Glycosidic bonds, starch

Polysaccharides have many monosaccharide units linked by glycosidic bonds. Starch and glycogen are polymers of D-glucose, mainly linked 1,4 or 1,6 and a. Cellulose consists of D-glucose units linked 1,4 and p. 

Maltose obtained by the hydrolysis of starch and cellobiose by the hydrolysis of cellulose are isomenc disaccharides In both maltose and cellobiose two d glucopyra nose units are joined by a glycosidic bond between C 1 of one unit and C 4 of the other The two are diastereomers differing only m the stereochemistry at the anomeric carbon of the glycoside bond maltose is an a glycoside cellobiose is a (3 glycoside... 

A slurry of the starch is cooked in the presence of a heat-stable bacterial endo-a-amylase. The enzyme hydrolyzes the a-l,4-glycosidic bonds in pregelatinized starch, the viscosity of the gel rapidly decreases, and maltodextrins are produced. The process may be terrninated at this point. The solution is purified and dried, and the maltodextrins are utilized as blandtasting functional ingredients in dry soup mixes, infant foods, sauces and gravy mixes, etc. 

The glycosidic bond to an anomeric carbon can be either a or (3. Maltose, the disaccharide obtained by enzyme-catalyzed hydrolysis of starch, consists of two cv-D-glucopyranose units joined by a 1->4-o-glycoside bond. Cellobiose, the disaccharide obtained by partial hydrolysis of cellulose, consists of two /3-o-glucopyranose units joined by a 1—>4-/3-glycoside bond. 

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). 

Polysaccharides are polymers made up of many monosaccharides joined together by glycoside bonds, and include cellulose, starch, vegetable mucilage and plant gums. 

Cellulose is a high molecular weight polymer of D-glucose with fi( 1 -4)-glycosidic bonds, found in plant fibres it is the major component of most plant tissues. Starch is another common polysaccharide, containing two polymers of glucose, amylose and amylopectin. It was used in some paint preparations and in the production of paper. Acid treatment of starch produces dextrins, which are used as adhesives and additives in water colour paintings. 

Especially in the US and European markets, amylases have been added to detergents along with proteases since 1973 to capitalize on the activity of the amylases toward starch-containing soils. From different amylases available, only a-amylases are used for detergents. They are able to catalyze the hydrolysis of the amylose and amylopectin fractions of starch, i.e., cleavage of the a-l,4-glycosidic bonds of the starch chain [15]. This facilitates the removal of starch-based stains by the detergent. 

Maltose (1) occurs as a breakdown product of the starches contained in malt ( malt sugar see p. 148) and as an intermediate in intestinal digestion. In maltose, the anomeric OH group of one glucose molecule has an a-glycosidic bond with C-4 in a second glucose residue. 

The important difference between a and 3 glycosidic bonds can be seen in the digestibility of the major plant polysaccharides cellulose and starch. 

The process starts in the mouth during mastication where salivary a-amy-lase cleaves some of the a-1,4 glycosidic bonds of starch. 

On the other hand, amylopectin accounts for about 80 per cent by weight of starch and consists of hundreds of glucose molecules linked together by l,4 -a- and also 1,6 -a-glycoside bonds. Amylopectin contains branches (nonlinear), approximately one in every 20 to 25 glucose units. Hydrolysis of amylopectin yields maltose. 

Glycogen and starch ingested in the diet are hydrolyzed by a-amylases, enzymes in saliva and intestinal secretions that break (al—>4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (fil—>4) linkages. Termites readily digest cellulose... 

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