Introduction to Carbohydrates

1. Introduction to Carbohydrates

2. Introduction • Carbohydratesare the most abundant organic molecules in nature. • They have a wide range of functions, including • Providing a significant fraction of the energy in the diet of most organisms, • Acting as a storageform of energy in the body, • Serving as cell membrane components that mediate some forms of intercellular communication. • Carbohydrates also serve as a structural component of many organisms, including the cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose of plants. • The general formula for many of the simpler carbohydrates is(CH2O)n, hence the name "hydrate of carbon."

3. Classification and structure ofcarbohydrates • Monosaccharides(simple sugars) can be classified according to the number of carbon atoms they contain.

4. Classification and structure ofcarbohydrates • Carbohydrateswith an aldehyde as their most oxidized functional group are called aldoses. • whereas those with a keto group as their most oxidizedfunctional group are called ketoses. • For example, glyceraldehyde is an aldose, whereas dihydroxyacetone is a ketose

5. Classification and structure ofcarbohydrates • Monosaccharidescan be linked by glycosidic bondsto create larger structures. • Disaccharidescontain two monosaccharide units. • Oligosaccharidescontain from three to about twelvemonosaccharide units. • whereas polysaccharidescontain more than twelvemonosaccharide units, and can be hundreds of sugar units in length.

6. A. Isomers and epimers • Compounds that have the same chemical formula but have differentstructuresare called isomers. • Forexample, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula C6H12O6 • If two monosaccharidesdifferin configuration around only one specific carbon atom (with the exception of the carbonyl carbon) they are defined as epimersofeach other. (they are also isomers!) • For example, glucose and galactose are C-4 epimers—their structures differ only in the position of the -OHgroup at carbon4.

7. A. Isomers and epimers • Glucose and mannose are C-2 epimers. • However, galactose and mannose are NOTepimers—they differ in the position of –OH groups at two carbons (2 and 4) and are, therefore, defined only as isomers . • [Note: The carbons in sugars are numbered beginning at the end that contains the carbonyl carbon—that is, the aldehyde or keto group]

8. B. Enantiomers • A special type of isomerism is found in the pairs of structures that are mirrorimages of each other. • These mirror images are called enantiomers, andthetwo members of the pair are designated as a D-and an L-sugar. • The vast majority of the sugars in humans are D-sugars

9. Cyclization of monosaccharides • Less than 1% of each of the monosaccharides with five or more carbons exists in the open-chain (acyclic) form. • Rather, they are predominantly found in a ring form, in which the aldehyde(or ketone) group has reacted with an alcoholgroup on the same sugar.

10. Cyclization of monosaccharides 1. Anomeric carbon • Formation of a ring results in the creation of an anomericcarbonat carbon1 of an aldose or at carbon2 of a ketose. • These structures are designated the α or βconfigureations of the sugar, for example α-D-glucose, andβ-D-glucose

11. Cyclization of monosaccharides 2. Reducing sugars • If the hydroxylgroup on the anomericcarbonof a cyclized sugar is notlinked to another compound by a glycosidicbond, the ring can open. • The sugar can act as a reducingagent, and is termed a reducingsugar. • A reducing sugar can react with chemical reagents (for example, Benedict's solution) and reduce the reactive component, with the anomeric carbonbecoming oxidized. • [Note: Only the state of the oxygenon the anomericcarbondetermines if the sugar is reducingor nonreducing-the other hydroxylgroups on the molecule are not involved.]

12. Joining of monosaccharides • Monosaccharidescan be joined to form disaccharides, oligosaccharides, and polysaccharides. • Important disaccharides include • Lactose (glucose + galactose) • Sucrose (glucose + fructose) • Maltose (glucose + glucose) • Important polysaccharides include branched glycogen(from animal source) and starch(plant source) and unbranched cellulose(plant source), each is a polymer of glucose. • The bonds that link sugars are called glycosidic bonds • These are formed by enzymes known as glycosyltransferases

13. 1. Naming glycosidic bonds • Glycosidicbondsbetween sugarsare named according to the numbers of the connected carbons, and with regard to the position of the anomerichydroxylgroup of the sugar involved in the bond. • If this anomerichydroxylis in the α-configuration, the linkage is an α-bond. • If it is in the β-configuration, the linkage is a β-bond. • Lactose, for example, is synthesized by forming a glycosidicbondbetween carbon 1 of β-galactoseand carbon 4 of glucose. • The linkage is, therefore, a β (1→4) glycosidicbond.

15. D. Complex carbohydrates • Carbohydratescan be attached by glycosidic bondsto non-carbohydratestructures, including; • Purines and pyrimidines (found in nucleic acids), • Aromaticrings (such as those found in steroids and bilirubin), • Proteins (found in glycoproteins and glycosaminoglycans), • Lipids (found in glycolipids).

