title: The phosphorylation site in double helical amylopectin as investigated by a combined approach using chemical synthesis, crystallography and molecular modeling.
abstract: The only known in planta substitution of starch is phosphorylation. Whereas the function of starch phosphorylation is poorly understood, phosphorylated starch possesses improved functionality in vitro. Molecular models of native crystalline starch are currently being developed and the starch phosphorylating enzyme has recently been discovered. Accordingly, it is desirable to obtain a more exact description of the molecular structures of phosphorylated starch. We have determined the crystal structure of methyl alpha-D-glucopyranoside 6-O-phosphate as its potassium salt which is thought to be the starch phosphate counterion in vivo. From this structure and previously known glucophosphate structures we describe the possible 6-O-phosphate geometries and through modeling extrapolate the results to the double helical structure of the crystalline part of amylopectin. The geometries of the existing crystal structures of 6-O-phosphate groups were found to belong to two main adiabatic valleys. One of these conformations could be fitted into the double helical amylopectin part without perturbing the double helical amylopectin structure and without creating steric problems for the hexagonal chain-chain packing.
authors.affiliation: Food Technology, Department of Dairy and Food Science, The Royal Veterinary and Agricultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark. se@kvl.dk
authors.forename: Søren Balling
authors.initials: SB
authors.lastname: Engelsen
authors.forename: Anders Østergaard
authors.initials: AØ
authors.lastname: Madsen
authors.forename: Andreas
authors.initials: A
authors.lastname: Blennow
authors.forename: Mohammed Saddik
authors.initials: MS
authors.lastname: Motawia
authors.forename: Birger Lindberg
authors.initials: BL
authors.lastname: Møller
authors.forename: Sine
authors.initials: S
authors.lastname: Larsen
mesh_terms: D000687:Amylopectin / Q000737:chemistry* / Q000378:metabolism*; D002236:Carbohydrate Conformation; D018360:Crystallography, X-Ray; D008757:Methylglucosides / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009005:Monosaccharides / Q000138:chemical synthesis / Q000737:chemistry; D010766:Phosphorylation; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D008757:Methylglucosides; D009005:Monosaccharides; C473885:methyl-glucopyranoside-6-O-phosphate; C027020:methylglucoside; D000687:Amylopectin