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Title
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X-ray structure of recombinant ricin A-chain at 1.8 A resolution.
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Authors
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S.A.Weston,
A.D.Tucker,
D.R.Thatcher,
D.J.Derbyshire,
R.A.Pauptit.
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Ref.
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J Mol Biol, 1994,
244,
410-422.
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PubMed id
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Abstract
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Ricin is a potent plant toxin which acts by removing a specific adenine residue
from the ribosome. The X-ray crystal structure of a new, tetragonal crystal form
of the recombinant ricin A-chain diffracting to 1.8 A resolution has been
determined via molecular replacement methods and refined to a crystallographic
R-factor of 18.6%. The higher resolution electron density allowed improvements
to be made upon previously published models, resulting in an increase in the
assigned secondary structure of the protein. The enzyme adopts the same global
conformation in this crystal form with differences in detail due only partly to
crystal packing. The active site superimposes closely with those of previously
published models but the locations of the active-site water molecules differ in
this structure. To address the current mechanistic model, an additional two
structures are presented: recombinant ricin A-chain complexed with the substrate
analogue formycin monophosphate as well as with adenosine monophosphate, which
is cleaved by the crystalline enzyme. The formycin monophosphate displaces a
putative catalytic water molecule. This supports the notion that the analogue
does not bind in a transition state conformation and that contacts from other
elements of the 28 S RNA natural substrate are required to achieve full
reactivity. The structure of the adenosine monophosphate complex suggests a
mechanism for the release of the adenine product via of the side-chain Tyr80.
The structures suggest that Glu177 is better positioned for the activation of
the catalytic water molecule than Arg180.
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