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PDBsum entry 1a9p
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Pentosyltransferase
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PDB id
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1a9p
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References listed in PDB file
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Key reference
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Title
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Calf spleen purine nucleoside phosphorylase complexed with substrates and substrate analogues.
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Authors
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C.Mao,
W.J.Cook,
M.Zhou,
A.A.Federov,
S.C.Almo,
S.E.Ealick.
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Ref.
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Biochemistry, 1998,
37,
7135-7146.
[DOI no: ]
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PubMed id
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Abstract
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Purine nucleoside phosphorylase (PNP) is a key enzyme in the purine salvage
pathway, which provides an alternative to the de novo pathway for the
biosynthesis of purine nucleotides. PNP catalyzes the reversible phosphorolysis
of 2'-deoxypurine ribonucleosides to the free bases and 2-deoxyribose
1-phosphate. Absence of PNP activity in humans is associated with specific
T-cell immune suppression. Its key role in these two processes has made PNP an
important drug design target. We have investigated the structural details of the
PNP-catalyzed reaction by determining the structures of bovine PNP complexes
with various substrates and substrate analogues. The preparation of
phosphate-free crystals of PNP has allowed us to analyze several novel
complexes, including the ternary complex of PNP, purine base, and ribose
1-phosphate and of the completely unbound PNP. These results provide an atomic
view for the catalytic mechanism for PNP proposed by M. D. Erion et al. [(1997)
Biochemistry 36, 11735-11748], in which an oxocarbenium intermediate is
stabilized by phosphate and the negative charge on the purine base is stabilized
by active site residues. The bovine PNP structure reveals several new details of
substrate and inhibitor binding, including two phosphate-induced conformational
changes involving residues 33-36 and 56-69 and a previously undetected role for
His64 in phosphate binding. In addition, a well-ordered water molecule is found
in the PNP active site when purine base or nucleoside is also present. In
contrast to human PNP, only one phosphate binding site was observed. Although
binary complexes were observed for nucleoside, purine base, or phosphate, ribose
1-phosphate binding occurs only in the presence of purine base.
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