Amylose branching amylopectin

Amylase occurs in many plants, such as barley, wheat, rye, soy beans, and potatoes, where it is generally accompanied by some a-amylase. [ -Amylase initiates hydrolysis at the nonreducing end of an amylose or amylopectin chain, and removes maltose units successively until the reducing end of the molecule is encountered in amylose or a branch is met in amylopectin. ( -Amylase is used commercially in the preparation of maltose symps. After P-amylase hydrolysis of amylopectin there remains a P-amylase limit dextrin. ( -Amylase has been used as a probe of the fine stmcture of amylopectin (43-46). 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. cx-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary stmcture. 

FIGURE 7.21 Amylose and amylopectin are the two forms of starch. Note that the linear linkages are o (1 4), but the branches in amylopectin are o (1 6). Branches in polysaccharides can involve any of the hydroxyl groups on the monosaccharide components. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues. 

Starches can be separated into two major components, amylose and amylopectin, which exist in different proportions in various plants. Amylose, which is a straight-chain compound and is abundant in potato starch, gives a blue colour with iodine and the chain assumes a spiral form. Amylopectin, which has a branched-chain structure, forms a red-purple product, probably by adsorption. 

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... 

These synthetic linear and branched molecules may be important as type polymers, particularly if the interconversion of amylose to amylopectin is intramolecular, in which case the initial molecular weight and molecular-weight distribution would be retained. There is the possibility that the in vitro synthesis may even result in a truly three-dimensional structure, as distinct from that of the natural component. 

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. 

A second reason for the turn-over in the osmotic modulus may arise from a decrease in A2 until it becomes zero or even negative. This would be the classical situation for a phase separation. The reason why in a good solvent such a phase separation should occur has not yet been elucidated and remains to be answered by a fundamental theory. In one case the reason seems to be clear. This is that of starches where the branched amylopectin coexists with a certain fraction of the linear amylose. Amylose is well known to form no stable solution in water. In its amorphous stage it can be brought into solution, but it then quickly undergoes a liquid-solid transition. Thus in starches the amylose content makes the amylopectin solution unstable and finally causes gelation that actually is a kinetically inhibited phase transition [166]. Because of the not yet fully clarified situation this turn-over will be not discussed any further. 

The (branched) amylopectin is generally considered more alkali-stable than amylose. Amylopectin produced a very small proportion of glucoiso-saccharinic acid after exposure to M KOH at 25° for nearly 2 months."" However, glycogen, which has a similar structure, but more a-D-(1 6) bonding, undergoes the peeling process where a-o-(1 4) bonds are pre-... 

While starch is the major structural polysaccharide, plant energy storage and regulation utilize a combination of similar polysaccharides that are referred to as starch. Starch can be divided into two general structures, largely linear amylose (structure 9.17) and largely branched amylopectin (structure 9.18). 

Figure 4.1 Schematic representation of the structures of (a) linear amylose and (b) branched amylose and amylopectin.

Most potato starches are composed of a mixture of two polysaccharides, a linear fraction, amylose, and a highly branched fraction, amylopectin. The content of amylose is between 15 and 25% for most starches. The ratio of amylose to amylopectin varies from one starch to another. The two polysaccharides are homoglucans with only two types of chain linkage, a-(l 4) in the main chain and a-(l 6)-linked branch chains. Physicochemical properties of potato and its starch are believed to be influenced by amylose and amylopectin content, molecular weight, and molecular weight distribution, chain length and its distribution, and phosphorus content (Jane and Chen, 1992). 

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... 

Figure E2.3.1 Plot of absorbance at 600 nm against percentage amylose (w/w) for mixtures of potato amylose and amylopectin with iodine. The absorbance of 0% amylose is due to the l2 affinity of the long outer branches of amylopectin.

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