|Yoo, Sang Ho|
|Wong, Kit Sum|
Submitted to: Applied Glycoscience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2003
Publication Date: 6/1/2003
Citation: JANE, J., YOO, S., WONG, K., DUVICK, S.A., GARDNER, C.A. STRUCTURES OF AMYLOPECTIN AND STARCH GRANULES-HOW ARE THEY SYNTHESIZED?. APPLIED GLYCOSCIENCE. 2003. V. 50. PP. 167-172. Interpretive Summary: Chemical cross-linking reactions were applied to intact native maize starch granules to investigate whether amylase molecules are interspersed among amylopectin molecules or are present in bundles. Results show that amylose molecules are interspersed among amylopectin instead of being present in bundles. The two molecules are synthesized side by side by two different starch synthases, granular bound starch synthase and soluble starch synthase, respectively. Molecular weights and structures of amylopectin molecules differ, based on their origin, and their properties differ. Arrangements of starch molecules in granules were proposed, based on the shape of the granule. Pinholes are commonly found on the granule surface of the A-type crystalline starches from maize and sorghum, but not on the B-type crystalline starches of potato. Starch granules with pinholes can be isolated from the region around the germ of the seed. It is postulated that pinholes are the results of enzyme degradation of starch. SEM microscopy studies show that maize kernels, at a dormant state, display significant enzyme attack on starch, particularly in the region of the germ. Results suggest that starch is hydrolyzed by enzymes to produce energy for seed germination, even in the dormant state. This work advances understanding of maize starch structure and that of it's componenets. Advancing the understanding of the mechanisms of starch synthesis and degradation of maize starch, and the location of these activities within the kernel, enables researchers to understand how starch quality traits can be utilized for industrial or nutritional purpose.
Technical Abstract: Amylopectin and amylose are synthesized simultaneously by soluble starch synthase and granular bound starch synthase. Both are synthesized radially by apposition to developing starch granules. Cross-linking reactions introduced in intact starch granules show that amylose is cross-linked onto amylopectin instead between amylose molecules. Results suggest that amylose molecules are interspersed among amylopectin molecules in starch granules, not isolated in bundles. Amylopectin is the major component of most starches, has a much larger molecular weight (MW) than amylose and a branched structure. External branch chains of amylopectin are present in double helices, which form the crystalline structure of starch granules. Recent studies show that the MW of waxy starch amylopectin is larger than that of normal and high-amylose starch counterparts. Amylopectin of starch displaying the B-type polymorphism has, in general, a smaller MW and lower dispersed molecular density than does the A-type starch. B-type amylopectin carries fewer short, B1 chains and has a larger radius of gyration than does the A-type amylopectin. Results suggest that there are more (alpha 1-6) branch linkages present within the crystalline region of branch-chain clusters of amylopectin that display A-type polymorphism. In contrast, branch linkages of the B-type starch are present mostly in the amorphous region and are hydrolyzed during acid hydrolysis to produce Naegeli dextrins. The large proportion of short, B1 chains with scattered branch linkages of the A-type amylopectin produce many short double helices consisting of B1 and A chains, which extend within a single cluster and are more easily rearranged into closely packed orthorhombic unit cells, reflecting the A-type diffraction pattern. The lack of B1 chains in the B-type amylopectin is related to the deficiency of certain branching enzymes, such as SBEIIb for high amylose maize starch. Pinholes are observed on the surface of many A-type starch granules, such as those of maize and sorghum, but not on B-type starch. A-type starches are also more susceptible to enzyme hydrolysis than B-type starches. The nature of the pinholes is unknown. A recent study using freeze-fracture techniques on maize kernels and scanning electron microscopy reveals that starch granules with holes are mostly located beneath the germ. This suggests that pinholes may result from enzyme hydrolysis of starch, providing energy for seed germination.