The attachment of carbohydrates to noncarbohydrate structures, such as nucleic acids, steroids, bilirubin, glycoproteins, proteoglycans, and glycolipids, is facilitated by glycosidic bonds. If the sugar is attached to an –NH2 group on the noncarbohydrate molecule, it forms an N-glycosidic link. On the other hand, if the attachment is to an –OH group, it forms an O-glycosidic link. It is important to note that all sugar-sugar glycosidic bonds are O-type linkages.

In terms of dietary carbohydrate digestion, the mouth and intestinal lumen are the main sites. This process occurs rapidly and is catalyzed by enzymes called glycoside hydrolases (glycosidases), which break down glycosidic bonds. Since diets typically contain minimal amounts of monosaccharides from mixed animal and plant sources, the enzymes primarily target polysaccharides and oligosaccharides, as well as tri- and disaccharides, breaking them down into their reducing sugar components. Glycosidases are usually specific to the structure, configuration, and type of bond they target. The final products of carbohydrate digestion are glucose, galactose, and fructose, which are absorbed by the cells of the small intestine known as enterocytes.

One of the enzymes involved in carbohydrate digestion is salivary α-amylase, which acts briefly on dietary starch and glycogen during mastication. It hydrolyzes random α(1→4) bonds. However, it is worth noting that humans lack the ability to produce β(1→4)-endoglucosidases, which are necessary for digesting cellulose, a plant-derived carbohydrate containing β(1→4) glycosidic bonds. Amylopectin and glycogen, which have α(1→6) bonds that α-amylase cannot hydrolyze, result in the formation of a mixture of short, branched, and unbranched oligosaccharides called dextrins after digestion. Disaccharides are also present in the digest as they are resistant to amylase.

Carbohydrate digestion temporarily ceases in the stomach due to the high acidity that inactivates salivary α-amylase. However, when the acidic stomach contents reach the small intestine and are neutralized by bicarbonate from the pancreas, pancreatic α-amylase takes over the starch digestion process.

Additionally, intestinal disaccharidases play a role in carbohydrate digestion.
Carbohydrates can be attached by glycosidic bonds to noncarbohydrate structures, including purine and pyrimidine bases (found in nucleic acids), aromatic rings (such as those found in steroids and bilirubin), proteins (found in glycoproteins and proteoglycans), and lipids (found in glycolipids). If the group on the noncarbohydrate molecule to which the sugar is attached is an –NH2 group, then the bond is called an N-glycosidic link. If the group is an –OH, then the bond is an O-glycosidic link (Fig. 7.7). [Note: All sugar-sugar glycosidic bonds are O-type linkages.]
III. DIETARY CARBOHYDRATE DIGESTION
The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. This digestion is rapid and is catalyzed by enzymes known as glycoside hydrolases (glycosidases) that hydrolyze glycosidic bonds (Fig. 7.8). Because little monosaccharide is present in diets of mixed animal and plant origin, the enzymes are primarily endoglycosidases that hydrolyze polysaccharides and oligosaccharides and disaccharidases that hydrolyze tri-and disaccharides into their reducing sugar components. Glycosidases are usually specific for the structure and configuration of the glycosyl residue to be removed as well as for the type of bond to be broken. The final products of carbohydrate digestion are the monosaccharides glucose, galactose, and fructose that are absorbed by cells (enterocytes) of the small intestine.
A. Salivary α-amylase
The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication (chewing), salivary α-amylase acts briefly on dietary starch and glycogen, hydrolyzing random α(1→4) bonds. [Note: There are both α(1→4)-and β(1→4)-endoglucosidases in nature, but humans do not produce the latter. Therefore, we are unable to digest cellulose, a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannot hydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins (Fig. 7.9). [Note: Disaccharides are also present as they, too, are resistant to amylase.]
Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase.
B. Pancreatic α-amylase
When the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas, and pancreatic αamylase continues the process of starch digestion.
C. Intestinal disaccharidases