On the cluster structure of amylopectin

Perspectives on Starch Structure, Function, and Synthesis in Relation to the Backbone Model of Amylopectin

Understanding functionality of polysaccharides such as starch requires molecular representations that account for their functional characteristics, such as those related to gelatinization, gelation, and crystallization. Starch macromolecules are inherently very complex, and precise structures can only be deduced from large data sets to generate relational models. For amylopectin, the major, well-organized, branched part of starch, two main molecular representations describe its structure: the classical cluster model and the more recent backbone model. Continuously emerging data call for inspection of these models, necessary revisions, and adoption of the preferred representation. The accumulated molecular and functional data support the backbone model and it well accommodates our present knowledge related to the biosynthesis of starch. This Perspective focuses on our current knowledge of starch structure and functionality directly in relation to the backbone model of amylopectin.

A Review of the Impact of Starch on the Quality of Wheat-Based Noodles and Pasta: From the View of Starch Structural and Functional Properties and Interaction with Gluten

Starch, as a primary component of wheat, plays a crucial role in determining the quality of noodles and pasta. A deep understanding of the impact of starch on the quality of noodles and pasta is fundamentally important for the industrial progression of these products. The starch structure exerts an influence on the quality of noodles and pasta by affecting its functional attributes and the interaction of starch-gluten proteins. The effects of starch structure (amylopectin structure, amylose content, granules size, damaged starch content) on the quality of noodles and pasta is discussed. The relationship between the functional properties of starch, particularly its swelling power and pasting properties, and the texture of noodles and pasta is discussed. It is important to note that the functional properties of starch can be modified during the processing of noodles and pasta, potentially impacting the quality of the end product, However, this aspect is often overlooked. Additionally, the interaction between starch and gluten is addressed in relation to its impact on the quality of noodles and pasta. Finally, the application of exogenous starch in improving the quality of noodles and pasta is highlighted.

Structural, Thermal, Pasting and Digestion Properties of Starches from Developing Root Tubers of Sweet Potato

Three sweet potato varieties with white-, yellow- and purple-fleshed root tubers were harvested at 100, 120, 140 and 160 days after planting (DAP). Their starch structural, thermal, pasting and digestion properties were measured to reveal the influences of harvesting dates on the physicochemical properties of sweet potato root tuber starch. Though starches from different varieties displayed some differences in physicochemical properties due to their different genetic backgrounds, they were influenced by harvesting date in similar ways. Starches isolated from root tubers at 100 and 160 DAP exhibited lower granule sizes than those at 120 and 140 DAP. The amylose content was higher in root tubers at 100 and 120 DAP than at 140 and 160 DAP. Starches from root tubers at 100 DAP exhibited CA-type X-ray diffraction patterns, and then the B-type crystallinity gradually increased at later harvesting dates. The different harvesting dates had no significant effects on the short-ranged ordered structure and lamellar thickness of starch, but the lamellar peak intensity decreased significantly at later harvesting dates. Starch had a lower gelatinization temperature and a wider gelatinization temperature range in root tubers at 140 and 160 DAP than at 100 and 120 DAP. The higher peak viscosity and lower pasting temperature were associated with the late harvesting date. The digestion of starch had slight differences among root tubers at different harvesting dates. The harvesting dates of root tubers played more important roles in starch properties than the variety. This study would be helpful for breeders, farmers and sweet potato starch users.

Glucan, water dikinase (GWD) penetrates the starch granule surface and introduces C6 phosphate in the vicinity of branching points

Starch phosphorylation mediated by α-glucan, water dikinase is an integral part of starch metabolism. So far however, it is not fully understood. For getting deeper insights, several in vitro assays and intensive mass spectrometry analyses were performed. Such analyses allowed us to determine the phosphorylation position within the amylopectin in detail. Thus, unique features of the starch structure and GWD action were correlated. Therefore, recombinant potato GWD (Solanum tuberosum L.; StGWD) was used for detailed analyses of the phosphorylation pattern of various starches. Additionally, oil palm (Elaeis guineensis Jacq.; EgGWD) GWD was cloned and characterized, representing the first characterization of GWD of a monocot species. The distribution patterns of single phosphorylated glucan chains catalyzed by both GWDs were compared. The phosphorylation distribution patterns of both GWDs varied for different starches. It was proven that GWD phosphorylates different positions within the amylopectin of native starch granules. GWD enters the starch granule surface and phosphorylates the glucosyl units in the proximity of branching points to convert the highly ordered glucan chains into a less ordered state and to render them accessible for the downstream acting hydrolases. This enables deciphering the GWD actions and the related structural properties of starch granules.

