Topic 8.13 Starch Architecture
Starch is a semicrystalline biopolymer stored in many plant locations, such as leaves, grains, tubers, and roots. Numerous studies led to the view that the structure of this polysaccharide can best be visualized at four different levels: molecules, lamellae, blocklet and granules. Although starch consists almost solely of glucose molecules, the composition, the shape, and the size of the granules depend on the plant source.
Amylopectin and Amylose Pack Tightly in Starch Granules
The major part of the granule mass, approximately 98–99%, is a complex homopolymer made up of two components, amylose and amylopectin (Web Figure 8.13.A). The α-D-glucosyl units associate in long linear chains linked through α-D-1,4 glycosidic linkages, wherein α-D-1,6 glycosidic linkages are formed as branch points. The contribution of α-D-1,6 glycosidic linkages to total bonds is extremely low in amylose (less than 1%) and moderately extensive in amylopectin (ca. 5–6%) resulting, respectively, in essentially linear and branched homopolymers. Amylose, which is relatively extended, has a lower molecular weight (500–20,000 glucose units) than amylopectin (ca. 106 glucose units) which has a compact shape.
Web Figure 8.13.A The chemical composition of starch. Starch is made up of amylose and amylopectin. Glucose units are linked almost exclusively through α-D-1,4-glucosidic bonds in amylose. Amylopectin also contains α-D-1,4-linked glucose chains (6 < n,m < 100 glucose residues) but interspersed with α-D-1,6-glycosidic bonds (branch points) that give a tree-like structure to the macromolecule.
Native starches vary widely among sources due to their content of different proportions of amylose (10–20%) and amylopectin (80–90%). But more importantly, the ratio of amylose to amylopectin defines the architecture of the regular semicrystalline arrays of amylopectin in the supramolecular complex called the starch granule (Web Figure 8.13.B). In chloroplasts and amyoplasts (seed endosperm plastids), amylose and amylopectin are organized in relatively dense spheroidal granules (0.1 to > 50 μm in diameter) that vary in shape and size.
Web Figure 8.13.B Concentric layers of the starch granule. Light microscopy of iodine-stained sections of pea seed starch. Iodine reacts primarily with the amylose component due to its open structure. (From Ridout et al. 2003.)
When observed by optical microscopy, most of the granules exhibit concentric shells that originate from internal lamellar structures made up of alternating semicrystalline and amorphous layers (Web Figure 8.13.C).
Web Figure 8.13.C Levels of organization of the starch granule. Four levels of organization make up the starch granule: the cluster of amylopectin molecules (0.1–1 nm), the lamella (~10 nm), the blocklet (20–250 nm) and the whole granule (> 1μm). Amylopectin molecules are closely packed together to form clusters of double helices. The crystalline lamella is created by the association of amylopectin double helices interspersed with amorphous regions. The blocklet is the ordered aggregation of several crystalline-amorphous lamellae into an asymmetric structure with an axial ratio of 3:1 (named “normal blocklets”). Amylose and other materials (e.g., water, lipids) disturb the regular formation of blocklets introducing “defects” (named “defective blocklets”). The ordered aggregation of normal and defective blocklets form the concentric rings of hard (crystalline) and soft (semi-crystalline) shells in the starch granule.
Although Nägeli suggested in 1858 that organized units of starch are surrounded and held together by less-organized material, it was Badenhuizen in 1937 who first described structures composed of crystalline units embedded in amorphous material. These starch units that are refractory to enzymatic breakdown led to the current concept of the “blocklet” (initially called “blöckchen Strüktur”) (Buleon et al. 1998). The blocklet is a supramolecular structure intermediate between the macromolecules and the organization of the large starch granule (Figure 3). The reconstruction of transmission electron microscopy images of sectioned starches reveals roughly ellipsoidal regions of 20 to 500 nm in diameter. These particular structures result from the superposition of alternating semi-crystalline and amorphous amylopectin lamellae within the blocklets. A blocklet is made up of the semi-crystalline lamellae, which are formed from clusters of amylopectin molecules, and the amorphous lamellae, which incorporate lower-branching molecules such as amylose and lipids. Frequently, some amylopectin molecules assembly loosely, disturbing the semi-crystalline array and causing “defects” in the amorphous rings. These two clusters of blocklets, “normal” and “defective”, appear regularly one after another in the radial growth of the starch granule, yielding two different formations: the heterogeneous and homogenous shells. Consequently, the amylopectin molecule plays a major role in the architecture of the blocklet in which other components modulate the strength and flexibility of the whole starch granule (Tang et al. 2006). Earlier studies on starch granules found that the less crystalline parts are more easily digested by enzymes than the more crystalline counterparts. The differential response of starch to enzymatic attack made clear that the structure of the semicrystalline shells modulates the catalytic capacity of hydrolyzing enzymes.
In recent years, methods by which specific enzymes can be localized demonstrated that the starch granule is usually associated with biosynthetic enzymes for building the complex architecture of the starch granule and with degradative enzymes for breakdown of the polysaccharide. Proteins associated with starch granules can be grouped into two categories, surface-bound proteins and internal granule-associated proteins. The small degree of association between surface-bound proteins and starch granules facilitates the separation by digestion with proteases (e.g., thermolysin, proteinase K) or by extensive washing in aqueous buffer. On the contrary, removal of internal granule-associated proteins requires drastic conditions (e.g., sodium dodecylsulfate).
In the diurnal accumulation of starch, the biosynthesis of amylose is attributed essentially to the granule-bound starch synthase found entirely within the granule matrix. The low activity observed when the enzyme lacks the polysaccharide environment reveals that starch synthase requires the starch granule to be active. Similarly, the surface of the granule is critical for the catalytic capacity of certain enzymes that participate in the nocturnal breakdown of starch. For example, plastidic starch phosphorylase cannot use the intact starch granule as substrate, but isoamylase is functional in glucan debranching at the granule surface. In almost all land plants, light controls the distribution and activity of chloroplast enzymes that partition between the soluble phase and the granule surface. In summary, the starch granule in chloroplasts changes alternately not only in size, but also in composition with a regular periodicity of about 24 hours.
Harnessing Starch for Tomorrow’s World
One of society’s most important plant products, the synthesis of starch occurs in specialized organelles: chloroplasts for transient starch in leaves, and amyloplasts for stored starch in heterotrophic tissues. As plant tubers (e.g., potato) and seed endosperms (e.g., maize, wheat, rice) are by far the largest source of starch for humankind, it is not only an essential foodstuff that contributes largely to the daily caloric intake, but also an important intermediate in industrial processes, such as adhesives and biodegradable plastics.
Starch is insoluble in cold water because crystalline regions of the granule do not allow water entry. However, crystalline regions begin to separate into swelled amorphous forms when the temperature is increased (Damager et al 2010). Starch gelatinization refers to the concerted action of water and heat that breaks down the intermolecular bonds of starch molecules, driving the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. Both the amount of water and the temperature condition granule so that the penetration of water decreases the number and size of crystalline regions with a concurrent increase in the randomness of the general structure. Gelling properties are an important issue of processing requirements and texture controls when: (i) cooking pastry, roux sauce, and custard; as well as (ii) producing raw materials for the chemical, pharmaceutical and textile industries.
As a consequence, two aspects have to be implemented for improving these activities: sufficient quantity for feeding the world and adequate quality for particular uses. The former relies upon the farmer, but the responsability of both rests with the processing chain, which is represented by the food and chemical industries. Hence, the challenge of the future will be to develop improved varieties with the ultimate goal of feeding the world and releasing chemical products less harmful to the environment.