V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose).
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.
Gessler
K
K
Institut für Kristallographie, Freie Universität Berlin, Takustrasse 6, D-14195 Berlin, Germany.
Usón
I
I
Takaha
T
T
Krauss
N
N
Smith
S M
SM
Okada
S
S
Sheldrick
G M
GM
Saenger
W
W
D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D003196:Computer Graphics; D018360:Crystallography, X-Ray / Q000379:methods; D003505:Cyclodextrins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data
D003505:Cyclodextrins