16. N- and O-glycosides: • If the group on the non-carbohydrate molecule to which the sugar is attached is an –NH2 group, the structure is an N-glycoside and the bond is called an N—glycosidiclink. • If the group is an –OH, the structure is an O-glycoside, and the bond is an O-glycosidic link.

17. Digestion of carbohydrates • The principal sites of dietary carbohydratedigestion are the mouth and intestinallumen. • There is little monosaccharidepresent in diets of mixed animal and plant origin. • Therefore, the enzymesneeded for degradation of most dietary carbohydratesare primarily disaccharidasesand endoglycosidases(that break oligosaccharides and polysaccharides).

18. Digestion of carbohydrates • Hydrolysis of glycosidicbondsis catalyzed by a family of (glycoside hydrolases) glycosidasesthat degrade carbohydratesinto their reducingsugar components.

19. A. Digestion of carbohydrates begins in the mouth • The major dietary polysaccharides are of animal (glycogen) and plant origin (starch, composed of amylose and amylopectin). • During mastication, salivary α-amylaseacts briefly on dietary starch in a random manner, breaking some α (14) bonds. • Because branched amylopectin and glycogenalso contain α (16) bonds, the digest resulting from the action of α-amylasecontainsamixture of smaller, branched oligosaccharide molecules (dextrins) .

20. Digestion of carbohydrates • Carbohydratedigestion halts temporarily in the stomach, because the high acidity inactivates the salivary α-amylase. • There are both α(14) andβ(14)-endoglucosidasesin nature, but humans do not produce and secrete the latter in digestive juices. • Therefore, they are unable to digest cellulose—acarbohydrateof plant origin containingβ(14)glycosidic bonds between glucoseresidues.

21. B. Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine • When the acidicstomach contents reach the smallintestine, they are neutralizedby bicarbonate secreted by the pancreas, and pancreatic α-amylase continues the process of starch digestion.

22. C. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal cells • The final digestive processes occur at the mucosal lining of the upper jejunum • For example,isomaltasecleaves the α(16) bond in isomaltose and maltasecleaves maltose, both producing glucose. • sucrasecleaves sucroseproducing glucose and fructose, • lactase(β-galactosidase) cleaves lactoseproducinggalactoseand glucose. • These enzymes are secreted through, and remain associated with, the luminal side of the brush border membranes of the intestinalmucosalcells.

23. D. Absorption of monosaccharides by intestinal mucosal cells • The duodenum and upper jejunum absorb the bulk of the dietary sugars. • Insulinis not required for the uptake of glucose by intestinal cells. However, different sugars have different mechanisms of absorption. • For example, galactoseand glucoseare transported into the mucosal cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions.

24. D. Absorption of monosaccharides by intestinal mucosal cells • Fructoseuptake requires a sodium-independent monosaccharide transporter (GLUT-5) for its absorption. • All three monosaccharidesare transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2.

25. Abnormal degradation of disaccharides • Any defect in a specific disaccharidaseactivity of the intestinalmucosa causes the passage of undigested carbohydrateinto the largeintestine. • As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the largeintestine, causing osmotic diarrhea. • This is reinforced by the bacterial fermentation of the remaining carbohydrateto two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO2 and H2 gas, causing abdominalcramps, diarrhea, and flatulence.

26. 1. Digestive enzyme deficiencies • Genetic deficiencies of the individual disaccharidasesresult in disaccharideintolerance. • Alterations in disaccharidedegradation can also be caused by a variety of intestinaldiseases, malnutrition, or drugs that injure the mucosa of the small intestine. • For example, brush border enzymes are rapidly lost in normal individuals with severediarrhea, causing a temporary, acquired enzyme deficiency.

27. Lactose intolerance Lactase-deficient • It is thought to be caused by small variations in the DNA sequence of a region on chromosome2 that controls expression of the gene for lactase. • Treatment for this disorder is to reduce consumption of milk while eating yogurts and cheeses, as well as green vegetables such as broccoli, to ensure adequate calcium intake; to use lactase-treated products; or to take lactasein pill form prior to eating.

28. Sucrase-isomaltase complex deficiency • This deficiency results in an intolerance of ingested sucrose. • Treatment includes the dietary restriction of sucrose, and enzyme replacement therapy.

29. Diagnosis • Identification of a specific enzyme deficiency can be obtained by performing oral tolerance tests with the individual di -saccharides. • Measurement of hydrogen gas in the breath is a reliable test for determining the amount of ingested carbohydrate not absorbed by the body, but which is metabolized instead by the intestinal flora