Starch branching enzymes contributing to amylose and amylopectin fine structure in wheat.

Starch branching enzymes (SBEs) play a major role in determining starch molecular structure in cereal endosperm. This study investigates how SBEIIs contribute to the chain-length distributions (CLDs) of both amylopectin and amylose, obtained by enzymatic debranching of native starch. Wheat starches with low (37%) to high (93%) amylose content were obtained through altering SBEII in planta. Multiple components were detected in both amylose and amylopectin CLDs. Model fitting of these CLDs reveals a quantitative association between the enzyme activities in amylopectin and amylose. SBEIIa modifies shorter branches (degree of polymerization DP ≲ 12) in amylopectin and longer amylose chains with a CLD peak at ˜3000 DP. SBEIIb acts on longer branches (DP≲ 32) in amylopectin, while its effect on amylose fine structure is not significant. Using both the amylose and amylopectin models to analyze the CLD reveals connections between amylose and amylopectin in wheat starch biosynthesis.