.Babu,
W.J.Cook,
J.Habash,
J.R.Helliwell,
J.D.Stoeckler,
R.E.Parks.
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Ref.
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J Biol Chem, 1990,
265,
1812-1820.
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PubMed id
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Abstract
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The three-dimensional structure of human erythrocytic purine nucleoside
phosphorylase has been determined at 3.2 A resolution using x-ray diffraction
data. Intensity data were measured using radiation from the Synchrotron
Radiation Source, Daresbury, England, and oscillation film techniques. Phases
were determined by using multiple isomorphous replacement methods with four
heavy-atom derivatives and were improved using solvent flattening techniques.
Purine nucleoside phosphorylase exists in the crystal as a trimer in which
subunits are related by a crystallographic 3-fold axis. Each subunit contains an
eight-stranded mixed beta-sheet and a five-stranded mixed beta-sheet which join
to form a distorted beta-barrel structure. This core beta-structure is flanked
by seven alpha-helices in a manner that generates a novel folding pattern. The
active site, which was characterized from binding of the substrate analogs
8-iodoguanine and 5'-iodoformycin B, is located near the subunit-subunit
boundary within the trimer and involves seven different segments from one
subunit and an additional short segment from an adjacent subunit. In the
crystal, the phosphate-binding site is probably occupied by a sulfate ion. The
specificity of purine nucleoside phosphorylase for guanine, hypoxanthine, and
their analogs can be explained on the basis of the arrangement of hydrogen bond
donors and acceptors in the active site.
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