Carbohydrates PDF

CARBOHYDRATES CARBOHYDRATES CLASSIFICATION OF CARBOHYDRATES Definition Monosaccharides Is a polyhydroxy aldehyde, a polyhydroxy...

CARBOHYDRATES CARBOHYDRATES CLASSIFICATION OF CARBOHYDRATES Definition Monosaccharides Is a polyhydroxy aldehyde, a polyhydroxy contain single polyhydroxy aldehyde or ketone, or a compound that yields ketone unit polyhydroxy aldehydes or polyhydroxy are simple sugars that can no longer be ketones upon hydrolysis broken down to simpler forms of Most abundant class of bioorganic carbohydrates by hydrolysis reactions (E.g. molecules on planet Earth glucose, fructose) Relatively low in humans contain 3-7 C atoms Constitute about 75% by mass of dry plant Disaccharides materials are formed from the linkage of two monosaccharides (E.g. sucrose, lactose) FUNCTIONS OF CARBOHYDRATES upon hydrolysis, they produce 2 Functions monosaccharide units Carbohydrate oxidation provides energy Oligosaccharides Carbohydrate storage, in the form of contain three to ten monosaccharide units glycogen, provides a short-term energy covalently bonded to each other (E.g. reserve. raffinose and inulin) Supply carbon atoms for the synthesis of usually found associated with proteins and other biochemical substances (proteins, lipids in complex molecules lipids, and nucleic acids). Polysaccharide Form part of the structural framework of are polymers in which monosaccharides are DNA and RNA molecules the monomers (E.g. starch, cellulose) Carbohydrates linked to lipids are structural contain many monosaccharide units components of cell membranes covalently bonded Carbohydrates linked to proteins function in number of monosaccharide units varies from a variety of cell-to-cell and cell-to-molecule a few 100 units to 50,000 units recognition processes. Page 1 of 14  nonsuperimposable. (Chiral molecules have CHIRALITY: HANDEDNESS IN MOLECULE handedness) Definition An achiral molecule is a molecule whose mirror images are superimposable. (Achiral a form of isomerism (Isomerism is the molecules do not possess handedness) phenomenon in which more than one Examples: compounds have the same chemical a. Chiral objects- Hands (cannot be formula but different chemical structures) superimposed) The structural requirement for handedness b. Achiral objects- Flask (can be is the presence of a carbon atom that has superimposed) four different groups bonded to it in a tetrahedral orientation. Bromochloroiodomethane is a chiral organic Most monosaccharides exist in two forms: a molecule “left handed” and “right handed” form. The relationship between these two forms is that of mirror images. A mirror image is the reflection of an object in a mirror. TWO CLASSES OF MIRROR IMAGES Superimposable Mirror Images are images that coincide at all points when IMPORTANCE OF CHIRALITY the images are laid upon each other. In human-body chemistry, it is found that Nonsuperimposable Mirror Images right-handed and left-handed forms of are images where not all points coincide molecules usually elicit different responses when the images are laid upon each other. and that our bodies can normally use only one of the two forms of a chiral compound. CHIRALITY Monosaccharides and the building block for more complex types of carbohydrates, are Chiral Center almost always “right-handed” molecules. is a molecule that has four different groups Amino acids are always left-handed. bonded to it in a tetrahedral orientation A molecule that contains a chiral center is said to be chiral. Chiral molecules are Page 2 of 14  The four groups attached to the atom at the STEREOISOMERS chiral center assume a tetrahedral geometry Definition governed by the following conventions: ○ Vertical lines from the chiral center are isomers that have the same molecular represent bonds to groups directed and structural formulas but differ in the into the printed page. orientation of atoms in space. ○ Horizontal lines from the chiral center represent bonds to groups TWO TYPES OF ISOMERS directed out of the printed page. Enantiomers are stereoisomers whose molecules are non-superimposable mirror images of each other. Molecules with chiral center Diastereomers Are stereoisomers whose molecules are not mirror images of each other. Example: Cis–trans isomers D AND L SYSTEMS D and L system used to designate the FISCHER PROJECTION FORMULA handedness of enantiomers. Definition is a