Monosaccharides for MBBS, BDS, Nursing Students

General Description of monosaccharides

1. Carbohydrates Dr. Apeksha Niraula Assistant professor Clinical Biochemistry Institute of Medicine

2. Carbohydrates are polyhydroxy aldehydes or ketones , or substances that yield such compounds on hydrolysis Empirical formula (CH 2 O)n where n = no. of carbons which is always n ≥ 3

3. Functions: Sources of energy Intermediates in the biosynthesis of other basic biochemical entities (fats and proteins) Associated with other entities such as glycosides, vitamins, and antibiotics Form structural tissues in plants and in microorganisms (cellulose, lignin, murein) Participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system

4. Classification: Based on the number of saccharides (sugar) units: Monosaccharides: That cannot be hydrolyzed into simpler carbohydrates They may be subdivided based on the number of Carbon atoms Trioses : Glyceraldehyde, Dihydroxyacetone Tetroses : Erythrose, Erythrulose Pentoses : Ribose, Ribulose Hexoses : Glucose, Fructose, Mannose, Galactose

5. Disaccharides: Yield two molecules of monosaccharides when hydrolyzed Glycosidic bond Can be divided into: Reducing Disaccharide : Lactose (Glucose + Galactose), Maltose (Glucose + Glucose) Non-Reducing Disaccharide : Sucrose (Glucose + Fructose)

6. Oligosaccharides: Yield two to ten molecules of monosaccharides on hydrolysis Maltotriose ( Glu +Glu + Glu) Raffinose (Gal + Glu+ Fru) Stachyose (Gal +Gal + Glu+ Fru)

7. Polysaccharides Yield more than ten molecules of monosaccharides on hydrolysis Can be divided into: Homopolysaccharides: made up of only one type of repeating unit E.g. Starch, Glycogen, Cellulose, Inulin Heteropolysaccharides: Consists of chains of complex carbohydrates E.g. Hyaluronic acid, Heparin, Chondroitin sulfate

8. Monosaccharide Glucose is the most important monosaccharide Structure of Glucose can be represented in three ways: Straight-chain Cyclic structure Simple ring proposed by Haworth Chair form

9. Fischer and Haworth Projection Formula of Glucose and Fructose

10. Sugars exhibit various forms of Isomerism Stereoisomers: same structural formula but differ in spatial configuration Depends on presence of asymmetric carbons (2 n ) Glucose with 4 asymmetric carbons, has 16 isomers D and L isomerism: Orientation of -H and –OH groups around the penultimate carbon atom (e.g. C5 in glucose) When –OH group is on the right: D series When –OH group is on the left: L series Enzymes are specific for this configuration

11. Epimers: Two sugars that differ only in the configuration around one carbon atom D- glucose and D- mannose , which differ only in the stereochemistry at C-2 , D- glucose and D- galactose which differ at C-4

12. Pyranose and furanose ring structures : In an aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain For example: D- fructofuranose or D- fructopyranose D- glucopyranose or D- glucofuranose

13. Alpha and beta Anomers: Result of a reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals D-glucose’s free hydroxyl group at C-5 reacts with aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers: α & β

14. If -OH group at C1 and CH 2 OH at C5 is in the same side: β anomer If -OH group at C1 and CH 2 OH at C5 is in the opposite side: α anomer

15. Optical activity: When a beam of plane-polarized light is passed through a solution of optical isomer, it will be rotated either to the right or to the left Right: dextrorotatory (d or +) Left: levorotatory (l or -) Racemic mixture: equal amounts of (+) and (-) isomers thus no optical activity Optical rotation of glucose in solution is dextrorotatory, hence called dextrose in clinical practice Naturally occurring form of fructose is levorotatory

16. Mutarotation: Interconversion of α and β anomers During this process hemiacetal ring opens and reforms with the change of position of –H and –OH groups on C1 α -glucopyranose 38% and β -glucopyranose 62%

17. Physiological importance of monosaccharides: Trioses : Glyceraldehyde and DHAP are formed in the glycolysis pathway Pentoses : D-Ribose: Nucleic acids, coenzymes (ATP, NAD), flavoproteins , intermediate in pentose phosphate pathway D- Arabinose and Xylose : Constituents of glycoproteins D- Lyxose : human heart muscle L- Xylulose : Intermediate in uronic acid pathway, found in urine in essential pentosuria

18. Hexoses : D-Glucose: sugar of the body, present in urine ( glycosuria ) in DM owing to raised blood glucose (hyperglycemia) D-Galactose: present in milk sugar, constituents of glycolipids and glycoproteins, failure to metabolize leads to galactosemia and cataract D-Mannose: constituent of glycoproteins D-Fructose: present in Honey and fruit juices, hereditary fructose intolerance

19. Sugar Derivatives Deoxy sugars: Deoxy sugars are those in which a hydroxyl group has been replaced by hydrogen e.g. deoxyribose in DNA Deoxy sugar L- fucose occurs in glycoproteins 2-deoxyglucose is used experimentally as an inhibitor of glucose metabolism

20. Amino sugars: D-glucosamine: a constituent of hyaluronic acid D-galactosamine: constituent of chondroitin Several antibiotics (e.g., erythromycin) contain amino sugars important for their antibiotic activity

21. Glucosamine (2-amino-D-glucopyranose) (α form)

22. Sugar acids: Formed by oxidation of aldehyde or primary alcohol group in monosaccharide. E.g. Glucuronic acid : oxidation of C6 Gluconic acid : oxidation of C1

23. α- D- Glucuronate

24. Sugar alcohol: Derived from sugars by reduction of aldehyde group into –OH group k/a glycitols or polyhydric alcohols For example: Sorbitol, Mannitol Xylitol Cause bloating and diarrhea due to their  osmotic  effect, thus used in constipation as a stool softener ( laxative ) Used to decrease pressure in the eyes, as in glaucoma , and to lower  increased intracranial pressure

25. Glycosides: formed by condensation between hydroxyl group of anomeric carbon of a monosaccharide, or its residue, and a second compound that may or may not (in the case of an aglycone ) be another monosaccharide If hemiacetal portion is glucose, the resulting compound is glucoside ; if galactose, galactoside Aglycone may be methanol, glycerol, a sterol, a phenol, or a base such as adenine Cardiac glycosides : Digitalis and Ouabain (inhibitor of Na + -K + ATPase of cell membranes) A glycoside streptomycin is an antibiotic

26. Disaccharides Two monosaccharides joined by a glycosidic bond Reducing disaccharides: Maltose and Lactose Non-reducing disaccharides: Sucrose and Trehalose Trehalose : major sugar of insect hemolymph . Found in fungi and yeasts. Two glucose units joined by α (1 1 ) bond

27. Found in germinating cereals and malt. Digestion by amylase or hydrolysis of starch

28. In lactase deficiency, malabsorption leads to diarrhea and flatulence

29. K/A Table sugar Both the anomeric carbon of glucose and fructose residue of sucrose are engaged in gycosidic bond formation: Non-reducing sugar. After hydrolysis changes to reducing sugar: Invert sugar

30. Thank You