Digestion, Absorption, and Transport of Carbohydrates

University:
Kentucky State University
Course:
CHE 315 | Biochemistry
Academic year:

2021
Views:

214

Pages:

36
Author:

Carissa Thomas

Digestion, Absorption, and Transport of Carbohydrates Carbohydrates are the largest source of dietary calories for most of the world's population. The major carbohydrates in the US diet are starch, lactose, and sucrose. The starches amylose and amylopectin are polysaccharides composed of hundreds to millions of glucosyl units linked together through a-1,4-and a-1,6-glycosidic bonds (Fig. 21.1). Lactose is a disaccharide composed of glucose and galactose, linked together through a ẞ-1,4-glycosidic bond. Sucrose is a disaccharide composed of glucose and fructose, linked through an a-1,2- glycosidic bond. The digestive processes convert all of these dietary carbohydrates to their constituent monosaccharides by hydrolyzing glycosidic bonds between the sugars. No. This polysaccharide is cellulose, which contains ẞ-1,4- glycosidic bonds. Pancreatic and salivary a-amylase cleave only a-1,4-bonds between glucosyl units. Glucoamylase is an exoglycosidase that is specific for the a-1,4-bonds between glucosyl residues (Fig. 21.4A). It begins at the nonreducing end of a polysaccharide or limit dextrin, and it sequentially hydrolyzes the bonds to release glucose monosaccharides. It will digest a limit dextrin down to isomaltose, the glucosyl disaccharide with an a-1,6-branch, that is subsequently hydrolyzed principally by the isomaltase activity in the sucrase-isomaltase complex. A Maltose 1.4 bond B C HO 1,8 bond HOA Matace LOH HO Reouding HO- CH Maltotrione Trehalose CHOH HO OH OH Glucose Ticht activity CH OH H HOHC H Glucose OH OH FIGURE 21.4 A. Glucoamylase activity. Glucoamylase is an -1,4-xoglycosidase that initiates cleavage at the nonreducing end of the sugar. Thus, for maltatriose, the bond labeled in hydrolyzed first, which then allows the bond at position 2 to be the next one hydrolyzed B. Isomaltase activity. Arrows indicate the o 1,6-bonds that are cleavod. C. Trehalose. This disaccharide contains two glucose moreties linked by an unusual bond that joins their anomeric carbons. It is cleaved by trehsize 2. Sucrase-Isomaltase Complex The structure of the sucrase-isomaltase complex is similar to that of glucoamylase, and these two proteins have a high degree of sequence homology. However, after the single polypeptide chain of sucrase-isomaltase is inserted through the membrane and the protein protrudes into the intestinal lumen, an intestinal protease clips it into two separate subunits that remain attached to each other through noncovalent interactions. Each subunit has a catalytic site that differs in substrate specificity from the other through noncovalent interactions. The sucrase-maltase site accounts for approximately 100% of the intestine's ability to hydrolyze sucrose in addition to maltase activity; the isomaltase- maltase site accounts for almost all of the intestine's ability to hydrolyze a-1,6- bonds (see Fig. 21.4B), in addition to maltase activity. Together, these sites account for approximately 80% of the maltase activity of the small intestine. The remainder of the maltase activity is found in the glucoamylase complex. Individuals with genetic deficiencies of the sucrase-isomaltase complex show symptoms of sucrose intolerance but are able to digest normal amounts of starch in a meal without problems. The maltase activity in the glucoamylase complex, and residual activity in the sucrase-isomaltase complex (which is normally present in excess of need), is apparently sufficient to digest normal amounts of dietary starch. 3. Trehalase Trehalase is only half as long as the other disaccharidases and has only one catalytic site. It hydrolyzes the glycosidic bond in trehalose, a disaccharide composed of two glucosyl units linked by an a-bond between their anomeric carbons (see Fig. 21.4C). Trehalose, which is found in insects, algae, mushrooms, and other fungi, is not currently a major dietary component in the United States. However, unwitting consumption of trehalose can cause nausea, vomiting, and other symptoms of severe gastrointestinal distress if consumed by an individual deficient in the enzyme. Trehalase deficiency was discovered when a woman became very sick after eating mushrooms and was initially thought to have a-amanitin poisoning. 4. B-Glycosidase Complex (Lactase-Glucosylceramidase) The B-glycosidase complex is another large glycoprotein found in the brush border that has two catalytic sites extending in the lumen of the intestine. However, its primary structure is very different from that of the other enzymes, and it is attached to the membrane through its carboxyl end by a phosphatidylglycan anchor (see Fig. 10.6). The lactase catalytic site hydrolyzes

Digestion, Absorption, and Transport of Carbohydrates

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