3rd lecture CARBOHYDRATES AND ITS CLASSIFICATION.pptx

carbohydrate, class of naturally occurring compounds and derivatives formed from them. Carbohydrates are probably the most abundant and widespread organic substances in nature and are essential constituents of all living things. The term carbohydrate means “watered carbon”; the general formula Cx(H2O)y is commonly used to represent many carbohydrates.


starch granulesWheat starch granules stained with iodine.
In the early part of the 19th century, substances such as wood, starch, and linen were found to be composed mainly of molecules containing atoms of carbon (C), hydrogen (H), and oxygen (O) and to have the general formula C6H12O6; other organic molecules with similar formulas were found to have a similar ratio of hydrogen to oxygen.

Carbohydrates are formed by green plants from carbon dioxide and water during the process of photosynthesis. They serve as energy sources and as essential structural components in organisms; in addition, part of the structure of nucleic acids, which contain genetic information, consists of carbohydrate.

General features
Classification and nomenclature
Learn about the structures and uses of the simple sugars glucose, fructose, and galactose
Learn about the structures and uses of the simple sugars glucose, fructose, and galactoseMonosaccharides play an important role in energy transfer.
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Although a number of classification schemes have been devised for carbohydrates, the division into four major groups—monosaccharides, disaccharides, oligosaccharides, and polysaccharides—used here is among the most common. Most monosaccharides, or simple sugars, are found in grapes, other fruits, and honey. Although they can contain from three to nine carbon atoms, the most common representatives consist of five or six joined together to form a chainlike molecule. Three of the most important simple sugars—glucose (also known as dextrose, grape sugar, and corn sugar), fructose (fruit sugar), and galactose—have the same molecular formula, (C6H12O6), but, because their atoms have different structural arrangements, the sugars have different characteristics; i.e., they are isomers.

Organic Chemistry: Carbohydrates and proteins. (Compton's 17:604) Fructose and Glucose.

Slight changes in structural arrangements are detectable by living things and influence the biological significance of isomeric compounds. It is known, for example, that the degree of sweetness of various sugars differs according to the arrangement of the hydroxyl groups (―OH) that compose part of the molecular structure. A direct correlation that may exist between taste and any specific structural arrangement, however, has not yet been established; that is, it is not yet possible to predict the taste of a sugar by knowing its specific structural arrangement. The energy in the chemical bonds of glucose indirectly supplies most living things with a major part of the energy that is necessary for them to carry on their activities. Gal

1. Carbohydrate Any biochemistry textbook may work, e.g. Lippincott’s Illustrated Reviews, Biochemistry, 3 rd ed, Chapters 7-8,10-14

2. CARBOHYDRATES Learning objectives: Classify carbohydrates according to their definitions Discuss isomeric properties of carbohydrates Draw structures of the most common carbohydrates Discuss digestion of dietary carbohydrates

3. CARBOHYDRATES The most abundant organic molecules in nature Provide a significant fraction of the energy in the diet of most organisms Important source of energy for cells Can act as a storage form of energy Can be structural components of many organisms Can be cell-membrane components mediating intercellular communication Can be cell-surface antigens Can be part of the body’s extracellular ground substance Can be associated with proteins and lipids Part of RNA, DNA, and several coenzymes (NAD + , NADP + , FAD, CoA)

4. CARBOHYDRATES Polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis Carbohydrate with an aldehyde group: Aldose Carbohydrate with a ketone group: Ketose O H C H- C - OH CH 2 OH C H 2 OH C O CH 2 OH Glyceraldehyde Dihydroxyacetone Aldehyde group Keto group

5. CARBOHYDRATES Polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis Empirical formula of many simpler carbohydrates: (CH 2 O) n (hence the name hydrate of carbon) Both can be written C 3 H 6 O 3 or (CH 2 O) 3 Glyceraldehyde Dihydroxyacetone O H C H- C - OH CH 2 OH C H 2 OH C O CH 2 OH

6. Monosaccharides Polyhydroxy aldehydes or ketones that can’t easily be further hydrolyzed “Simple sugars” Number of carbons Name Example 3 Trioses Glyceraldehyde 4 Tetroses Erythrose 5 Pentoses Ribose 6 Hexoses Glucose, Fructose 7 Heptoses Sedoheptulose 9 Nonoses Neuraminic acid

