Carbohydrates 1 PDF

# CARBOHYDRATE ## IV - CHEMISTRY OF CARBOHYDRATES OF PHYSIOLOGIC IMPORTANCE - Biomedical importance - Carbohydrate functions, nomenclature, classification and structures, glycosidic linkages, cyclization and isomerization of monosaccharides, derivatives of monosaccharides (sugar phosphate, deoxy s...

# CARBOHYDRATE ## IV - CHEMISTRY OF CARBOHYDRATES OF PHYSIOLOGIC IMPORTANCE - Biomedical importance - Carbohydrate functions, nomenclature, classification and structures, glycosidic linkages, cyclization and isomerization of monosaccharides, derivatives of monosaccharides (sugar phosphate, deoxy sugars, sugar alcohol, sugar acid and ascorbic acid.) - Reducing and non reducing disaccharides. - Polysaccharides: homopolysaccharides and heteropolysaccharides. - Glycoconjugate: Proteoglycans, Glycoproteins and Peptidoglycans. - Digestion and absorption of carbohydrates and clinical correlations. ## Definition of Carbohydrates or saccharides - Compounds with formula, $(CH_2O)n$, are called as carbohydrates (hydrates of carbons). With the discoveries of many diverse carbohydrates it was noticed that many, but not all, carbohydrates have the above empirical formula; some also contain nitrogen, phosphorus or sulfur. - There are some carbohydrates (derivatives) that do not possess $(CH_2O)n$. On the other hand, there are a few non-carbohydrate compounds like lactic acid with empirical formula $(CH_2O)n$. Hence, Carbohydrates are chemically defined as polyhydroxy aldehydes or ketones, their derivatives and their polymer. | Aldehyde group | Ketone group | | --------------- | --------------- | | CHO | CH2OH | | H-C-OH | C=0 | | HO-C-H | HO-C-H | | H-C-OH | H-C-OH | | H-C-OH | H-C-OH | | CH2OH | CH2OH | | Glucose (a polyhydroxy aldehyde) | Fructose (a polyhydroxy ketone) | ## Functions of carbohydrates - Provide the body with energy: cells convert carbohydrates into the fuel molecule ATP through a process called cellular respiration. During periods of starvation when carbohydrates aren't available, the body can convert amino acids from muscle into glucose to provide the brain with energy. - Energy storage: in human 'glycogen', and in plants 'starch'. - Structural components: in plants (cellulose), in insects (Chitin). - Carbohydrate derivatives are vitamins, antibiotics and drugs. - Carbohydrate are components of connective tissues, bone, cartilage, skin, membranes and nerve tissue e.g. GAGS. - Carbohydrate are components of blood group substances, nucleic acids ‘ribose & deoxyribose.. etc. - Key intermediates of metabolism (sugars). ## Classification of Carbohydrates - Modern chemical classification of carbohydrates is based upon understanding of the products of their hydrolysis, i.e., on the number of sugar units. <br> - **Monosaccharides** - (also called 'simple' sugars) are those which cannot be hydrolysed further into simpler forms. - **Disaccharide** - Those sugars which yield two molecules of the same or different molecules of monosaccharide on hydrolysis. 'Attached together via a glycosidic linkage'. - **Oligosaccharide** - Those sugars which yield 3 to 10 monosaccharide units on hydrolysis. - **Polysaccharides (Complex Carbohydrates)** - Those sugars which yield more than ten molecules of monosaccharides on hydrolysis. Polysaccharides are further divided into two groups: Homo and Hetero- polysaccharides. ## Monosaccharides: an Organic chemistry review - **Monosaccharides Classification** - They can be subdivided further: - **I. Position of carbonyl group** - at C1, carbonyl is an aldehyde: aldose - at any other carbon, carbonyl is a ketone: ketose - 'all based on the structure of dihydroxyacetone' - **II. Number of carbons** - three carbons: triose - four carbons: