Starch phosphorylation—Maltosidic restrains upon 3′‐ and 6′‐phosphorylation investigated by chemical synthesis, molecular dynamics and NMR spectroscopy

Abstract

Phosphorylation is the only known in vivo substitution of starch, yet no structural evidence has been provided to explain its implications of the amylosidic backbone and its stimulating effects on starch degradation in plants. In this study, we provide evidence for a major influence on the glucosidic bond in starch specifically induced by the 3-O-phosphate. Two phosphorylated maltose model compounds were synthesized and subjected to combined molecular dynamics (MD) studies and 950 MHz NMR studies. The two phosphorylated disaccharides represent the two possible phosphorylation sites observed in natural starches, namely maltose phosphorylated at the 3'- and 6'-position (maltose-3'-O-phosphate and maltose-6'-O-phosphate). When compared with maltose, both of the maltose-phosphates exhibit a restricted conformational space of the alpha(1-->4) glycosidic linkage. When maltose is phosphorylated in the 3'-position, MD and NMR show that the glucosidic space is seriously restricted to one narrow potential energy well which is strongly offset from the global potential energy well of maltose and almost 50 degrees degrees from the Phi angle of the alpha-maltose crystal structure. The driving force is primarily steric, but the configuration of the structural waters is also significantly altered. Both the favored conformation of the maltose-3'-phosphate and the maltose-6'-phosphate align well into the 6-fold double helical structure of amylopectin when the effects on the glucosidic bond are not taken into account. However, the restrained geometry of the glucosidic linkage of maltose-3'-phosphate cannot be accommodated in the helical structure, suggesting a major local disturbing effect, if present in the starch granule semi-crystalline lattice.

Leaf Starch Turnover Occurs in Long Days and in Falling Light at the End of the Day

“…The obvious candidate in this context is the degree of phosphorylation of the starch granule surface. The presence of phosphate groups on Glc residues within amylopectin chains profoundly influences the rate of starch degradation by b-amylases (Hejazi et al, 2008), probably because these groups reduce the level of crystalline organization of the granule matrix and thus open the surface to attack (Hansen et al, 2009). Phosphate groups are added by two glucan water dikinases: water dikinase and PWD.…”

The Phosphoglucan Phosphatase Like Sex Four2 Dephosphorylates Starch at the C3-Position in Arabidopsis

“…The presence of phosphate induces structural changes in amylopectin, promoting the solubility of the glucan chains (Blennow and Engelsen, 2010;Hejazi et al, 2010) and confers a high swelling power to starch gels (Blennow et al, 2001). Moreover, it has recently been proposed that, while C6-phosphorylation causes only minor changes in amylopectin structure, phosphorylation at the C3-position imposes significant steric effects and is predicted to induce conformational changes in the glucan backbone that can disrupt starch crystallinity (Hansen et al, 2009). Given that C3-phosphate esters are scarce in nature, the lsf2-like starches with elevated amounts of C3-bound phosphate may potentially have interesting biotechnological applications.…”

A Putative Phosphatase, LSF1, Is Required for Normal Starch Turnover in Arabidopsis Leaves

“…Although only about 20% of phosphate groups are in this position, molecular modeling studies indicate that they are more disruptive of the packing of double helices of amylopectin chains than phosphate groups on the six-position (Blennow et al, 2002;Hansen et al, 2009); hence, they may have a disproportionate effect on the accessibility of the granule surface for starch-degrading enzymes. However, we found no difference in the ratio of Glc 6-P to Glc 3-P residues in amylopectin from wild-type and lsf1 leaves.…”