A model for the reproduction of amylopectin cluster by coordinated actions of starch branching enzyme isoforms

Amylopectin is a highly branched glucan which accounts for approximately 65-85% of starch in most plant tissues. It is crucially important to understand the biosynthetic process of this glucan in regulating the structure and functional properties of starch granules. Currently, the most accepted ideas of structural feature and biosynthesis of amylopectin are that amylopectin is composed of a branched element called "cluster" and that the essential process of amylopectin biosynthesis is to reproduce a new cluster from the existing cluster. The present paper proposes a model explaining the whole process of amylopectin biosynthesis as to how the new cluster is reproduced by concerted actions of multiple isoforms of starch biosynthetic enzymes, particularly by combinations of distinct roles of starch branching enzyme (BE) isoforms. This model proposes for the first time the molecular mechanism as to how the formation of a new cluster is initiated, and the reason why BEI can play a major role in this step. This is because BEI has a rather broad chain-length preference compared to BEIIb, because a low preference of BEI for the substrate chain-length is advantageous for branching a couple of elongated chains that are not synchronously formed and thus these chains having varied lengths could be safely attacked by this isoform. On the contrary, it is unlikely that BEIIb is involved in this reaction because it can react to only short chains having degree of polymerization of 12-14. BEIIa is possibly able to complement the role of BEI to some extent, because BEIIa can attack basically short chains but its chain-length preference is lower compared with BEIIb. The model implies that the first branches mainly formed by BEI to construct the amorphous lamellae whereas the second branches predominantly formed by BEIIb are located mainly in the crystalline lamellae. This paper provides new insights into the roles of BEI, BEIIb, and BEIIa in amylopectin biosynthesis in cereal endosperm.

Interfacial Catalysis during Amylolytic Degradation of Starch Granules: Current Understanding and Kinetic Approaches

Enzymatic hydrolysis of starch granules forms the fundamental basis of how nature degrades starch in plant cells, how starch is utilized as an energy resource in foods, and develops efficient, low-cost saccharification of starch, such as bioethanol and sweeteners. However, most investigations on starch hydrolysis have focused on its rates of degradation, either in its gelatinized or soluble state. These systems are inherently more well-defined, and kinetic parameters can be readily derived for different hydrolytic enzymes and starch molecular structures. Conversely, hydrolysis is notably slower for solid substrates, such as starch granules, and the kinetics are more complex. The main problems include that the surface of the substrate is multifaceted, its chemical and physical properties are ill-defined, and it also continuously changes as the hydrolysis proceeds. Hence, methods need to be developed for analyzing such heterogeneous catalytic systems. Most data on starch granule degradation are obtained on a long-term enzyme-action basis from which initial rates cannot be derived. In this review, we discuss these various aspects and future possibilities for developing experimental procedures to describe and understand interfacial enzyme hydrolysis of native starch granules more accurately.

The functional implications of high-amylose wholegrain wheat flours: An in vitro digestion and fermentation approach combined with metabolomics

Wheat flour is one of the most prevalent foodstuffs for human consumption, and novel strategies are underway to enhance its nutritional properties. This work evaluated wholegrain flours from bread wheat lines with different amylose/amylopectin ratios through in vitro starch digestion and large intestine fermentation. High-amylose flours presented a higher resistant starch content and lower starch hydrolysis index. Moreover, UHPLC-HRMS metabolomics was carried out to determine the profile of the resulting in vitro fermentates. The multivariate analysis highlighted distinctive profiles between the flours derived from the different lines compared to the wild type. Peptides, glycerophospholipids, polyphenols, and terpenoids were identified as the main markers of the discrimination. The high-amylose flour fermentates showed the richest bioactive profile, containing stilbenes, carotenoids, and saponins. Present findings pave the way toward applying high-amylose flours to design novel functional foods.

