Starch molecule, branched amylopectin

Starch is the most important polysaccharide in the human diet. It consists of two t)q)es of molecules fhe linear and helical amylose and fhe branched amylopectin. Depending on fhe source, sfarch generally contains 20-25% amylose and 75-80% amylopectin. In contrast to the two polysaccharides mentioned in the Section III.C, it is a major energy... 

More complex starches are termed amylopectins and contain branched chains. Starches can be very large molecules indeed, with molecular masses ranging up into the millions. 

Starch synthase in chloroplasts and amyloplasts catalyzes the addition of single glucose residues, donated by ADP-glucose, to the reducing end of a starch molecule by a two-step insertion mechanism. Branches in amylopectin are introduced by a second enzyme. 

Because the varieties of starch that contain only amylopec-tin are also crystalline, exhibiting the same diffraction patterns as starches containing amylose, there is a strong likelihood that the extensively branched amylopectin molecule also crystallizes in a double-helical form. In turn, this implies that linear sequences in amylopectin remain sufficiently long to... 

Chains of starch molecules can branch by forming [l<->6] glycoside bonds, as shown in Figure 12.21. This branching leads to more compact forms of starch, such as glycogen and amylopectin. 

Intermediate material is found in starch granules this material has structures and properties in between those of the essentially linear amylose molecules and those of the larger and more highly branched amylopectin molecules. The intermediate material has a heterogeneous nature.101 Lansky et al.102 fractionated amylose and reported that some amylose molecules were branched and had a lower affinity for iodine. 

Amylose is synthesized by granular-bound starch synthase, whereas amylopectin is synthesized by soluble starch synthase (Chapter 4).334,339 Because amylose is synthesized by the granular-bound starch synthase in a progressive manner,340 the amylose molecule is likely confined in the granule and has little opportunity to interact and form double helices with other starch molecules to facilitate branch formation. Branching reactions do occur on some amylose molecules, but at a much lower frequency than with amylopectin, and result in slightly branched amylose molecules. 

Starch molecules are polymers of anhydroglucose and occur in both linear and branched form. The degree of polymerization and, accordingly, the molecular weight of the naturally occurring starch molecules vary radically. Furthermore, they vary in the ratio of branched-chain polymers (amylopectin) to linear-chain polymers (amylose), both within a given type of starch and from one type to another. These factors, in addition to any type of chemical modification used, affect the viscosity, texture, and stability of the starch sols significantly. 

Assuming that potato starch contains 23 % amylose, the amylopectin must yield 49 % maltose by action of the /3-amylase. If the potato starch contains 5% end groups, the amylopectin should contain 6.5%. Taking this value, /3 = 0.065, we find = 0.56 and fs = 0.51. On the other hand, if we assume that potato starch contains branched molecules with an average degree of ramification expressed by /3 = 0.05, then ts = 0.59 and fs = 0.54. 

Fig. 1 (A) Linear amylose starch molecule and (B) branched amylopectin starch molecule.

Starch is a semicrystalline polymer. The linear amylose molecules are amorphous in nature, but the branched amylopectin portion has been reported as partially crystalline. It is believed that the crystalline regions in the starch granule are interspersed in a continuous amorphous phase. ° ... 

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