two-dimensional structural notation for showing the spatial arrangement of groups about chiral centers in molecules. In a Fischer projection formula, a chiral center is represented as the intersection of vertical and horizontal lines. The enantiomer with the chiral center -OH group on the right is the right-handed isomer (D-glyceraldehyde) TETRAHEDRAL ARRANGEMENTS The enantiomer with the chiral center -OH group on the left is the left-handed isomer (L-glyceraldehyde) Page 3 of 14  The D,L nomenclature gives the same extent but in opposite configuration (handedness) only at the directions. highest-numbered chiral center. PROPERTIES OF ENANTIOMERS Constitutional Isomers and Diastereomers Constitutional isomers differ in most chemical and physical properties (have different boiling points and melting points) Diastereomers also differ in most chemical Dextrorotatory and Levorotatory Compounds and physical properties (have different Enantiomers are optically active, i.e., they boiling points and freezing points) are compounds that rotate the plane of In contrast, nearly all the properties of a pair polarized light of enantiomers are the same Dextrorotatory compound: Chiral The differences are: compound that rotates light towards right ○ Their interaction with plain polarized (clockwise; +) light Levorotatory compound: Chiral compound ○ Their interaction with other chiral that rotates light towards left substances (counterclockwise; -) There is no correlation between D, L and +,- Interaction with Plain-Polarized Light ○ In D and L system, the structure is Properties of Light: viewed ○ Ordinary light waves- vibrate in all ○ + and - can be determined using a directions polarimeter ○ Plane polarized light waves- vibrate only in one direction Interactions between Chiral Compounds Right- and Left-handed baseball players Plain-polarized light is rotated clockwise (to cannot use the same glove (chiral) but can right) or counterclockwise (to left) when use the same hat (achiral) passed through enantiomers ○ two members of the enantiomer ○ Direction and extent of rotation will pair (chiral) react differently with depend upon the concentration of other chiral molecules the enantiomer Enantiomeric pairs have some solubility in ○ Same concentration of two achiral solvents like ethanol and have enantiomers rotates light to the Page 4 of 14  different solubility in chiral solvent like IMPORTANT MONOSACCHARIDES D-2-butanol Enantiomers have the same boiling points, D-Glucose melting points, and densities. An aldohexose ○ All these are dependent upon Six-membered cyclic form intermolecular forces, whereas Most abundant in nature chirality doesn’t depend on such Most important source of human nutrition forces. Also known as dextrose, blood sugar Our body responds differently to different (70-100 mg/dL) enantiomers ○ One may give higher rate or one may be inactive Example: Body response to D isomer of hormone epinephrine is 20 times greater than its response to L isomer CLASSIFICATION OF MONOSACCHARIDES D-Galactose Monosaccharides A diastereomer of D-glucose Classification based on number of carbon Used as component of lactose (milk sugar) atoms: Used to differentiate between blood type ○ Triose - 3 carbon atoms Also called brain sugar (uses for energy of ○ Tetrose - 4 carbon atoms the brain, also part of brain and nerve ○ Pentoses - 5 carbon atoms tissue) ○ Hexoses - 6 carbon atoms Six-membered cyclic form Classification based on functional groups: ○ Aldoses - Monosaccharides with one aldehyde group ○ Ketoses - Monosaccharides with one ketone group Combined number of C atoms and functional group ○ Aldohexose - Monosaccharide with aldehyde group and 6 C atoms ○ Ketopentose - Monosaccharide with ketone group and 5 C atoms Page 5 of 14 D-Fructose ○ Cyclic structures are in equilibrium Most important ketohexose in humans with open chain forms Mostly found in fruit and in honey Cyclic structures are formed by the reaction Sweetest tasting in all sugar of carbonyl group (C=O) with hydroxyl Also known as levulose, fruit sugar. (-OH) group on carbon 5 Five-membered cyclic form 2 forms of D-Glucose: ○ α-form where the -OH of C1 and CH2OH of C5 are on opposite sides ○ β-form where the -OH of C1 and CH2OH of C5 are on the same side D-Ribose Anomers An aldopentose Cyclic monosaccharides that differ only in Found in DNA and RNA (also ATP) the position of the substituents