Binding of endoamylase to native starch grains from poplar wood

Abstract

Starch grains were isolated from parenchyma cells of poplar wood (Populus × canadensis Moench ‘robusta’). The grains contained tightly bound amylase activity, which was classified as α-amylase (EC 3.2.1.1) by substrate specificity. The enzyme differs in some kinetic properties and the native electrophoresis profile from soluble amylase forms of the same material. The binding of α-amylase to isolated grains was affected by temperature and by maltose and malto-oligosaccharides in high concentrations. The amylase content of the grains was reduced to 18% as the temperature was increased from 1 °C to 31 °C, and 97% of the activity was released by a treatment with maltose at 20°C. The binding was fully reversible. The release of oligoglucans from starch grains was inhibited by removing the starch-associated amylase or by preventing the binding by addition of exogenous glucans, while the degradation of a soluble substrate — starch azure — was not affected by carbohydrates. These observations indicate that the granule-bound amylase is involved in the dissolution of starch and that binding to the grains is a prerequisite of the amylolytic activity. The results are consistent with the assumption that the amylase contains a raw starch binding site in addition to the active site. The exposure of excised twigs harvested in late autumn or winter to a low temperature regime led to an increase of the granule-bound amylase activity and to a simultaneous decrease of the starch level. It is therefore suggested that the reversible binding of the α-amylase to starch granules as affected by temperature and glucans might take part in the regulation of the starch content.