Carbohydrates in Biology (PDF)

# Carbohydrates - A major source of energy from our diet. - Composed of C, H, and O - Also called saccharides, which means "sugars" - Are produced by photosynthesis in plants. - Glucose are synthesized in plants using CO2, H2O, and energy from the sun. - Are oxidized in living cells (respiration) t...

# Carbohydrates - A major source of energy from our diet. - Composed of C, H, and O - Also called saccharides, which means "sugars" - Are produced by photosynthesis in plants. - Glucose are synthesized in plants using CO2, H2O, and energy from the sun. - Are oxidized in living cells (respiration) to produce CO2, H2O, and energy. - "hydrate of carbon" - Derived from the formula $C_n(H_2O)_m$ - Glucose: $C_6H_{12}O_6$ or $C_6(H_2O)_6$ - Polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield them after hydrolysis. ## Function - Provides energy. - Glycogen - provides short term energy reserve. - Supplies Carbon for synthesis of other biochemical substances. - Part of the structure of DNA and RNA. - Linked to lipids - in cell membrane. - Linked to proteins - in biological recognition processes. - Regulation of blood sugar. - Spare the use of Protein for energy. - Breakdown of fatty acids and preventing ketosis. - Provide flavor and sweeteners. - Source of dietary fibers. ## Isomerism ### Isomers: * Compounds possessing identical molecular formula but different structures ### Types of Isomerism: 1. Structural Isomerism 2. Stereoisomerism ## Structural Isomerism - Same molecular formulae but differ from each other by having different structures. #### An example of Structural Isomers: * **D-Glucose:** CH2OH CHO H-C-OH HO - C - H H-C-OH H-C-OH CH2OH * **D-Fructose:** CH2OH C = O HO - C - H H-C-OH H-C-OH CH2OH ## Stereoisomerism - Same molecular formula and same structure but differ in configuration. - Differ in arrangement of their atoms in space. - Presence of chiral centers allow the formation of stereoisomers. ### Types of stereoisomerism associated with glucose are: 1. D and L isomerism 2. Optical isomerism 3. Epimerism 4. α and β anomerism ## Enantiomers - Stereoisomers whose molecules are nonsuperimposable mirror images of each other. ## Diastereomers - Stereoisomers whose molecules are not mirror images of each other. ## D and L Isomerism <start_of_image> * **L-Glucose:** O = C - H HO - 2C - H H - 3C - OH HO - 4C - H HO - 5C - H CH2OH Left-handed * **D-Glucose:** O = C - H H - 2C - OH HO - 3C - H H - 4C - OH H - 5C - OH CH2OH Right-handed ## Learning Check ### Identify each as the D or L isomer. **A:** O = C - H HO - 2C - H HO - 3C - H HO - 4C - H CH2OH _Ribose_ **B:** O = C - H H - 2C - OH HO - 3C - H HO - 4C - H CH2OH _Threose_ **C:** CH2OH C = O HO - 2C - H H - 3C - OH H - 4C - OH CH2OH _Fructose_ ## Solution **A:** O = C - H HO - 2C - H HO - 3C - H HO - 4C - H CH2OH _Ribose_ (**L**) **B:** O = C - H H - 2C - OH HO - 3C - H HO - 4C - H CH2OH _Threose_ (**D**) **C:** CH2OH C = O HO - 2C - H H - 3C - OH H - 4C - OH CH2OH _Fructose_ (**D**) ## Optical Isomerism - Optical activity is the capacity of a substance to rotate the plane polarized light passing through it. ### Clockwise direction: * Dextrorotatory (d) or (+) ### Counterclockwise direction: * Levorotatory (l) or (-) ## Epimerism - If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric) atom. ### Examples of Epimers: * **D-Mannose:** CHO HO - 2C - H HO - 3C - H H - 4C - OH H - 5C - OH CH2OH (epimer at C-2) ***D-Glucose:** CHO H - 2C - OH HO - 3C - H H - 4C - OH H - 5C - OH CH2OH * **D-Galactose:** CHO H - 2C - OH HO - 3C - H HO - 4C - H H - 5C - OH CH2OH (epimer at C-4) ## Anomerism - Isomers obtained from the change of position of hydroxyl group attached to the anomeric carbon e.g. α and β glucose are 2 anomers. - Also α and β fructose are 2 anomers. ### Anomers of Glucose: * **α-D-Glucopyranose:** 6CH2OH H H OH H HO OH H OH α * **β-D-Glucopyranose:** 6CH2OH H H OH H HO OH H OH β ## Mutarotation - The change in the specific optical rotation by the interconversion of α and β forms of D glucose to an equilibrium mixture. ## Classification (Based on Size) 1. **Monosaccharides:** The simplest carbohydrates. 