on the Five-membered cyclic form anomeric carbon atom Cyclic Forms of Other Monosaccharides Intramolecular cyclic hemiacetal formation and the equilibrium between various forms are not restricted to glucose All aldoses with five or more carbon atoms CYCLIC FORMS OF MONOSACCHARIDES establish similar equilibria, but with different Cyclic Hemiacetal Forms of D-Glucose percentages of the alpha, beta, and open-chain forms Dominant forms of monosaccharides with 5 or more C atoms Page 6 of 14  Fructose and other ketosis with a sufficient ○ In a β configuration, both of these number of carbon atoms also cyclize groups point in the same direction. ○ In an α configuration, the two Pyranose and Furanose groups point in opposite directions. Pyranose- cyclic monosaccharide containing a six-atom ring Furanose- cyclic monosaccharide containing a five-atom ring Their ring structures resemble the ring structures in the cyclic ethers pyran and furan, respectively -OH GROUP POSITION The scientific identity of a monosaccharide is determined by the positioning of the other -OH groups in the Haworth projection formula HAWORTH PROJECTION FORMULA ○ Any -OH group at a chiral center that is to the right in a Fischer Definition projection formula points down in Two-dimensional structural notation that the Haworth projection formula specifies the three-dimensional structure of ○ Any -OH group to the left in a a cyclic form of a monosaccharide Fischer projection formula points up in the Haworth projection formula REACTIONS OF MONOSACCHARIDES Five important reactions of α AND β CONFIGURATION monosaccharides: ○ Oxidation to acidic sugars ○ Reduction to sugar alcohols Determine by the position of the -OH group ○ Glycoside formation on C1 relative to the -CH2OH group that ○ Phosphate ester formation determines D or L series ○ Amino sugar formation Page 7 of 14  Glucose will be used as the monosaccharide The carbonyl group in a monosaccharide reactant (either an aldose or a ketose) is reduced to Other aldoses, as well as ketosis, undergo a hydroxyl group using hydrogen as the similar reactions reducing agent ○ The product is the corresponding OXIDATION TO ACIDIC SUGARS polyhydroxyl alcohol called sugar alcohol or alditol The redox chemistry of monosaccharide is ○ Sorbitol - used as a moisturizing closely linked to the the alcohol and agent in foods and cosmetics and as aldehyde functional groups a sweetening agent in chewing gum Oxidation can yield three different types of (to prevent tooth decay) acidic sugars depending on the typeof oxidizing agent used GLYCOSIDE FORMATION ○ Aldonic Acid - formed when weak oxidizing agents such as Tollens and Benedict’s solutions oxidize the Glycoside: Acetal formed from a cyclic aldehyde and monosaccharide by replacement of the ○ Reducing Sugar - carbohydrate that hemiacetal carbon -OH group with an -OH gives a positive test with Tollens and group Benedict’s Solutions ○ Glucoside - Glycoside produced Strong oxidizing agents can oxidize both from glucose ends of a monosaccharide at the same time ○ Galactoside - Glycoside produced (the carbonyl group and the terminal from galactose primary alcohol group) to produce a ○ Exist in both α and β forms dicarboxylic acid ○ Such polyhydroxy dicarboxylic acids PHOSPHATE ESTER FORMATION are known as aldaric acids In biochemical systems, enzymes can oxidize the primary alcohol end of an aldose The hydroxyl groups can react with such as glucose, without oxidation of the inorganic oxyacids to form inorganic esters aldehyde group, to produce and alduronic Phosphate esters of various acid monosaccharides are stable in aqueous REDUCTION TO PRODUCE SUGAR solution and play important roles in the ALCOHOLS metabolism of carbohydrates Page 8 of 14  Baby foods are rich in maltose AMINO SUGAR FORMATION CELLOBIOSE Amino Sugar - formed when one of the hydroxyl groups of a monosaccharide is replaced with an amino group Produced as an intermediate in the In naturally occurring amino sugars, the C2 hydrolysis of the polysaccharide cellulose. hydroxyl group is replaced by an amino Contains two D-Glucose monosaccharide group units (must have a Beta configuration Amino sugars and their N-acetyl derivatives instead of Alpha). are important building blocks of Cannot be digested by humans polysaccharides such as chitin and hyaluronic acid LACTOSE DISACCHARIDES Made up of β-D-galactose unit and a D-glucose