this chapter covers several aspects of carbohydrate digestion. first it goes over some basics about carbohydrates and digestion in general, then talks about the specific enzymes at work in the brush border of the small intestine, then a brief section about lactose intolerance, and finally, a section about the transport of glucose from the intestine into the blood.
the normal american diet is made of 40-50% carbohydrates, and of this, 50-60% is made of the starch molecules (10,000 to 1 million glucosyl units long) amylose and amylopectin. amylose consists of glucose molecules bonded with alpha 1-4 bonds, while amylopectin is the same, but with alpha 1-6 bonds (and therefore branches) as well. digestion of these starches begins in the mouth with alpha amylase, which is an example of an endoglucosidase-- an enzyme which cleaves alpha 1-4 bonds at random intervals. this initial digestion leaves chunks of polysaccharides called alpha-dextrins. stomach acid deactivates the amylase, and carbohydrate digestion continues in the small intestine.
in the small intestine, carbohydrate digestion takes place mainly on the "brush border" of the intestinal mucosa, which have enzymes that are embedded in the intestinal membrane that poke into the intestinal lumen. the four major "glucosidases" are glucoamylase, trehalase, beta-glucosidase, and sucrase-isomaltase. each of these enzymes has a specific structure and catalytic sites that are specific to certain types of carbohydrates. glucoamylase breaks down alpha 1-4 bonds from the non reducing end ("tail end") of the sugar, until isomaltase remains-- isomaltase is basically just two sugar units branched together in an alpha 1-6 bond. the action of glucoamylase on polysaccharides seems similar to the action of glycogen phosphorylase in the breakdown of glycogen in that both are removing glucose residues one at a time from the tail end, and both can not remove the last glucose unit (or in glucose phosphorylase's case, the last 4).
sucrase-isomaltase is another brush border glucosidase with two catalytic sites. one is specific to isomaltose and maltose (and thus can break down the isomaltose from glucoamylase's activity) and one is specific to sucrose and maltose. beta glucosidase also has two catalytic sites. one is specific to breaking the beta 1-4 bond between glucose and galactose in lactose, and the other is specific to cleaving beta 1-4 bonds in glycolipids. the last glucosidase in the brush border is trehalase, which has only one catalytic site which is specific to trehalose, a sugar found only in some insects, mushrooms, and algae. these four enzymes work in tandem to break down the different types of carbohydrates that are dumped into the duodenum. the relative concentration of these enzymes change depending on the location in the gut. for example, pancreatic alpha amylase is secreted mainly in the duodenum, sucrase-maltase and beta-glucosidase is mainly in the jejunum, and glucoamylases is most common in the ileum.
a couple other notes about carbohydrate digestion. lactose intolerance occurs with either a lactase deficiency or intestinal damage. most adults only have 10% of the lactase activity that they had as children. lactose intolerance can ultimately lead to malabsorption of nutrients: lactose is undigested in the gut, and is instead metabolized by the bacterial flora in the colon, producing gases and lactic acid. the lactic acid can increase the intestinal lumen's osmolarity and cause water to distend the abdomen, which increases peristalsis and potentially causes diarrhea and malabsorption of other nutrients.
the last section of the chapter focused on the membrane channels that transport glucose across the intestinal lining into the blood. there are two types of such transport proteins, regular facilitated glucose channels and Na+ facilitated channels. Na+ facilitated channels use an ATPase Na+/K+ pump to establish a low concentration of Na+ inside the intestinal cell. the concentration gradient that is formed from the higher Na+ concentration in the intestinal lumen is then coupled with glucose transport in these membrane proteins. facilitative glucose transporters (also called "GLUT" transporters) do not require ATP and simply allow glucose to flow down its concentration gradient from the intestinal lumen, into the epithelium, and out into the serosa side ("blood side").
a few interesting notes relating to these glucose transporting membrane proteins. the liver's GLUT transporters have a higher than usual Km (the concentration at which the substrate is half-saturated with enzyme-- generally representing the enzyme's affinity for substrate and in this case the GLUT for glucose) for glucose, because it will only accept glucose into its cells (to be converted into glycogen for storage) when the blood glucose concentration is very high, like after a high carb meal. also, in muscle and fat cells, insulin stimulates glucose absorption by means of recruiting intracellular vesicles of glucose transport proteins to the cell membrane, where they can facilitate glucose transport into the cell.
questions
1. what percentage of a normal american diet consists of carbohydrates?
2. what percentage of the carbohydrate calories consists of amylose and amylopectin?
3. how many glucosyl units do amylose and amylopectin have?
4. what types of bonds do amylose and amylopectin have?
5. what are the major natural sweeteners found in fruit, honey, and vegetables?
6. what is the major dietary carb found from animal sources?
7. how much liquid do the salivary glands secrete per day?
8. what is an "endoglucosidase" and what is an example of one?
9. what is salivary amylase inactivated by?
10. how much digestive enzyme is secreted by the pancreas per day?
11. what are in the pancreatic secretions?
12. what are oligosaccharides?
13. what are the glucosidases found in the brush border of the small intestine?
14. describe the digestion of lactose and sucrose in the small intestine.