Microstructures of Starch Granules with Different Amylose Contents and Allomorphs as Revealed by Scattering Techniques

In this study, as-is (ca. 12% moisture by mass) and hydrated (50% water by mass) granules of waxy potato (WP), waxy wheat (WW), waxy maize, normal maize, and high-amylose maize (HAM) starches were investigated by using small-angle neutron and X-ray scattering (SANS and SAXS), wide-angle X-ray scattering, and ultra-small-angle neutron scattering. The SANS and SAXS data were fitted using the two-phase stacking model of alternating crystalline and amorphous layers. The partial crystalline lamellar structures inside the growth rings of granules were analyzed based on the inter-lamellar distances, thicknesses of the crystalline lamellae and amorphous layers, thickness polydispersities, and water content in each type of layer. Despite having a longer average chain length of amylopectin, the WP and HAM starches, which had B-type allomorph, had a shorter inter-lamellar distance than the other three starches with A-type allomorph. The WP starch had the most uniform crystalline lamellar thickness. After hydration, the amorphous layers were expanded, resulting in an increase of inter-layer distance. The low-angle intensity upturn in SANS and SAXS was attributed to scattering from interfaces/surfaces of larger structures, such as growth rings and macroscopic granule surfaces. Data analysis methods based on model fitting and 1D correlation function were compared. The study emphasized─owing to inherent packing disorder inside granules─that a comprehensive analysis of different parameters was essential in correlating the microstructures with starch properties.

Cluster Size of Amylopectin and Nanosized Amylopectin Fragments Characterized by Pyrene Excimer Formation

Amylopectin from waxy corn and the three nanosized amylopectin fragments (NAFs)—NAF(56), NAF(20), and NAF(8)—from waxy corn starch with a hydrodynamic diameter of 227, 56, 20, and 8 nm, respectively, were randomly labeled with 1-pyrenebutyric acid. The efficiency of these pyrene-labeled amylopectin-based polysaccharides (Py-AbPS) for pyrene excimer formation (PEF) upon diffusive encounter between an excited and a ground-state pyrene increased with increasing concentration of unlabeled NAF(56) in Py-AbPS dispersions in DMSO. Fluorescence decay analysis of the Py-AbPS dispersions in DMSO prepared with increasing [NAF(56)] yielded the maximum number (N_blob^exp) of anhydroglucose units (AGUs) separating two pyrene-labeled AGUs while still allowing PEF. Comparison of N_blob^exp with N_blob^theo, obtained by conducting molecular mechanics optimizations on helical oligosaccharide constructs with HyperChem, led to a relationship between the interhelical distance (d_h-h) in a cluster of oligosaccharide helices, [NAF(56)], and the number of helices in a cluster. It was found that the AbPSs were composed of building blocks made of 3.5 (±0.9) helices that self-assembled into increasingly larger clusters with increasing [NAF(56)]. The ability of PEF-based experiments to yield the cluster size of AbPSs provides a new experimental means to probe the interior of AbPSs at the molecular level.

Improving Agricultural Traits While Maintaining High Resistant Starch Content in Rice

BACKGROUND

Resistant starch (RS) is beneficial for human health. Loss of starch branching enzyme IIb (BEIIb) increases the proportion of amylopectin long chains, which greatly elevates the RS content. Although high RS content cereals are desired, an increase in RS content is often accompanied by a decrease in seed weight. To further increase the RS content, genes encoding active-type starch synthase (SS) IIa, which elongates amylopectin branches, and high expression-type granule-bound SSI (GBSSI), which synthesizes amylose, were introduced into the be2b mutant rice. This attempt increased the RS content, but further improvement of agricultural traits was required because of a mixture of indica and japonica rice phonotype, such as different grain sizes, flowering times, and seed shattering traits. In the present study, the high RS lines were backcrossed with an elite rice cultivar, and the starch properties of the resultant high-yielding RS lines were analyzed.

