Starch Degrading Enzymes

Starch degradation during seed germination is one of the most important physiologic processes in higher plants currently receiving attention from plant physiologists, geneticists, and molecular biologists. In applied science, this interest is due to the importance of cereal crops as dietary components in human alimentation. We describe the processes underlying the hydrolysis of starch molecules in cereal seeds during germination.

Starch is stored in the endosperm of cereal seeds in two related forms, amylose and amylopectin, both of which are polymers of glucose. Amylose is a straight-chain polymer of adjoining glucose molecules with a-1,4 glucosidic linkages. Amylopectin consists of many amylose chains linked via a-1,6 bonds to produce a multiple-branched molecule. Starch breakdown during germination in cereal seeds is caused by the synergistic action of hydrolytic enzymes, i.e., a-amylase, ß-amylase, debranching enzyme, and a-glucosidase. It is generally accepted that phosphorylase is not involved in this process. a-amylase plays a major role during the degradation of native starch granules. Even though other amylolytic enzymes participate in the process of starch breakdown, the contribution of a-amylase is prerequisite for the initiation of this process. a-amylase is not present in the dry cereal seed, but is rapidly induced by the action of gibberellins (GAs), which are produced by the embryo and trigger the promotion of the gene expression in the aleurone layers surrounding the endosperm. Scutellar epithelium cells also play an important role in the production of a-amylase. This enzyme catalyzes the hydrolysis of the a-1,4 glucan bonds of the starch molecule. Under anoxic conditions, only rice seeds are able to induce a-amylase, whereas barley, wheat, oat, and rye do not.

ß-amylase (1,4 a-glucan maltohydrolase) catalyzes the liberation of a -maltose from the non-reducing ends of starch-related a-1,4 glucans. In cereals, ß-amylase helps mobilize the starch in germinating grains. In barley, the enzyme is already present in the dry seeds, where it accumulates during the process of grain development and is mainly bound to the starch granules. In rice, ß-amylase is synthesized de novo during seed germination and is almost absent in dry seeds. Debranching enzymes exclusively hydrolyze a-1,6 bonds in those glucans that contain both a-1,4 and a-1,6 linked glucosyl moieties. a-glucosidase (maltase) catalyzes the hydrolysis of a-1,4 glucan bonds of irreducible ends of the starch molecule and maltose as to produce glucose.

Carbohydrates from starch such as glucose and maltose are used not only for energy during germination but also for growth of the young shoots and roots. These sugars are changed to energy molecules such as ATP through the TCA cycle and are used as materials for the cell wall and various physiologic substances.

Shigeo Ohtsuki and Junji Yamaguchi
(Bioscience Center, Nagoya University)