7. Oligosaccharides Hydrolyzable polymers of 2-6 monosaccharides Disaccharides composed of 2 monosaccharides Examples: Sucrose, Lactose Polysaccharides Hydrolyzable polymers of > 6 monosaccharides Homopolysaccharides : polymer of a single type of monosaccharide Examples: Glycogen, Cellulose Heteropolysaccharides : polymer of at least 2 types of monosaccharide Example: Glucosaminoglycans

8. ISOMERISM Structural isomers Compounds with the same molecular formula but with different structures Functional group isomers with different functional groups E.g. glyceraldehyde and dihydroxyacetone Positional isomers with substituent groups on different C-atoms E.g. COO - -CHOPO 3 - -CH 2 OH and COO - -CHOH-CH 2 OPO 3 - 2-Phosphoglycerate 3-Phosphoglycerate

9. ISOMERISM Stereoisomers Compounds with the same molecular formula, functional groups, and position of functional groups but with different conformations cis-trans isomers with different conformation around double bonds H COOH H COOH C C C C HOOC H H COOH Fumaric acid ( trans ) Maleic acid ( cis )

10. Reference compound for optical isomers is the simplest monosaccharide with an asymmetric carbon: glyceraldehyde D -Glyceraldehyde is assigned to be the isomer that has the hydroxyl group on the right when the aldehyde group is at the top in a Fischer projection formula. It is also dextrorotatory, so it is also D (+)-Glyceraldehyde C-atom 1 C-atom 2 (an asymmetric carbon) C-atom 3 O H C H- C - OH CH 2 OH D-Glyceraldehyde L-Glyceraldehyde Dihydroxyacetone O H C H- C - OH CH 2 OH C H 2 OH C O CH 2 OH O H C HO- C - H CH 2 OH

11. Isomers and epimers Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C 6 H 1 2 O 6 . Carbohydrate isomers that differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see “anomers” below) are defined as epimers of each other. For example, glucose and galactose are C-4 epimers—their structures differ only in the position of the –OH group at carbon 4. Glucose and mannose are C-2 epimers. However, galactose and mannose are NOT epimers—they differ in the position of –OH groups at two carbons (2 and 4) and are, therefore, defined only as isomers

12. Enantiomers A special type of isomerism is found in the pairs of structures that are mirror images of each other. These mirror images are called enantiomers, and the two members of the pair are designated as a D - and an L -sugar (Figure 7.5). The vast majority of the sugars in humans are D -sugars. In the D isomeric form, the –OH group on the asymmetric carbon (a carbon linked to four different atoms or groups) farthest from the carbonyl carbon is on the right, whereas in the L -isomer it is on the left. Enzymes known as racemases are able to interconvert D - and L -isomers.

13. Optical isomers that are not enantiomers are diastereomers Diastereomers that differ by their configuration on a single asymmetric carbon are epimers CHO Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH D-Glucose CHO Ι HO – C – H Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH D-Mannose CHO Ι H – C – OH Ι HO – C – H Ι HO – C – H Ι H – C – OH Ι CH 2 OH D-Galactose CH 2 OH Ι C = O Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH D-Fructose CHO Ι HO – C – OH Ι H – C – OH Ι H – C – OH Ι CH 2 OH D-Ribose C 6 H 12 O 6 C 6 H 12 O 6 C 6 H 12 O 6 C 6 H 12 O 6 C 5 H 10 O 5

14. Cyclization of monosaccharides Less than 1 % of each of the monosaccharides with five or more car- bons exists in the open-chain (acyclic) form. Rather, they are pre- dominantly found in a ring (cyclic) form, in which the aldehyde (or keto) group has reacted with an alcohol group on the same sugar, making the carbonyl carbon (carbon 1 for an aldose or carbon 2 for a ketose) asymmetric. [Note: Pyranose refers to a six-membered ring consisting of five carbons and one oxygen, for example, glucopyranose (Figure 7.6), whereas furanose denotes a five-membered ring with four carbons and one oxygen.] 6-membered ring: Pyranose 5-membered ring: Furanose

