Amylose branching

Borovsky, D., Smith, E. E., and Whelan, W. J. 1975. Purification and properties of potato 1,4-a-D-glucan 1,4-a-D-glucan, 6-a-(l,4-a-D-glucano)-transferase. Evidence a dual catalytic function in amylose branching enzyme. Eur. J. Biochem. 59, 615-625. 

The ratio of amylose and amylopectin varies according to starch source. Chemical structures of starch, such as molecular sizes of amylose, branch-chain lengths... 

Amylopectin and glycogen are saccharides similar to amylose, except with branched chains. 

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. 

Normal com starch is composed of 20—30% of the linear polysaccharide amylose and 70—80% of the branched polysaccharide amylopectin. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

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. 

FIGURE 16.7 Native starch ( ) and fractions of native starch differing in their branching characteristics (nb/lcb amylose -type fraction scb amylopectin -t/pe fraction ) separated on semipreparative... 

Amylopectin (Section 25.15) A polysaccharide present in starch. Amylopectin is a polymer of a(l,4)-linked glucose units, as is amylose (see aniylose). Unlike amylose, amylopectin contains branches of 24-30 glucose units connected to the main chain by an a(l,6) linkage. 

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. 

In starch, glucose molecules are joined head-to-tail through oxygen atoms. A thousand or more glucose molecules may be linked in this way. either in long single chains (amylose) or branched chains (amylopectin). 

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. 

There are two types of glucose chains in starch. One is a simple chain called amylose, and the other is a complex branched form called amylopectin. In the starch grains in a plant, amylopectin makes up the bulk of the material, from 50 to 80 percent by weight, made up of several million amylopectin molecules per starch grain. The rest is a much larger number of the smaller amylose chains, made up of 500 to 20,000 glucose units in each chain. Amylopectin molecules are made of several million glucose units. 

FIGURE 19.25 The amylopectin molecule is another component of starch. It has a more highly branched structure than amylose, as emphasized in the inset. 

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