Feeder Pathways for Glycolysis:- Dietary Polysaccharides and Disaccharides Undergo Hydrolysis to Monosaccharides
For most humans, starch is the major source of carbo hydrates in the diet. Digestion begins in the mouth, where salivary -amylase (Fig. 14–9) hydrolyzes the in ternal glycosidic linkages of starch, producing short poly saccharide fragments or oligosaccharides. (Note that in this hydrolysis reaction, water, not Pi, is the attacking species.) In the stomach, salivary -amylase is inactivated by the low pH, but a second form of α-amylase, secreted by the pancreas into the small intestine, continues the breakdown process. Pancreatic -amylase yields mainly maltose and maltotriose (the di- and trisac charides of (α1→4) glucose) and oligosaccharides called limit dextrins, fragments of amylopectin containing (α1→6) branch points. Maltose and dextrins are de graded 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:
The monosaccharides so formed are actively trans ported into the epithelial cells, then passed into the blood to be carried to various tissues, where they are phosphorylated and funneled into the glycolytic sequence.
Lactose intolerance, common among adults of most human populations except those originating in Northern Europe and some parts of Africa, is due to the disappearance after childhood of most or all of the lactase activity of the intestinal cells. Lactose cannot be completely digested and absorbed in the small intestine and passes into the large intestine, where bacteria con vert it to toxic products that cause abdominal cramps and diarrhea. The problem is further complicated because undigested lactose and its metabolites increase the osmolarity of the intestinal contents, favoring the retention of water in the intestine. In most parts of the world where lactose intolerance is prevalent, milk is not used as a food by adults, although milk products predigested with lactase are commercially available in some countries. In certain human disorders, several or all of the intestinal disaccharidases are missing. In these cases, the digestive disturbances triggered by dietary disaccharides can sometimes be minimized by a controlled diet.
FIGURE 14–10 Glycogen breakdown by glycogen phosphorylase. The enzyme catalyzes attack by inorganic phosphate (pink) on the terminal glucosyl residue (blue) at the nonreducing end of a glycogen molecule, releasing glucose 1-phosphate and generating a glycogen molecule shortened by one glucose residue. The reaction is a phosphorolysis (not hydrolysis).
