Biochemistry Chapter 3: Chemistry of Carbohydrates PDF

# Chapter (3) Chemistry of Carbohydrates ## Chapter (III) Chemistry of Carbohydrates - Carbohydrates are widely distributed in plants and animals. They have important structural and metabolic roles. - They are poly hydroxyl aldehydes, poly hydroxyl ketones or compounds that can be hydrolyzed to th...

# Chapter (3) Chemistry of Carbohydrates ## Chapter (III) Chemistry of Carbohydrates - Carbohydrates are widely distributed in plants and animals. They have important structural and metabolic roles. - They are poly hydroxyl aldehydes, poly hydroxyl ketones or compounds that can be hydrolyzed to them. ## Classification: 1. Monosaccharides: are simple sugars that cannot be broken down (or hydrolyzed) into smaller molecules. 2. Disaccharides can be hydrolyzed to give two monosaccharides. 3. Oligosaccharides are polymers made up of two to ten monosaccharide units. 4. Polysaccharides are condensation products (polymers) of many monosaccharides (more than ten). ## Monosaccharides can be classified according to the number of carbon atoms to: - triose = three carbons - tetrose= four carbons - pentose = five carbons - hexose = six carbon - heptose = seven carbon ## Monosaccharides can be classified according to the reactive group as: - **aldehydes**: are monosaccharides with an aldehyde group. - **ketoses**: are monosaccharides containing ketone group. ## Nomenclature also of monosaccharides can be combined in designating compounds, e.g. glucose is an aldohexose that is, it is a six-carbon monosaccharide (-hoxose) containing an aldehyde group (-aldo). - The carbons are numbered sequentially with the aldehyde or katone group being on the carbon with the lowest possible number. ## Examples: * **D-Glucose (Aldohexose)** ``` C-H 1 H-C-OH 2 HO - C - H 3 H-C-OH 4 H-C-OH 5 6 CH₂ OH ``` * **D-Fructose (keto hexose)** ``` 1CH2 OH C = O 2 HO - C - H 31 H-C-OH 4 H-C - OH 5| CH2 OH ``` ## Cyclic forms of sugars: - **A)** The cyclic form of glucose is a six-numbered ring; such sugars are called **pyranoses** because they resemble pyran. - **B)** Fructose forms a five-numbered ring called a **furanose**. * **Pynanose:** ``` CH2OH 5 1 4 2 3 ``` * **Furanose:** ``` CH2OH 5 6 1 O 2 OH/CH2OH 3 4 OH ``` - **c)** The intermolecular hemiacetals occur when an alcohol reacts with an aldehyde. These are unstable compounds. ``` OH R₁- C + R2 - OH R₁-C-H H O-R2 ``` - **D)** Cyclized forms of hemiacetals can be formed by similar intramolecular reaction. In glucose, the - OH on carbons (C5) can react with the carbonyl group (on carbon 1) to form a stable, cyclic hemiacetal. ## Glucose ``` H OH C-H C H-C-OH H-C-OH HO - C - H HO - C - H H-C-OH H-C-OH H-C-OH CH₂ OH Open-chain form H-C CH₂ OH Cyclic form ``` ## Isomerism - **A)** Optical activity is the ability of sugars to rotate the plane of polarized light. This is due to the presence of asymmetric carbon atoms in carbohydrates. - **Definition of asymmetric carbon atom**: Is a carbon which is attached (or bonded) to four different atoms or groups. - e.g. carbons No. 2, 3, 4 and 5 in glucose are asymmetric. - **B)** Specific rotation: 1. If plane polarized light is rotated to the right (clockwise), the compound is dextrorotatory (indicated by d or +). 2. If plane polarized light is rotated to the left (anticlockwise), the compound is levorotatory (indicated by L or - ). - e.g.: glucose is dextro rotatory sugar. - Fructose is levorotatory sugar. - **C)** D and L isomerism - The arrangement of the - H and – OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. - When the - OH group of this carbon is on the right, the sugar is the D – isomer; when it is on the left, it is the L-isomer. - This is in relation to the smallest monosaccharide (glyceraldehydes), which is known as a parent carbohydrate. - Most of the monosaccharides occurring in mammals are D - sugars. * **L-Glyceraldehyde**: ``` C-H HO - C - H CH₂ OH ``` * **D - Glyceraldehydes:** ``` C-H H-C-OH CH₂ OH ``` * **L-Glucose:** ``` CH₂OH C-H HO-C-H H-C-OH