2. **Disaccharides:** Consist of 2 monosaccharides. 3. **Oligosaccharides:** Consist of 3-10 monosaccharides. 4. **Polysaccharides:** Contain many monosaccharides. ## Monosaccharides - Consist of 3 to 6 carbon atoms, typically. - A carbonyl group (aldehyde or ketone). - Several hydroxyl groups. - Colorless, crystalline solids. ## Aldose - Monosaccharides with an aldehyde group and many hydroxyl (-OH) groups. ### Examples of Aldoses: * **Triose:** (3 C atoms) * **Tetrose:** (4 C atoms) * **Pentose:** (5 C atoms) * **Hexose:** (6 C atoms) ## Ketose - Monosaccharides with a ketone group and many hydroxyl (-OH) groups. ### Examples of Ketoses: * **Triose:** (3 C atoms) * **Tetrose:** (4 C atoms) * **Pentose:** (5 C atoms) * **Hexose:** (6 C atoms) ## Learning Check ### Identify each as aldo- or keto- and as tetrose, pentose, or hexose: **A: ** O = C - H H - 2C - OH H - 3C - OH H - 4C - OH H - 5C - OH CH2OH **B: ** CH2OH C = O HO - 2C - H H - 3C - OH CH2OH ## Solution **A:** _Aldohexose_ **B:** _Ketopentose_ ## D-Glucose - Found in fruits, corn syrup, and honey. - An aldohexose with the formula $C_6H_{12}O_6$. - Known as blood sugar in the body. - The monosaccharide in polymers of starch, cellulose, and glycogen. ## Blood Glucose Level - Glucose has a normal blood level of 70-90 mg/dL. - Glucose tolerance test measures blood glucose for several hours after ingesting glucose. ## D-Fructose - Is a ketohexose $C_6H_{12}O_6$ - Is the sweetest carbohydrate. - Is found in fruit juices and honey. - Converts to glucose in the body. ## D-Galactose - An aldohexose $C_6H_{12}O_6$. - Not found free in nature. - Obtained from lactose, a disaccharide. - A similar structure to glucose except for the -OH on C4. ## Structure of Monosaccharides ### Structures of Monosaccharides: 1. Fisher projection: - The straight chain structural formula. 2. Haworth projection: - Cyclic formula or ring structure. 3. X-ray diffraction analysis: - Boat and chair form. ## Dissacharides - Glycosides formed by the condensation of 2 simple sugars. - If the glycosidic linkage involves the carbonyl groups of both sugars (as in sucrose) the resulting disaccharide is non-reducing. - If the glycosidic linkage involves the carbonyl group of only one of the 2 sugars (as in maltose and lactose) the resulting disaccharide is reducing. ## Important Dissaccharides - Monosaccharides + Disaccharide + H2O - Glucose + Glucose -> Maltose - Glucose + Galactose -> Lactose - Glucose + Fructose -> Sucrose ## Maltose - Also known as malt sugar. - Composed of two D-glucose molecules. - Obtained from the hydrolysis of starch. - Linked by an α-1,4-glycosidic bond formed from the α -OH on C1 of the first glucose and -OH on C4 of the second glucose. - Used in cereals, candies, and brewing. - Found in both the α- and β- forms. ## Formation of Maltose - 6HOCH2 - (5)H/H - (4)HOOH - (3)H - (2)OH - (1)*OH - (6)HOCH2 - (5)H/H - (4)OH - (3)H - (2)OH - (1)*H - O ## Lactose - Is a disaccharide of β-D-galactose and α- or β-D-glucose. - Contains a β-1,4-glycosidic bond. - Is found in milk and milk products. ## Formation of Lactose - 6HOCH2 - (5)HO/H - (4)HOH - (3)H - (2)OH - (1)*H - (6)HOCH2 - (5)H/H - (4)OH - (3)H - (2)OH - (1)*OH - O ## Sucrose - Also known as table sugar. - Obtained from sugar cane and sugar beets. - Consists of α-D-glucose and β-D-fructose. - Has an α, β-1,2- glycosidic bond. ## Formation of Sucrose - CH2OH - H - O - H - (4)HOOH - (3)H - (2)HO - (1)H - (6)HOCH2 - (5)H - (4)HO - (3)H - ( 2)CH2OH - ( 1)H - O - α - β ## Relative Sweetness - **Fructose:** Sweetest - even sweeter than sucrose. - **Honey:** D-fructose and D-glucose. - **Lactose:** Almost no sweetness and is sometimes added to food as a filler. ## Classification (Based on oxidising agents) 1. **Reducing Sugar:** Carbohydrate that gives a positive result with Benedict's and Tollen's test. - Lactose - Maltose 2. **Non-reducing Sugar:** Negative with Benedict's and Tollen's test. - Sucrose ## Reducing Sugars - Carbohydrates with a carbonyl group that oxidizes to give a carboxylic acid. - Undergo reaction with Benedict's reagent (Cu2+) to give the corresponding carboxylic acid. - Include the monosaccharides glucose, galactose, and fructose. ## Oxidation of D-Glucose - O = C - H - H - 2C - OH - HO - 3C - H - H - 4C - OH - H - 5C - OH - CH2OH - D-glucose - Glucose is a reducing sugar. - O = C - OH - H - 2C - OH - HO - 3C - H - H - 4C - OH - H - 5C - OH - CH2OH - D-gluconic acid - Glucose is oxidized to a carboxylic acid. ## Reduction of D-Glucose - Involves the carbonyl group. - Produces sugar alcohols called alditols. - Such as D-glucose gives D-glucitol also called sorbitol. ## Polysaccharides - Formeed by the condensation of n molecules of monosaccharides with the removal of n-1 molecules of water. - Since condensation involves the carbonyl groups of the sugars, leaving only one free carbonyl group at the end of a big molecule, polysaccharides arre non-reducing. ## Polysaccharides (continued) - Polymers of D-glucose. - Includes amylose and amylopectin, starches made of α-D-glucose. - Includes glycogen (animal starch in muscle), which is made of α-D-glucose. - Includes cellulose (plants and wood), which is made of β-D-glucose. ## Classification (Based on Repeating Units) 1. **Homopolysaccharide:** Only one type of monosaccharide monomer unit is present. 2. **Heteropolysaccharide:** More than one (usually two) type of monosaccharide monomer units are present. ## Classification (Based on Branching) 1. **Branched:** Polysaccharides formed from two major types of biochemical polymers therefore forming branch/es 2. **Unbranched:** Linear polymers. ## Structures of Amylose and Amylopectin - Amylose (20%) - Glucose monomers - Amylopectin (80%) - CH2OH - OH - OH - (α-1, 6-Glycosidic bond to branch) - OH - (a-1, 4-Glycosidic bond) - CH2OH - OH - (α-1, 4-Glycosidic bond) - CH2OH - OH - OH - OH - (a) Unbranched chain of amylose - (b) Branched-chain of amylopectin ## Amylose - A polymer of α-D-glucose molecules. - Linked by α-1,4-glycosidic bonds. - A continuous (unbranched) chain. ## Glycogen - Polysaccharide that stores α-D-glucose in muscle. - Similar to amylopectin, but is more highly branched. ## Cellulose - Polysaccharide of glucose units in unbranched chains. - Has β-1,4-glycosidic bonds. - Cannot be digested by humans because humans cannot break down β-1,4-glycosidic bonds. ## Comparison of Starch and Cellulose - Starch (α-bonds) - C - O - C - O - C - L - Alpha bonds between glucose molecules in starch are easily broken by human digestive enzymes. - Human digestive enzymes easily break alpha bonds in starch. - Cellulose (β-bonds) - C O C - V V - C - Beta bonds in dietary fibers are indigestible by human enzymes. - Human digestive enzymes cannot break beta bonds in cellulose. - V