unit joined by a β(1>4) glycosidic Two monosaccharides can react to form linkage disaccharide. Milk is rich in the disaccharide lactose In Disaccharides formation, one of the Lactose intolerant- irritates large intestine monosaccharides reactants serves as then causes diarrhea hemiacetal, and the other one as alcohol The bond that links the two mono of a disaccharide together is called glycosidic SUCROSE (TABLE SUGAR) linkage The most abundant of all disaccharides and MALTOSE (MALT SUGAR) found in plants Produced commercially from the juice of sugar cane and sugar beets Produced in break down of starch Nonreducing sugar Made up to two D-glucose units, one of which must be a-D-glucose Also called malt sugar OLIGOSACCHARIDES The most important reaction of maltose is hydrolysis Page 9 of 14  Carbohydrates that contain three (3) to ten Polysaccharide that is a storage form for (10) monosaccharide units via glycosidic monosaccharides and used as an energy linkages. source in cells Type of carbohydrate naturally found in an Starch array of plant foods. ○ Glucose is the monomeric unit Functions including cell recognition and cell ○ Storage polysaccharide in plants adhesion. The Oligosaccharides responsible for blood Types of Polysaccharides Isolated from Starch groups are D-galactose and its derivatives Amylose ○ Unbranched-chain polymer and Raffinose accounts for 15%–20% of the starch a trisaccharide in which glucose acts as a ○ Has α(14) glycosidic bonds monosaccharide bridge between galactose ○ Glucose has alpha configuration and fructose. ○ Spiral like structures It has both α and β glycosidic bonds Amylopectin Stachyose ○ Branched chain polymer and a tetrasaccharide consisting of two accounts for 80%–85% of the starch α-D-galactose units, one α-D-glucose unit, ○ Has α(14) and α(16) glycosidic and one β-D-fructose unit. bonds ○ Up to 100,000 glucose units are present ○ Amylopectin is digested more POLYSACCHARIDES readily by humans (can The Polymer Chain ○ Hydrolyze α linkages but not β Polysaccharides are polymers of many linkages) monosaccharide units bonded with glycosidic linkages Glycogen Two types: Glucose storage polysaccharide in humans ○ Homopolysaccharide and animals ○ Heteropolysaccharide Contains only glucose units Branched chain polymer with α(14) glycosidic bonds in straight chains and α(16) in branches STORAGE POLYSACCHARIDES Three times more highly branched than Starch amylopectin in starch Contains up to 1,000,000 glucose units Page 10 of 14  Excess glucose in blood is stored in the dietary supplement to help with joint form of glycogen problems. STRUCTURAL POLYSACCHARIDES Cellulose Found in plant cell walls Woody portions such as stems, stalks and trunks have high concentrations of fibrous water insoluble substances. ACIDIC POLYSACCHARIDES Naturally occurring polysaccharides Linear homopolysaccharide with β(14) glycosidic bond A polysaccharide with a disaccharide Humans do not have enzymes that repeating unit in which one of the hydrolyze β(14) linkages and so they cannot disaccharide components is amino sugar digest Cellulose and has a negative charge due to the Animals also lack these enzymes, but they sulfate group or carboxyl group. can digest cellulose due to the presence of Heteropolysaccharides: 2 monosaccharides cellulase-producing bacteria present in alternating patterns. (Example:. 5,000 glucose units and a molecular mass of Hyaluronic acid and Heparin) 900,000 amu Hyaluronic Acid Chitin Alternating residues of N-acetyl- Second most abundant polysaccharides β-D-glucosamine and D-glucuronate Similar to cellulose structurally and Highly viscous and serve as lubricants in the functionally fluid of joints as well as vitreous humor of Linear polymer with all β(14) glycosidic the eye linkages Vitreous humor: jelly like consistency of the It has an N-acetyl amino derivative of eye due to the glass like appearance (Greek glucose word “hyalos” means glass) Function is to give rigidity to the exoskeletons of crabs, lobster Complete hydrolysis of chitin produces D-glucosamine, which is marketed as a Page 11 of 14 Heparin ○ Dietary monosaccharides or Small highly sulfated polysaccharide disaccharides Contains 15-90 disaccharides residues per ○ Simple carbohydrates are usually chain. sweet to the taste and are Blood anticoagulant commonly referred to as sugars Released at the site of tissue injury. ○ Constitute 