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PDBsum entry 1ypc
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Proteinase inhibitor(chymotrypsin)
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PDB id
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1ypc
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Direct observation of better hydration at the n terminus of an alpha-Helix with glycine rather than alanine as the n-Cap residue.
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Authors
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Y.Harpaz,
N.Elmasry,
A.R.Fersht,
K.Henrick.
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Ref.
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Proc Natl Acad Sci U S A, 1994,
91,
311-315.
[DOI no: ]
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PubMed id
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Note In the PDB file this reference is
annotated as "TO BE PUBLISHED".
The citation details given above were identified by an automated
search of PubMed on title and author
names, giving a
percentage match of
85%.
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Abstract
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The structural basis for the stability of N termini of helices has been analyzed
by thermodynamic and crystallographic studies of three suitably engineered
mutants of the barley chymotrypsin inhibitor 2 with Ser, Gly, or Ala at the
N-cap position (residue 31). Each mutant has a well-organized shell of hydration
of the terminal NH groups of the helix. The three structures are virtually
superimposable (rms separations for all atoms, including the common water
molecules, are 0.15-0.17 A) and show neither changes in conformation at the site
of substitution nor changes in the crystal packing. The only changes on going
from Ser-31 to Ala-31 to Gly-31 are in the position of a water molecule
(Wat-116). This is bound to the Ser-O gamma atom in the Ser-31 structure but is
in a weak hydrogen bonding position with the NH of residue 34 (O ... N = 3.28 A)
in the Ala-31 mutant, partly replacing the strong Ser-31-O gamma ... N34
hydrogen bond (O ... N = 2.65 A). The corresponding water molecule completely
replaces the Ser hydroxyl hydrogen bond to N34 on mutation to Gly (2.74 A). The
only other change between the three structures is an additional water molecule
in the Ala-31 structure (Wat-150) that partly compensates for the weak Wat-116
... N34 hydrogen bond. Perturbation of solvation by the side chain of Ala is
consistent with earlier hypotheses on the importance of exposure of the termini
of helices to the aqueous solvent.
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