HO-C-H H-C-OH HO-C-H CH₂ OH ``` * **D-Fructose:** ``` CH₂OH C = O HO-C-H H-C-OH H-C - OH H-C - OH CH₂ OH ``` - N.B. A sugar may be dextrorotatory or levorotatory irrespective. - **D)** Anomeric Carbon - Is a new asymmetric carbon that is created by cyclization at the carbon bound to oxygen in hemiacetal formation e.g. C₁ in glucose. - If the - OH group in the anomeric carbon is to the right, it is in the a-position. If it is to the left, it will be in the ß-position. * **a - D-Glucose:** ``` H OH / HO H C C 1 H-C-OH H-C-OH HO-C-H 0 HO-C-H H-C-OH 0 H-C-OH H-C H-C CH₂ OH CH₂ OH ``` * **β-D-glucose:** ``` H OH / HO H C C 1 H-C-OH H-C-OH HO-C-H 0 HO-C-H H-C-OH 0 H-C-OH H-C H-C CH₂ OH CH₂ OH ``` - In solution, a-and ß-sugars slowly change into an equilibrium mixture of both. This process is known as mutarotation. * **a -D- (+) Glucose:** ``` CH2OH 0 OH OH OH OH OH ``` * **B-D-(+)- glucose:** ``` CH₂OH 0 OH OH OH OH OH ``` - **Haworth formula** ## E) Aldose-Ketose Isomerism - Fructose has the same molecular formula as glucose (the same number of C, H and O), but differs in its structural formula; there is a keto group in position 2; which is the anomeric carbon of fructose Fig. (30), while in glucose there is an aldehyde group in position 1, the anomeric carbon of glucose. ## F) Epimers - These are isomers which differ in the orientation or arrangement of - H and - OH groups around C2, C3 and C4 of glucose. - The most important epimers of glucose are mannose (in C₂) and galactose (in C4). Fig. (30) * **D-Glucose:** ``` 0 C-H H-C-OH HO-C-H | H-C-OH HO-C-H | H-C-OH CH₂ OH ``` * **D-Mannose** ``` C-H HO-C-H HO-C-H | H-C-OH HO-C-H | H-C-OH CH₂ OH ``` * **D-Galactose** ``` C-H H-C-OH HO-C-H | H-C-OH H-C-OH | H-C-OH CH₂ OH ``` - **Epimers (important hexoses)** ## Fig. (30): Important monosuccharides ## Examples of important monosaccharides ### Trioses: glyceraldehydes and dihydroxy-acetone * **D - Glyceraldehyde** ``` C-H H-C-OH CH₂ OH ``` * **Dihydroxyaletone** ``` CH₂OH | C = 0 = CH₂ OH ``` ## Tetroses : erythrose ## Pentoses: - **aldopentose:** ribose - **ketopentose:** ribulose, xylulose * **D-ribose** ``` C-H H-C-OH H-C-OH H-C-OH H-C-OH CH₂ OH ``` * **D-ribulose** ``` CH2OH C = O H-C-OH H-C-OH H-C-OH CH₂ OH ``` * **D-xylulose** ``` CH₂OH C = O HO - C - H H-C-OH H-C-OH CH₂ OH ``` - Pentoses are important in nucleotides, nucleic acids (DNA & RNA) and many coenzymes. ## Derived Monosaccharides: ### 1. Sugar acids: e.g. sugar acids of glucose are formed by oxidation of: - **Aldehyde carbon (C₁) → gluconic acid** (example of uronic acid). - **Hydroxyl carbon (C6) → glucuronic acid** (example of uronic acid) - **Both → saccharic acid (glucaric).** * **Guconic acid:** ``` COOH H-C-OH HO-C-H H-C-OH H-C-OH CH₂ OH ``` * **Glucuronic acid:** ``` C-H H-C-OH HO-C-H H-C-OH H-C-OH COOH ``` * **L-ascorbic acid (Vitamin C)** ``` O = C HO - C Ο HO - C H-C HO-C-H CH₂OH ``` ### 2. Aminosugars In which, a hydroxyl group is replaced by an amino group. ### Examples: 1. **Glucosamine derivative of glucose, is aconstituent of hyaluronic acids.** 2. **Galactosamine is found in polysaccharide of cartilage, chondroitin sulfate.** * **Glucosamine:** ``` Ο C-H H-C - NH₂ HO-C-H H-C-OH H-C-OH CH₂ OH ``` * **Galactosamine:** ``` Ο C-H H-C-NH2 HO-C-H HO-C-H H-C-OH CH₂ OH ``` - It may occur in acetylated form e.g. glycoproteins. ### 3. Deoxy sugars (removal of oxygen) include 2-deoxyribose, found in DNA. * **Deoxy ribose: ** ``` C-H CH2 H-C-OH H-C-OH | CH₂ OH ``` ### 4. Sugar alcohols - Formed by reduction of monosaccharides at the carbonyl carbon: ### Examples: - **D-glucose → D- sorbitol** - **D-mannose → D-mannitol** - **D-galactose → D-dulcitol** - **D-fructose → D-mannitol and D-sorbitol.** - **D-ribose → D-ribitol** * **D-ribose** ``` C-H H - C - OH H - C - OH H - C - OH H - C - OH CH₂ OH ``` * **D-ribitol** ``` CH2OH H-C - OH H-C - OH H-C - OH H-C - OH CH₂ OH ``` - D-ribitol is a constituent of riboflavin (vitamin B2) and in hydrogen carriers FAD and FMN. - Important mono saccharine are shown in Fig. (30)