Atp

triose phosphate isomerase

-> Glyceraldehyde

-> 3-phosphate phosphomannose isomerase

FIGURE 14-9 Entry of glycogen, starch, disaccharides, and hexoses into the preparatory stage of glycolysis.

Dietary Polysaccharides and Disaccharides Undergo Hydrolysis to Monosaccharides

For most humans, starch is the major source of carbohydrates in the diet. Digestion begins in the mouth, where salivary «-amylase (Fig. 14-9) hydrolyzes the internal glycosidic linkages of starch, producing short poly-saccharide fragments or oligosaccharides. (Note that in this hydrolysis reaction, water, not Pj, is the attacking species.) In the stomach, salivary a-amylase is inactivated by the low pH, but a second form of a-amylase, secreted by the pancreas into the small intestine, continues the breakdown process. Pancreatic a-amylase yields mainly maltose and maltotriose (the di- and trisac-charides of a(1n4) glucose) and oligosaccharides called limit dextrins, fragments of amylopectin containing a(1n6) branch points. Maltose and dextrins are degraded by enzymes of the intestinal brush border (the fingerlike microvilli of intestinal epithelial cells, which greatly increase the area of the intestinal surface). Dietary glycogen has essentially the same structure as starch, and its digestion proceeds by the same pathway.

Disaccharides must be hydrolyzed to monosaccha-rides before entering cells. Intestinal disaccharides and dextrins are hydrolyzed by enzymes attached to the outer surface of the intestinal epithelial cells:

Dextrin + nH2O-> n D-glucose dextrinase

Maltose + H2O-> 2 D-glucose maltase

Lactose + H2O-> D-galactose + D-glucose lactase

Sucrose + H2O-> D-fructose + D-glucose sucrase

Trehalose + H2O-> 2 D-glucose trehalase

The monosaccharides so formed are actively transported into the epithelial cells (see Fig. 11-44), then passed into the blood to be carried to various tissues, where they are phosphorylated and funneled into the glycolytic sequence.

T Lactose intolerance, common among adults of most human populations except those originating

Nonreducing end

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