15. Mutarotation: Spontaneous conversion of one anomer to the other Equilibrium: 36% α -anomer, 63% β -anomer, <1% open-chain form O H OH OH H CH 2 OH H OH H H OH O H OH OH H CH 2 OH H OH H OH H CHO Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH D-Glucose α -anomer β -anomer

16. Learn (know) these structures O H OH OH H CH 2 OH H OH H OH H O H H OH OH CH 2 OH H OH H OH H O H OH OH H CH 2 OH H H OH OH H O OH OH H H H OH H CH 2 OH D-Glucopyranose D-Mannopyranose D-Galactopyranose D- Ribofuranose

17. Reducing sugars Carbohydrate with a free or potentially free aldehyde or ketone group Benedict’s solution CHO Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH COOH Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH 2 OH Cu 2+ Cu +  Cu 2 O H 2 O, OH - Reducing sugars: If the hydroxyl group on the anomeric carbon of a cyclized sugar is not linked to another compound by a glycosidic bond, the ring can open. The sugar can act as a reducing agent, and is termed a reducing sugar. Such sugars can react with chromogenic agents (for example, Benedict’s reagent or Fehling’s solution) causing the reagent to be reduced and colored , with the aldehyde group of the acyclic sugar becoming oxidized

18. Glycosidic bonds Bond formed between the anomeric carbon of a carbohydrate and the hydroxyl oxygen atom of an alcohol (O-glycosidic bond) or the nitrogen of an amine (N-glycosidic bond) Glycosidic bonds between monosaccharides yields oligo- and polysaccharides After glycosidic bond formation, the ring formation involving the anomeric carbon is stabilized with no potentially free aldehyde or keto groups

19. Joining of monosaccharides Monosaccharides can be joined to form disaccharides, oligosaccharides, and polysaccharides. Important disaccharides include lactose (galactose + glucose), sucrose (glucose + fructose), and maltose (glucose + glucose). Important polysaccharides include branched glycogen (from animal sources) and starch (plant sources) and unbranched cellulose (plant sources); each is a polymer of glucose. The bonds that link sugars are called glycosidic bonds. These are formed by enzymes known as glycosyltransferases.

20. Naming glycosidic bonds: Glycosidic bonds between sugars are named according to the numbers of the connected carbons, and with regard to the position of the anomeric hydroxyl group of the sugar involved in the bond. If this anomeric hydroxyl is in the α configuration, the linkage is an α - bond. If it is in the β configuration, the linkage is a β - bond. L actose , for example, is synthesized by forming a glycosidic bond between carbon 1 of β - galactose and carbon 4 of glucose. The linkage is, therefore, a β (1 → 4) glycosidic bond (see Figure 7.3). [Note: Because the anomeric end of the glucose residue is not involved in the glycosidic linkage it (and, therefore, lactose) remains a reducing sugar.

21. Glycosidic bonds Lactose: β -D- galactopyranosyl - (1→4)- D- glucopyranose Maltose: α -D-glucopyranosyl-(1→4)- D-glucopyranose Isomaltose: α -D-glucopyranosyl-(1→6)- D-glucopyranose Sucrose: α -D-glucopyranosyl-(1→2)- D- fructofuranose

22. Complex carbohydrates Carbohydrates can be attached by glycosidic bonds to non-carbohydrate structures, including purine and pyrimidine bases (found in nucleic acids), aromatic rings (such as those found in steroids and bilirubin), proteins (found in glycoproteins and proteoglycans), and lipids (found in glycolipids). N- and O-glycosides: If the group on the non-carbohydrate molecule to which the sugar is attached is an –NH 2 group, the structure is an N-glycoside and the bond is called an N-glycosidic link. If the group is an –OH, the structure is an O-glycoside, and the bond is an O-glycosidic link (Figure 7.7). [Note: All sugar–sugar glycosidic bonds are O-type linkages.]