20% of the energy in the Prevents formation of clots in the blood and US diet retards the growth of existing clots. Complex Carbohydrates Naturally present in mast cells: ○ The main complex carbohydrates ○ part of the immune system and are starch and cellulose, substances plays an important role in healing not generally sweet to the taste. and defense against pathogens ○ comes in contact with the GLYCOLIPIDS AND GLYCOPROTEINS: outside/external part of the body CELL RECOGNITION (skin, mouth, nose, mucosa of the lungs) It is now known that mono-, di-, and oligosaccharides attached through glycosidic linkages to lipid molecules and protein molecules have a wide range of biochemical functions (including allowing cells to interact with invading bacteria and viruses and enabling cells of differing function to recognize each other) DIETARY CONSIDERATIONS AND Note: The prefix glyco-,used in the terms CARBOHYDRATES glycolipid and gly-coprotein, is derived from Nutrition the Greek word glykys, which means Foods high in carbohydrate content “sweet” constitute over 50% of the diet of most people of the world GLYCOLIPID Balanced dietary food should contain about 60% of carbohydrate A glycolipid is a lipid molecule that has one Classes of Dietary Carbohydrates or more carbohydrate (or carbohydrate Simple Carbohydrates derivative) units covalently bonded to it. Page 12 of 14  Glycolipids are components of cellular Glycolipids are also important for the membranes comprising a hydrophobic lipid immune system. They act as antigens that tail and one or more hydrophilic sugar can be recognized by antibodies and groups linked by a glycosidic bond. activate an immune response. This Functions: recognition is important for the body's ○ One of their primary functions is defense against pathogens and foreign cell-cell recognition. The substances. carbohydrate chains on glycolipids act as markers that identify cells to GLYCOPROTEIN each other. (This recognition is important for a variety of biological processes such as tissue formation, A glycoprotein is a protein molecule that immune response and embryonic has one or more carbohydrate (or development.) carbohydrate derivative ) units covalently ○ Glycolipids are also involved in cell bonded to it. signaling. They can act as receptors A sugar component (glyco) linked to a or ligands and activate various protein describes the structure of signaling pathways within the cell. glycoproteins. Covalent bonds are used to This signaling is important for cell bind the two components together. growth, differentiation and survival. Functions: ○ Glycolipids are also involved in cell ○ Cell surface glycoproteins are crucial adhesion. They interact with other for cross-linking proteins (such as molecules on the cell surface to collagen) and cells to strengthen maintain the structure and integrity and stabilise a tissue. of the cell membrane. ○ Red blood cells also depend on Glycolipids are crucial for the proper glycoproteins for their function. The functioning of the cell membrane. The cell type of glycoprotein on human red membrane is composed of a lipid bilayer blood cells is referred to as the and glycolipids are an integral part of the blood type. Red blood cells with outer layer of the membrane. They help to type A blood have A antigens or A maintain the fluidity and flexibility of the cell glycoproteins. As a result, the body membrane, which is important for the learns that the blood is a movement of molecules in and out of the component of oneself and is cell. instructed not to fight it. Page 13 of 14  Glycoproteins are one of the major components of human pathogenic viruses. They have been demonstrated to have an important role(s) in infection and immunity. Concomitantly high titres of antibodies against these antigenic viral glycoproteins have paved the way for development of novel diagnostics. WHAT’S THE DIFFERENCE BETWEEN GLYCOPROTEINS AND GLYCOLIPIDS? Glycoproteins are found on the cell membrane and the blood whereas the glycolipids are only found on the cell membrane. Glycoproteins function as the receptors for chemical signaling whereas glycolipids facilitate cellular recognition. Glycolipids called cerebrosides and gangliosides occur extensively in brain tissue. Glycoproteins called immunoglobulins are key components of the body's immune system response to invading foreign materials. Page 14 of 14