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Title
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Crystal structures of the peanut lectin-lactose complex at acidic pH: retention of unusual quaternary structure, empty and carbohydrate bound combining sites, molecular mimicry and crystal packing directed by interactions at the combining site.
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Authors
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R.Ravishankar,
C.J.Thomas,
K.Suguna,
A.Surolia,
M.Vijayan.
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Ref.
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Proteins, 2001,
43,
260-270.
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PubMed id
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Abstract
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The crystal structures of a monoclinic and a triclinic form of the peanut
lectin-lactose complex, grown at pH 4.6, have been determined. They contain two
and one crystallographically independent tetramers, respectively. The unusual
"open" quaternary structure of the lectin, observed in the
orthorhombic complex grown in neutral pH, is retained at the acidic pH. The
sugar molecule is bound to three of the eight subunits in the monoclinic
crystals, whereas the combining sites in four are empty. The lectin-sugar
interactions are almost the same at neutral and acidic pH. A comparison of the
sugar-bound and free subunits indicates that the geometry of the combining site
is relatively unaffected by ligand binding. The combining site of the eighth
subunit in the monoclinic crystals is bound to a peptide stretch in a loop from
a neighboring molecule. The same interaction exists in two subunits of the
triclinic crystals, whereas density corresponding to sugar exists in the
combining sites of the other two subunits. Solution studies show that
oligopeptides with sequences corresponding to that in the loop bind to the
lectin at acidic pH, but only with reduced affinity at neutral pH. The reverse
is the case with the binding of lactose to the lectin. A comparison of the
neutral and acidic pH crystal structures indicates that the molecular packing in
the latter is directed to a substantial extent by the increased affinity of the
peptide loop to the combining site at acidic pH.
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