tetrose - five carbons: pentose - six carbons: hexose - seven carbons: heptose - etc. - Triose - Smallest monosaccharides have three carbon atoms (3C) - Tetrose (4C), Pentose (5C), Hexose (6C) etc... 7,9 ## Aldoses - **Configuration around C2** (red) distinguishes the members of each pair of monosaccharides - **Aldotriose** - CHO - HCOH - HCOH - CH2OH - D-Glyceraldehyde - **Aldotetroses** - CHO - HOCH - HCOH - CH2OH - D-Threose - CHO - HCOH - HCOH - HCOH - CH2OH - D-Erythrose - **Aldopentoses** - CHO - HCOH - HCOH - HCOH - CH2OH - D-Ribose (Rib) - CHO - HOCH - HCOH - HCOH - CH2OH - D-Arabinose (Ara) - CHO - HCOH - HOCH - HCOH - CH2OH - D-Xylose (Xyl) - CHO - HOCH - HOCH - HCOH - CH2OH - D-Lyxose (Lyx) - **Aldohexoses** - CHO - HCOH - HCOH - HCOH - HCOH - CH2OH - D-Allose - CHO - HOCH - HCOH - HCOH - HCOH - CH2OH - D-Altrose - CHO - HCOH - HOCH - HCOH - HCOH - CH2OH - D-Glucose (Gle) - CHO - HOCH - HOCH - HCOH - HCOH - CH2OH - D-Mannose (Man) - CHO - HOCH - HCOH - HOCH - HCOH - CH2OH - D-Gulose - CHO - HCOH - HOCH - HCOH - HOCH - CH2OH - D-Idose - CHO - HOCH - HCOH - HCOH - HOCH - CH2OH - D-Galactose (Gal) - CHO - HOCH - HCOH - HCOH - HCOH - CH2OH - D-Talose ## Ketoses - **Configuration around C3 (red)** distinguishes the members of each pair of monosaccharides - **Dihydroxyacetone** - CH2OH - C=0 - CH2OH - **D-Erythrulose** - CH2OH - C=0 - HCOH - CH2OH - **D-Ribulose** - CH2OH - C=0 - HCOH - HCOH - CH2OH - **D-Xylulose** - CH2OH - C=0 - HOCH - HCOH - CH2OH - **D-Psicose** - CH2OH - C=0 - HCOH - HCOH - HCOH - CH2OH - **D-Fructose** - CH2OH - C=0 - HOCH - HCOH - HCOH - CH2OH - **D-Sorbose** - CH2OH - C=0 - HCOH - HCOH - HCOH - CH2OH - **D-Tagatose** - CH2OH - C=0 - HOCH - HCOH - HCOH - CH2OH ## Stereochemistry - **Chirality in Monosaccharides** - **Asymmetric carbon:** A carbon atom is 'chiral' to which four different atoms or groups of atoms are attached is said to be asymmetric. - Note: chiral & achiral - Aldoses with 3C or more and ketoses with 4C or more are chiral. - The maximum number of possible stereoisomers is $2^n$, where n= number of chiral carbon atoms. - Half of stereoisomers are D and the other half are L. <br> - **Table of aldoses:** | # of Carbon Atoms | Name of Sugar Class | # of Chiral Carbon Atoms (n) | # of Stereoisomers (2^n) | # of D Stereoisomers | # of L Stereoisomers | | :------------------- | :------------------ | :---------------------- | :---------------------------- | :-------------------------- | :-------------------------- | | 3 | triose | 1 | 2 | 1 | 1 | | 4 | tetrose | 2 | 4 | 2 | 2 | | 5 | pentose | 3 | 8 | 4 | 4 | | 6 | hexose | 4 | 16 | 8 | 8 | ## Stereoisomerism - Same molecular formula, structures but differ in their configuration. - Asymmetric (Chirl) carbon allow the formation of stereoisomerism. - **Types of stereoisomerism:** - D and L isomerism - Optical isomerism - Epimerism - Anomerism ## Sugars exhibit various forms of isomerism: - The more important types of isomerism are as follows: ### 1. D and L isomerism (enantiomer) - Many carbohydrates exist as enantiomers (stereoisomers that are mirror images). <br> - *D-Glucose:* - H-C=0 - H-C-OH - HO-C-H - H-C-OH - H-C-OH - CH2OH - *L-Glucose:* - O=C-H - OH-C-H - H-C-HO - OH-C-H - H-C-OH - CH2OH <br> - D and L isomers are mirror images of each other. (These two forms are called Enantiomers). ### 2. Optical isomerism - Optical activity is the capacity of a substance to rotate the plane polarized light passing through it. - When light rotate to right (clockwise) direction, that substance is said to be dextrorotatory (D) (+). - When light rotate to left (anticlockwise) direction, that substance is said to be levorotatory (L) (-). - D and L enantiomers rotate polarized light in equal, but