Structure of Amylopectin

The average branch chain length of amylopectin has a bimodal distribution that differs from the single modal distribution of that of glycogen.257 The average branch chain 

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The structure of amylopectin according to Meyer is shown in Fig. 65. The single reducing end group is indicated by A all other terminal units are attached at the 1-position only. 

Amylo-1 —> 6-glucosidase obtained by Cori and Larner218 from rabbit muscles, and R-enzyme isolated by Hobson, Whelan and Peat219 from potatoes and broad beans, are typical debranching enzymes, which will hydrolyze the 6 — 1-a-D-glucosidic linkage rather than the normal 4 —> 1-a-D linkage. These enzymes will therefore be particularly important in determinations of the fine structure of amylopectin, if they can be sufficiently well purified. 

The Semicrystalline Structure of Amylopectin. Amylopectin crystallises according to a cluster molecule,25 as shown in Figure 8b. Within this structure there are crystalline and amorphous regions. An... 

Figure 8 The structure of amylopectin (a) organisation of the molecule (b) crystalline and amorphous regions (c) possible double helix structure...

Takeda, Y, Shibahara, S., Hanashiro, I. (2003). Examination of the structure of amylopectin molecules by fluorescent labeling. Carbohydr. Res., 338,471 75. 

Problem 22.44 Amylopectin, the water insoluble fraction of starch, behaves like amylose, except that more 2,3,4,6-tetra-O-methyl-D-glucose (5%), and an equal amount of 2.3-di-O-methyl-D-glucose, is formed. Deduce the structure of amylopectin. 

It has been estimated by Manners16 that 80-90% of the total number of chains in an amylopectin molecule are part of side chain clusters, while the remaining 10-20% of chains form inter-cluster connections. Thus, substantial progress in understanding the basic structure of amylopectin has been made. Although the three-dimensional structure of amylopectin in the granule is not yet known, there is evidence that it is a two-dimensional ellipsoid.17-19... 

Substances commonly found in starch granules are amylopectin, amylose, molecules intermediate between amylose and amylopectin, lipid (including phospholipids and free fatty acids), phosphate monoester and proteins/enzymes. The contents and the structures of amylopectin and amylose play major roles in the functional properties of starch. However, lipids, phospholipids and phosphate monoester groups have significant effects on starch functional properties, even though they are minor constituents. 

Amylopectin Amylopectin, the insoluble fraction of starch, is also primarily an a-1,4 polymer of glucose. The difference between amylose and amylopectin lies in the branched nature of amylopectin, with a branch point about every 20 to 30 glucose units. Another chain starts at each branch point, connected to the main chain by an a-1,6 glycosidic linkage. A partial structure of amylopectin, including one branch point, is shown in Figure 23-20. 

Partial structure of amylopectin. Amylopectin is a branched a-1,4 polymer of glucose. At the branch points, there is a single a-1,6 linkage that provides the attachment point for another chain. Glycogen has a similar structure, except that its branching is more extensive. 

Data on the fine structure of amylopectin from several sources have been obtained through the action of )8-amylase and pullulanase 104, 105, 106). Pullulanase removes specifically the a-D-(l- 6) linkages in amylo-... 

Hydrogenation of oxystarch with Eaney nickel catalyst, followed by hydrolysis, gave erythritol. Simultaneous reduction and hydrolysis gave erythritol in yields of up to 71 %. Oxyamylopectin has been reduced with sodium borohydride to the corresponding polyhydric alcohol, which has been methylated both derivatives have been hydrolyzed, to give fragments enabling the structure of amylopectin to be deduced. 

Isoamylases that exclusively hydrolyze the q -(1—>6) branch linkages of starch were first recognized in plants and first isolated from broad beans [92]. A bacterial isoamylase was obtained from the culturing of Pseudomonas amyloderamosa [93,94,95,96] and has found wide use in studying the structure of amylopectin and related polysaccharides [97]. 

For comparison, the steric structure of amylopectin is given below. Besides the (1 - 4)-a-glucosidic structure, the secondary arrangement and the beginning of the coiling tertiary structure can be seen. 

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