1. BIOC 460 - DR. TISCHLER - LECTURE 24
DIGESTION & ABSORPTION OF CARBOHYDRATES

2. OBJECTIVES
1. Principal dietary monosaccharides and disaccharides.
2. General structural features of amylose, amylopectin, glycogen and cellulose.
3. Products of starch digestion by pancreatic α-amylase.
Four glycoprotein brush-border enzyme complexes, their roles in digestion of carbohydrate, products of their actions.
How the primary nutritional monosaccharides (glucose, fructose, and galactose) are moved from the intestinal lumen to capillaries.
6. Tissue where each facilitative glucose transporters (GLUT 1
to GLUT 5) is found; describe their basic function

3. Figure 1. Structures of the most common dietary monosaccharides.
GLUCOSE
CH2OH OH
FRUCTOSE HO
Figure 1. Structures of the most common dietary monosaccharides.

4. Figure 1. Structures of the most common dietary disaccharides.
CH2OH
SUCROSE
(glucose -
fructose)
-1,2-bond OH O
MALTOSE
(glucose -
glucose)
CH2OH OH
-1,4-bond O
LACTOSE
(galactose -
glucose) OH
CH2OH
-1,4-bond O
TREHALOSE
(glucose -
glucose)
CH2OH OH
-1,1-bond
Figure 1. Structures of the most common dietary disaccharides.

5. O
-[1- 4] linkages
-[1-6] linkage
CH2OH
CH2
Figure 2. General structure of amylopectin (principal plant polysaccharide) and glycogen (principal animal polysaccharide)

6. TYPES OF POLYSACCHARIDES
Amylose:
plant CHO storage
linear chain with 1,4 glycosidic bonds
Amylopectin:
amylose+branches with 1,6 bonds
Glycogen
animal CHO storage
similar to amylopectin except more branches
Cellulose:
linear chain with 1,4 glycosidic bonds

7. Figure 3. Digestion of carbohydrates –
Top – Early digestion depicting the intake of dietary carbohydrates and the role of salivary amylase

8. Figure 3. Digestion of carbohydrates
Middle – Luminal digestion showing the combined roles of the stomach, pancreas and intestine.

9. 16 e t c .
14
-amylase
Maltotriose
Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

10. 16 e t c .
14
-amylase
-limit dextrin
(branched oligosaccharide; unbranched also produced)
Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

11. 16 e t c .
14
-amylase
Maltose
Glucose
Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

12. Figure 3. Digestion of carbohydrates
Bottom – Summary of the brush border glycoprotein complexes containing disaccharidases and oligosachharidases.

13. Lactase/-galactosidase Lactase
Table 1. Brush border complexes of the luminal membrane (brush border).
Enzyme Complex
Sucrase-isomaltase:
Sucrase
Maltase
Isomaltase
Glucoamylase
(exo)amylase
Lactase/-galactosidase
Lactase
Trehalase:
Trehalase
Natural Substrate
Sucrose (table sugar)
Maltose; maltotriose
-limit dextrins; isomaltose
Oligosaccharides; including -limit dextrins; not 1 6 bond
Lactose (milk sugar)
Trehalose (mushrooms)
Product(s)
Fructose + glucose
Glucose
Galactose + glucose

14. Primary –
defective lactase enzyme at birth
late onset appears gradually in late teens/adults
Secondary (acquired) –
consequence of diseases that damage brush border
tropical sprue – bacterial infection
celiac sprue – gluten-sensitive enteropathy

15. Normal
intestinal villi
Patient with
celiac sprue

16. Figure 5. Absorption of monosaccharides
Fructose;
Glucose
Na+
Galactose
Lumen of
also glucose
intestine
Galactose
Glucose
Intestinal
Na+
SGLT-1
GLUT-5
Epithelial cell
Brush border
Galactose
Glucose
Na+
Fructose
(+glucose)
Glucose +
Galactose
Na+
2K+
GLUT-2
ATP
3Na+
2K+
contraluminal membrane
ADP + Pi
2K+
3Na+
to capillaries
3Na+
2K+
= facilitated diffusion
= Na,K-ATPase
= Na + -
dependent co -
transport
Figure 5. Absorption of monosaccharides

17. Table 2. Types of Facilitative Glucose Transporters
Tissue
Function
GLUT-1
many mammalian tissues (e.g., red blood cell, brain, kidney)
glucose uptake
GLUT-2
Liver; pancreas -cell, contraluminal membrane of intestinal cell
high capacity; low affinity; regulates insulin release by pancreas; liver: removes glucose after a meal
GLUT-3
many mammalian tissues; especially brain, kidney
GLUT-4
skeletal muscle, heart, adipose
insulin-stimulated glucose uptake; helps prevent hyperglycemia through insulin action
GLUT-5
small intestine
primarily fructose absorption