opposite directions. - **Racemic:** When equal amounts of dextrorotatory and laevorotatory isomers are present, the resulting mixture has no optical activity, since the activities of each isomer cancels each other. - **Racemic mixture:** 50% D: 50% L is inactive and optical rotation zero. ### 3. Epimers - Epimers are two sugars that differ only in the configuration around one carbon atom of their structures. - D-Mannose differs from D-glucose only in its configuration around carbon 2. - D-Galactose differs from D-glucose only in its configuration around carbon 4. - D-Galactose and D-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. <br> - *D-Mannose (epimer at C-2):* - CHO - HO-C-H - HO-C-H - H-C-OH - H-C-OH - CH2OH - *D-Glucose:* - CHO - H-C-OH - HO-C-H - H-C-OH - H-C-OH - CH2OH - *D-Galactose (epimer at C-4):* - CHO - H-C-OH - HO-C-H - HO-C-H - H-C-OH - CH2OH ### Isomers - Compounds that have the same chemical formula but have different structures are called isomers. - **For example:** Glucose, galactose, mannose, and fructose have the same chemical formula $(C_6H_{12}O_6)$, but they differ in the organization of their atoms, making them isomers of one another. - Fructose is a structural isomer of glucose and galactose, meaning that its atoms are actually bonded together in a different order. ### 4. Anomerism - **Alpha (α) and Beta (β) anomers** - In solution glucose predominantly exist as closed-chain structure. - Because of cyclization of sugar, an additional asymmetric center is created at C-1 (anomeric carbon). This leads to formation of two isomers namely: - α - D-glucopyranose - β-D-glucopyranose <br> - **Anomeres** - *α-glucose:* - CH2OH - C - H - C - OH - C - H - OH - C - H - OH - C - H - OH - *β-glucose:* - CH2OH - C - H - C - OH - C - H - OH - C - OH - C - H - OH - C - H - OH <br> - α -glucose and β – glucose are two anomers of glucose. ## Mutarotation - The cyclic a and ẞ anomers of a sugar in solution are in equilibrium with each other, and can be spontaneously interconverted (a process called mutarotation). - An equilibrium mixture of 'A and B anomers'= mutarotation. - All reducing sugars exhibit mutarotation. - Mutarotation occurs spontaneously or with the help of an enzyme (generally called a mutase). <br> - **α- D - Glucose** - Specific rotation - $[a]_D$ = +112.2° - **Equi. mixture** - Specific rotation - $[a]_D$ = +52.7° - **β - D - Glucose** - Specific rotation - $[a]_D$ = +18.7° ## Important Monosaccharides - **(a) Trioses:** - Both D-glyceraldehyde and dihydroxyacetone occur in the form of phosphate esters, as intermediates in glycolysis. - They are also the precursors of glycerol, which the organism synthesises and incorporates into various types of lipids. - **(b) Tetroses:** - Erythrose-4-P occurs as an intermediate in hexosemonophosphate shunt which is an alternative pathway for glucose oxidation. - **(c) Pentoses:** - D-ribose is a constituent of nucleic acid RNA; also as a constituent of certain coenzymes, e.g. FAD, NAD, coenzyme A. - D-2-deoxyribose is a constituent of DNA. - **(d) Hexoses:** - **Glucose** is a hexose, the most nutritionally important monosaccharide. - sometimes called dextrose or blood sugar. - the compound to which other sugars absorbed into the body must be converted in the liver. - **Galactose** is a hexose, a component of lactose (milk sugar). - a component of substances present in nerve tissue. - **Fructose** is a ketohexose, the sweetest monosaccharide, sometimes called levulose or fruit sugar. - present in honey in a 1:1 ratio with glucose. - **(d) Heptoses:** - D-altro-heptulose (a ketose), an early intermediate in lipid A biosynthesis - L-glycero-D-manno-heptose (an aldose), a late intermediate in lipid A biosynthesis.