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Title
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Studies on crystal structures, active-centre geometry and depurinating mechanism of two ribosome-inactivating proteins.
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Authors
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Q.Huang,
S.Liu,
Y.Tang,
S.Jin,
Y.Wang.
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Ref.
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Biochem J, 1995,
309,
285-298.
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PubMed id
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Abstract
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Two ribosome-inactivating proteins, trichosanthin and alpha-momorcharin, have
been studied in the forms of complexes with ATP or formycin, by an
X-ray-crystallographic method at 1.6-2.0 A (0.16-0.20 nm) resolution. The native
alpha-momorcharin had been studied at 2.2 A resolution. Structures of
trichosanthin were determined by a multiple isomorphous replacement method.
Structures of alpha-momorcharin were determined by a molecular replacement
method using refined trichosanthin as the searching model. Small ligands in all
these complexes have been recognized and built on the difference in electron
density. All these structures have been refined to achieve good results, both in
terms of crystallography and of ideal geometry. These two proteins show
considerable similarity in their three-dimensional folding and to that of
related proteins. On the basis of these structures, detailed geometries of the
active centres of these two proteins are described and are compared with those
of related proteins. In all complexes the interactions between ligand atoms and
protein atoms, including hydrophobic forces, aromatic stacking interactions and
hydrogen bonds, are found to be specific towards the adenine base. The
relationship between the sequence conservation of ribosome-inactivating proteins
and their active-centre geometry was analysed. A depurinating mechanism of
ribosome-inactivating proteins is proposed on the basis of these results. The
N-7 atom of the substrate base group is proposed to be protonated by an acidic
residue in the active centre.
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