23. Model of cellulose molecules in a microfibril

24. Glucosaminoglycans Large complexes of negatively charged heteropolysaccharide chains Typically associated with a small (<5%) amount of protein forming proteoglycans Properties Can bind large amounts of water Gel-like matrix Viscous Lubricating Shock absorbing

25. Carbohydrates of glycoproteins Cell-surface molecules antigen determinants mediator of cell-cell interaction attachment sites for vira Most proteins in serum are glycosylated Example: Erythropoietin

26. Dietary carbohydrates Starch Sucrose Glucose and fructose Lactose Cellulose Other plant polysaccharides Digestible Non-digestible by humans Only monosaccharides are absorbed into the bloodstream from the gut. Digestion of carbohydrates involves their hydrolysis into monosaccharides

27. DIGESTION OF DIETARY CARBOHYDRATES The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. This digestion is rapid and is catalyzed by enzymes known as glycoside hydrolases (glycosidases) that hydrolyze glycosidic bonds. Because there is little monosaccharide present in diets of mixed animal and plant origin, the enzymes are primarily endoglycosidases that hydrolyze polysaccharides and oliosaccharides , and disaccharidases that hydrolyse tri- and disaccharides into their reducing sugar components (Figure 7.8). Glycosidases are usually specific for the structure and configuration of the glycosyl residue to be removed, as well as for the type of bond to be broken. The final products of carboh drate digestion are the monosaccharides, glucose, galactose and fruc - tose , which are absorbed by cells of the small intestine.

28. A. Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α - amylase acts briefly on dietary starch and glycogen, hydrolyzing random α (1 → 4) bonds. [Note: There are both α (1 → 4)- and β (1 → 4)- endoglucosidases in nature, but humans do not produce the latter. Therefore, we are unable to digest cellulose— a carbohydrate of plant origin containing β (1 → 4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α (1 → 6) bonds, which α - amylase cannot hydrolyze , the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides kown as dextrins (Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α - amylase.

29. B-Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine When the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas, and pan- creatic α - amylase continues the process of starch digestion. C. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal cells The final digestive processes occur primarily at the mucosal lining of the upper jejunum, and include the action of several disacchari dases. (Figure 7.1 0). For example, isomaltase cleaves the α( 1 → 6) bond in isomaltose and maltase cleaves maltose and maltotriose , each producing glucose, sucrase cleaves sucrose producing glucose and fructose, and

30. L actase ( β - galactosidase) cleaves lactose producing galactose and glucose. Trehalose, an α( 1 → 1 ) disaccharide of glucose found in mushrooms and other fungi, is cleaved by trehalase . These enzymes are secreted through, and remain associated with, the luminal side of the brush border membranes of the intestinal mucosal cells.

31. D. Absorption of monosaccharides by intestinal mucosal cells The duodenum and upper jejunum absorb the bulk of the dietary sugars. However, different sugars have different mechanisms of absorption. For example, galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that requires a concurrent uptake of sodium ions; the transport protein is the sodium-dependent glucose cotransporter 1 (SGL T-1 ). Fructose uptake requires a sodium-independent monosaccharide transporter (GL UT-5) for its absorption. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GL UT-2.

32. Digestive Enzymes Enzymes for carbohydrate digestion Enzyme Source Substrate Products α -Amylase Salivary gland Starch, glycogen Oligosaccharides Pancreas Dextrinase Small intestine Oligosaccharides Glucose Isomaltase Small intestine α -1,6-glucosides Glucose Maltase Small intestine Maltose Glucose Lactase Small intestine Lactose Galactose, glucose Sucrase Small intestine Sucrose Fructose, glucose Lactase deficiency produces lactose intolerance

33. Absorption of monosaccharides by intestinal mucosal cells Major monosaccharides Glucose, galactose, fructose Entry into mucosal cells from intestinal lumen Active transport of glucose and galactose with a concurrent uptake of Na + ions Facilitated transport of fructose via transporter protein GLUT-5 Entry into the portal circulation from mucosal cells Facilitated transport via transporter protein GLUT-2

34. Blood glucose concentrations Measured in mmol/L = mM or in mg/dL Conversion factor: 1 mM = 18 mg/dL Normal plasma glucose concentrations roughly 3.9 – 8.3 mM Hypoglycemia: < 2.2 mM Diabetes: > 7.0 mM (fasting) > 11.1 mM 2 h after ingestion of 75 g glucose All cells can use glucose as an energy source Brain cells and erythrocytes require glucose as an energy source

35. E. Abnormal degradation of disaccharides it is monosaccharides that are absorbed, any defect in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine. As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea . This is reinforced by the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO 2 and H 2 gas, causing abdominal cramps, diarrhea , and flatulence.