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Title
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Conformational changes in cubic insulin crystals in the pH range 7-11.
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Authors
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O.Gursky,
J.Badger,
Y.Li,
D.L.Caspar.
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Ref.
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Biophys J, 1992,
63,
1210-1220.
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PubMed id
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Abstract
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To determine the effect of variations in the charge distribution on the
conformation of a protein molecule, we have solved the structures of bovine
cubic insulin over a pH range from 7 to 11 in 0.1 M and 1 M sodium salt
solutions. The x-ray data were collected beyond 2-A resolution and the R factors
for the refined models ranged from 0.16 to 0.20. Whereas the positions of most
protein and well-ordered solvent atoms are conserved, about 30% of residues
alter their predominant conformation as the pH is changed. Conformational
switching of A5 Gln and B10 His correlates with the pH dependence of monovalent
cation binding to insulin in cubic crystals. Shifts in the relative positions of
the A chain NH2-terminal and B chain COOH-terminal groups are probably due to
titration of the A1 alpha-amino group. Two alternative positions of B25 Phe and
A21 Asn observed in cubic insulin at pH 11 are similar to those found in two
independent molecules of the 2Zn insulin dimer at pH 6.4. The conformational
changes of the insulin amino acids appear to be only loosely coupled at distant
protein sites. Shifts in the equilibrium between distinct conformational
substates as the charge distribution on the protein is altered are analogous to
the electrostatically triggered movements that occur in many functional protein
reactions.
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