15. describe the structure and activity of glucoamylase?
16. what are alpha-dextrins vs. limit-dextrins?
17. describe the structure and activity of the sucrase-isomaltase complex.
18. what percentage of maltase activity can be attributed to the sucrose-isomaltose complex?
19. describe the structure and activity of trehalase.
20. describe the structure and activity of the beta-glucosidase complex
21. pancreatic alpha-amylase activity is highest in...
22. sucrase-isomaltase activity is highest in...
23. beta-glucosidase activity is highest in...
24. glucoamylase activity is highest in...
25. what type of carbohydrates enter the colon?
26. what are the fatty acids that result from bacterial starch digestion in the colon?
27. what are the gases that result from bacterial starch digestion in the colon?
28. lactose intolerance can be caused by...
29. what are normal lactase levels of an adult as compared to a child?
30. what happens when lactose is ingested by a lactose intolerant person?
31. what does the glycemic index represent?
32. which sugars have the highest glycemic index?
33. what are the two types of glucose transport proteins?
34. describe the mechanism of the Na+ dependent glucose transporter.
35. what are facilitative glucose transporters?
36. compare the digestion of glucose with that of galactose and fructose.
37. in body tissues, why is glucose transport across membranes not the rate limiting step of glucose metabolism?
38. how does the high Km of glucose transport proteins in the liver relate to the liver's blood glucose regulation?
39. how is insulin related to glucose transport proteins in the liver?
answers
1. 40-45%
2. 50-60%
3. 10,000 to 1 million
4. amylose has alpha 1-4 bonds between glucosyl residues. amylopectin has alpha 1-4 bonds between glucosyl units as well as alpha 1-6 bonds between branches.
5. fructose, sucrose, glucose.
6. lactose, which is made of glucose and galactose.
7. ~1 liter a day
8. an enzyme that breaks internal alpha 1-4 bonds in a polysaccharide at random intervals, such as amylase.
9. acidity of the stomach
10. ~1.5 liters a day
11. trypsinogen, chymotrypsinogen, carboxypeptidase (for digestion of proteins), alpha-amylase (for carbohydrates), lipase (fat), and bicarbonate (neutralizing gastric acidity)
12. 4-9 glucosyl units long, contain one or more alpha-1,6 branches.
13. beta-glucoamylase, sucrase-isomaltase, beta-glycosidase, trehalase.
14. converted to monosaccharides by glucosidases attached to the brush border lining.
15. it has two domains that have different substrate specificity, and acts as an exoglucosidase by breaking alpha 1-4 bonds on the non reducing ("tail end") of the saccharides, releasing glucose units until only isomaltose remains.
16. alpha dextrins are the pieces of polysaccharides that result from salivary alpha-amylase's endoglucosidase activity. limit dextrins are oligosaccharides that have been formed from the further breakdown of polysaccharides by pancreatic alpha-amylase.
17. made of two subunits: sucrose-maltose subunit cleaves alpha 1-4 bonds in sucrose, maltose. isomaltose-maltose unit cleaves alpha 1-6 bonds in isomaltose, and also breaks down maltose.
18. 80%
19. trehalose is a smaller dissaccharidase that only has one catalytic site with specificity for trehalose, which is a relatively rare source of carbohydrate found in some insects, algae, and mushrooms.
20. has two catalytic sites: glucosyl-ceramidase site, which cleaves beta bonds in glycolipids, and lactase site, which breaks beta 1-4 bonds between glucose and galactose in lactose.
21. duodenum
22. jejunum
23. jejunum
24. ileum
25. any undigested starches: starches high in amylose, poorly hydrated starches (like in dried beans), dietary fiber.
26. acetic acid, propionic acid, butyric acid. 2,3,4 carbon.
27. hydrogen gas, CO2, methane
28. low lactase levels or intestinal injury
29. 10% of the level of a child
30. lactose is undigested by the lactase in the small intestine and is therefore metabolized by the colonic bacteria, which produces lactic acid, methane, and H2. the increased lactic acid increases osmolarity of the intestinal lumen, causing more water to be dumped into the lumen, causing excess peristalsis, causing malabsorption of other nutrients.
31. how quickly blood glucose levels rise after consumption of a food.
32. glucose and maltose.
33. sodium dependent glucose transporters and facilitative glucose transporters
34. a Na+ / K+ ATPase pump pumps Na+ out of the intestinal epithelium cells so that there is a low Na+ concentration within. the Na+ dependent transporter channels then use the resulting Na+ concentration gradient to power movement of glucose within the cell.
35. these are glucose channels that exist on both the luminal and serosal side of the intestinal epithelium that allow glucose to move down its concentration gradient without expenditure of energy.
36. galactose passes through the intestinal membrane in a similar way to glucose- via both Na+ facilitated channels and facilitative glucose transporters. fructose passes through by facilitated diffusion only.
37. because the transport proteins have a high affinity for glucose (a low Km) or are present in high numbers.
38. the liver will only transport glucose into its cells (and therefore convert glucose to glycogen for storage) when the blood glucose level is high, such as right after a high carb meal.
39. binding of insulin recruits GLUT proteins from intracellular vescicles onto the membrane.
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