RESULTS

The seed weight of high RS lines was greatly improved after backcrossing, increasing up to 190% compared with the seed weight before backcrossing. Amylopectin structure, gelatinization temperature, and RS content of high RS lines showed almost no change after backcrossing. High RS lines contained longer amylopectin branch chains than the wild type, and lines with active-type SSIIa contained a higher proportion of long amylopectin chains compared with the lines with less active-SSIIa, and thus showed higher gelatinization temperature. Although the RS content of rice varied with the cooking method, those of high RS lines remained high after backcrossing. The RS contents of cooked rice of high RS lines were high (27-35%), whereas that of the elite parental rice was considerably low (< 0.7%). The RS contents of lines with active-type SSIIa and high-level GBSSI expression in be2b or be2b ss3a background were higher than those of lines with less-active SSIIa.

CONCLUSIONS

The present study revealed that backcrossing high RS rice lines with elite rice cultivars could increase the seed weight, without compromising the RS content. It is likely that backcrossing introduced loci enhancing seed length and width as well as loci promoting early flowering for ensuring an optimum temperature during RS biosynthesis.

Effects of Various Allelic Combinations of Starch Biosynthetic Genes on the Properties of Endosperm Starch in Rice

Rice endosperm accumulates large amounts of photosynthetic products as insoluble starch within amyloplasts by properly arranging structured, highly branched, large amylopectin molecules, thus avoiding osmotic imbalance. The amount and characteristics of starch directly influence the yield and quality of rice grains, which in turn influence their application and market value. Therefore, understanding how various allelic combinations of starch biosynthetic genes, with different expression levels, affect starch properties is important for the identification of targets for breeding new rice cultivars. Research over the past few decades has revealed the spatiotemporal expression patterns and allelic variants of starch biosynthetic genes, and enhanced our understanding of the specific roles and compensatory functions of individual isozymes of starch biosynthetic enzymes through biochemical analyses of purified enzymes and characterization of japonica rice mutants lacking these enzymes. Furthermore, it has been shown that starch biosynthetic enzymes can mutually and synergistically increase their activities by forming protein complexes. This review focuses on the more recent discoveries made in the last several years. Generation of single and double mutants and/or high-level expression of specific starch synthases (SSs) allowed us to better understand how the starch granule morphology is determined; how the complete absence of SSIIa affects starch structure; why the rice endosperm stores insoluble starch rather than soluble phytoglycogen; how to elevate amylose and resistant starch (RS) content to improve health benefits; and how SS isozymes mutually complement their activities. The introduction of active-type SSIIa and/or high-expression type GBSSI into ss3a ss4b, isa1, be2b, and ss3a be2b japonica rice mutants, with unique starch properties, and analyses of their starch properties are summarized in this review. High-level accumulation of RS is often accompanied by a reduction in grain yield as a trade-off. Backcrossing rice mutants with a high-yielding elite rice cultivar enabled the improvement of agricultural traits, while maintaining high RS levels. Designing starch structures for additional values, breeding and cultivating to increase yield will enable the development of a new type of rice starch that can be used in a wide variety of applications, and that can contribute to food and agricultural industries in the near future.

Changes in fine structure of amylopectin and internal structures of starch granules in developing endosperms and culms caused by starch branching enzyme mutations of japonica rice

BEIIb plays a specific role in determining the structure of amylopectin in rice endosperm, whereas BEIIa plays the similar role in the culm where BEIIb is absent. Cereals have three types of starch branching enzymes (BEs), BEI, BEIIa, and BEIIb. It is widely known that BEIIb is specifically expressed in the endosperm and plays a distinct role in the structure of amylopectin because in its absence the amylopectin type changes to the amylose-extender-type (ae-type) or B-type from the wild-type or A-type and this causes the starch crystalline allomorph to the B-type from the wild-type A-type. This study aimed to clarify the role of BEIIa in the culm where BEIIb is not expressed, by using a be2a mutant in comparison with results with be2b and be1 mutants. The results showed that the amylopectin structure exhibited the B-type in the be2a culm compared with the A-type in the wild-type culm. The starch granules from the be2a culm also showed the B-type like allomorph when examined by X-ray diffraction analysis and optical sum frequency generation spectroscopy. Both amylopectin chain-length profile and starch crystalline properties were found to be the A-type at the very early stage of endosperm development at 4-6 days after pollination (DAP) even in the be2b mutant. All these results support a view that in the culm as well as in the endosperm at 4-6 DAP, BEIIa can play the role of BEIIb which has been well documented in maturing endosperm. The possible mechanism as to how BEIIa can play its role is discussed.