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Title
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Crystal structure of abrin-a at 2.14 A.
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Authors
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T.H.Tahirov,
T.H.Lu,
Y.C.Liaw,
Y.L.Chen,
J.Y.Lin.
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Ref.
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J Mol Biol, 1995,
250,
354-367.
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PubMed id
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Abstract
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The crystal structure of abrin-a, a type II ribosome-inactivating protein from
the seeds of Abrus precatorius, has been determined from a novel crystalline
form by the molecular replacement method using the coordinates of ricin. The
structure has been refined at 2.14 A to a R-factor of 18.9%. The
root-mean-square deviations of bond lengths and angles from the standard values
are 0.013 A and 1.82 degrees, respectively. The overall protein folding is
similar to that of ricin, but there are differences in the secondary structure,
mostly of the A-chain. Several parts of the molecular surface differ
significantly; some of them are quite near the active site cleft, and probably
influence ribosome recognition. The positions of invariant active site residues
remain the same, except the position of Tyr74. Two water molecules of
hydrogen-bonded active site residues have been located in the active site cleft.
Both of them may be responsible for hydrolyzing the N-C glycosidic bond. The
current abrin-a structure is lactose free; this is probably essential for
abrin-a crystallization. The B-chain is a glycoprotein, and the positions of
several sugar residues of two sugar chains linked to earlier predicted
glycosylation sites were determined. One of the sugar chains is a bridge between
two neighboring molecules, since one of its mannose residues is connected to the
galactose binding site of the neighboring molecule. Another sugar chain covers
the surface of the B-chain.
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