Amylopectin branching points

Amylases are exoen2ymes that attack amylose chains and result in the successive removal of maltose units from the nonreducing end. In the case of amylopectin, the cleaving stops two to three glucose units from the a-1,6-branching points. ( -Amylase [9000-91-3] is used for the production of maltose symps and for adjunct processing in breweries. The most important commercial products are made from barley or soybeans. 

A 0.2-g sample of amylopectin was analyzed to determine the fraction of the total glucose residues that are branch points in the structure. The sample was exhaustively methylated and then digested, yielding 50 /tmol of 2,3-dimethylglucose and 0.4 /tmol of 1, 2, 3, 6-tetramethylglucose. 

Figure 13-12. Structureofstarch. A Amylose, showing helical coil structure. B Amylopectin, showing 1 - 6 branch point.

The hydrolysis of starch by salivary and pancreatic amylases catalyze random hydrolysis of a(l—>4) glycoside bonds, yielding dextrins, then a mixmre of glucose, maltose, and isomaltose (from the branch points in amylopectin). 

FIGURE 6.15 AFM images of (A) potato amylose, (B) potato amylopectin (arrows branch points on the chains), and (C) rice amylose (arrows individual amylose structures). Reprinted with permission from Dang et al. (2006). 

Although it has been found that the separated amylose component can be readily orientated to yield fiber patterns, amylopectin usually gives poor or amorphous patterns. In the granule, however, amylopectin does exhibit crystallinity, since waxy maize starch gives a diffraction pattern and other waxy starches behave similarly.193 -195 (This suggests that the branch points in the amylopectin molecule may be in the amorphous part of the granule.)... 

Amylase operates exclusively on nonreducing, terminal units in amylose or on the branches in amylopectin, to produce maltose directly, and its hydrolytic action on 4 — 1-a-D linkages is stopped by any branch points. Enzyme action is greatly impeded by secondary valence forces, as retro-gradation, for example, is accompanied by an increased resistance to j8-amylolysis.19 The enzyme can be crystallized relatively easily. The mode... 

Chemically, both varieties of starch are polymers of glucose with the a-D-glucose units in the 4Ci conformation. The glucose units are linked -(1 ->4)- in both amylose and amylopectin but in amylopectin roughly one residue in twenty is linked -(1 ->6)-, which forms branch points. The proportion of branch points varies, depending on the source of the amylopectin. 

Figure 9.16 Structures of glucopyranose homopolysaccharides. Cellulose is a linear structure of the glucopyranose units linked /3( 1 —>4). Starch consists of amylose, which has a linear < (1—>4) structure, and amylopectin, which has a(l—>6) branch points on the linear a(l—>4) chains. Glycogen has a similar structure to amylopectin, but with a greater degree of < ( 1—>6) branching.

FIGURE 7-15 Amylose and amylopectin, the polysaccharides of starch, (a) A short segment of amylose, a linear polymer of o-glucose residues in (ctl —>4) linkage. A single chain can contain several thousand glucose residues. Amylopectin has stretches of similarly linked residues between branch points, (b) An (ctl —>6) branch point of amylopectin. (c) A cluster of amylose and amylopectin like that believed... 

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