V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose)

V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose)

^*Institut für Kristallographie, Freie Universität Berlin, Takustrasse 6, D-14195 Berlin, Germany; ^†Institut für Anorganische Chemie, Universität Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany; ^‡Biochemical Research Laboratory, Ezaki Glico Co., Ltd., 4-6-5 Utajima, Nishiyodogawa, Osaka 555, Japan; and ^§Institute of Cell and Molecular Biology, University of Edinburgh, The King’s Buildings, Mayfield Road, Edinburgh EH9 3JH, United Kingdom

Edited by Isabella L. Karle, Naval Research Laboratory, Washington, DC, and approved February 10, 1999 (received for review September 10, 1998)

Abstract

The amylose fraction of starch occurs in double-helical A- and B-amyloses and the single-helical V-amylose. The latter contains a channel-like central cavity that is able to include molecules, “iodine’s blue” being the best-known representative. Molecular models of these amylose forms have been deduced by solid state ¹³C cross-polarization/magic angle spinning NMR and by x-ray fiber and electron diffraction combined with computer-aided modeling. They remain uncertain, however, as no structure at atomic resolution is available. We report here the crystal structure of a hydrated cycloamylose containing 26 glucose residues (cyclomaltohexaicosaose, CA26), which has been determined by real/reciprocal space recycling starting from randomly positioned atoms or from an oriented diglucose fragment. This structure provides conclusive evidence for the structure of V-amylose, as the macrocycle of CA26 is folded into two short left-handed V-amylose helices in antiparallel arrangement and related by twofold rotational pseudosymmetry. In the V-helices, all glucose residues are in syn orientation, forming systematic interglucose O(3)_n⋅⋅⋅O(2)_n _+l and O(6)_n⋅⋅⋅O(2)_n ₊₆/O(3)_n ₊₆ hydrogen bonds; the central cavities of the V-helices are filled by disordered water molecules. The folding of the CA26 macrocycle is characterized by typical “band-flips” in which diametrically opposed glucose residues are in anti rather than in the common syn orientation, this conformation being stabilized by interglucose three-center hydrogen bonds with O(3)_n as donor and O(5)_n _+l, O(6)_n _+l as acceptors. The structure of CA26 permitted construction of an idealized V-amylose helix, and the band-flip motif explains why V-amylose crystallizes readily and may be packed tightly in seeds.

Footnotes

↵ ¶ To whom reprint requests should be addressed. e-mail: saenger{at}chemie.fu-berlin.de.
This paper was submitted directly (Track II) to the Proceedings Office.
Data deposition: The atomic coordinates have been deposited in the Cambridge Crystallographic Data Centre database. ID code CCDC 115146.
↵ ‖ In two complexes of the fully methylated heptakis-2,3,6-trimethyl-β-cyclodextrin, one glucose each was found in ¹C₄-chair (33) and ^3.0B-boat (34) forms.
↵ ** Some mean geometrical data for α-, β-, and γ-cyclodextrin taken from ref. 12: glycosidic bonds C( 1)_n ₊₁-O(4)_n-C(4)_n: α 119.0°, β 117.7⁰, γ 112.6°; virtual distances O(4)_n⋅⋅⋅ O(4)_n _-1: α 4.23 Å, β 4.36 Å, γ 4.48 Å; hydrogen bonds O(2)_n⋅⋅⋅ O(3)_n _-1: α 3.00 Å, β 2.86 Å, γ 2.81 Å.
↵ ‡‡ Definition of φ, ψ torsion angles: φ, O(5)_n-C(1)_n-O(4)_n _-l-C(4)_n _-l and ψ, C(1)_n-O(4)_n _-l-C(4)_n _-l-C(3)_n _-l; see International Union of Pure and Applied Chemistry recommendations (36).
ABBREVIATIONS:

CP/MAS,

cross-polarization/magic angle spinning;

CAn,

cycloamylose with n glucose residues