Carbohydrates and Their Properties

Carbohydrates: General Characteristics

• Carbohydrates are polyhydroxy aldehydes or ketones, and their derivatives.
• The term 'carbohydrate' comes from the French term 'hydrate de carbone'.
• Carbohydrates are composed of carbon (C), hydrogen (H), and oxygen (O).
• The empirical formula for carbohydrates is (CH₂O)ₙ.
• Not all carbohydrates follow this formula (e.g., deoxysugars and aminosugars).
• Carbohydrates are the most abundant compounds in nature. Cellulose, for example, accounts for approximately 100 billion tons annually.

Functions of Carbohydrates

• Source of energy.
• Involved in the biosynthesis of other biochemical entities (e.g., fats, proteins).
• Associated with other entities such as glycosides, vitamins, and antibiotics.
• Component of structural tissues in plants (e.g., cellulose) and microorganisms (e.g., lignin, murein)
• Involved in biological transport.
• Support cell-cell recognition.
• Part of the activation of growth factors.
• Important in immune system modulation.

Carbohydrates (Glycans) Composition

• Basic composition: (CH₂O)ₙ or H-C-OH.
• Monosaccharides: Simple sugars with multiple hydroxyl (OH) groups. Categories based on the number of carbons include trioses, tetroses, pentoses, and hexoses (3, 4, 5, 6 carbons respectively).
• Disaccharides: Two monosaccharides covalently linked.
• Oligosaccharides: A few monosaccharides (up to 9-10) covalently linked.
• Polysaccharides: Polymers consisting of monosaccharide or disaccharide units. Subdivided into homopolysaccharides (e.g., starch), heteropolysaccharides, and complex carbohydrates.

Monosaccharides

• Empirical formula: Cₙ(H₂O)ₙ.
• Exist in both open-chain and ring structures.
• Exhibit multiple structural isomers.
• Chiral carbon atoms lead to optical isomers.
• Typically have 3 to 6 carbon atoms.
• Common monosaccharides include pentoses (5-carbon sugars) and hexoses (6-carbon sugars).
• Monosaccharide chains have at least one carbonyl group (C=O).
• Aldoses have the carbonyl group at the end of the chain.
• Ketoses have the carbonyl group within the chain.
• Common examples include D-glucose and D-fructose.

Diagram of Isomeric Forms of Carbohydrates

• Isomers: Molecules with the same molecular formula but different structures.
• Constitutional isomers: Differ in the order of attachment of atoms.
• Stereoisomers: Atoms are connected in the same order, but differ in spatial arrangement.
  • Enantiomers: Nonsuperimposable mirror images.
  • Diastereoisomers: Not mirror images.
  • Epimers: Differ at one of several asymmetric carbon atoms.
  • Anomers: Differ at the anomeric carbon (the carbon that was involved in the cyclization).

Stereoisomers: D and L Forms

• D (dextrorotatory) and L (levorotatory) designate optical isomers based on configuration around the asymmetric carbon in glyceraldehyde.
• For sugars with more than one chiral center, D and L refer to the asymmetric carbon (often C5) furthest from the aldehyde or ketone group.
• Most naturally occurring sugars are D-isomers.

Optical Isomerism

• Property exhibited when mirror images of a molecule are non-superimposable.
• Asymmetric compounds rotate plane-polarized light.
• Measured using a polarimeter.
• Rotation can be dextrorotatory (+) or levorotatory (-).
• A racemic mixture (DL mixture) contains equal amounts of D and L isomers and does not exhibit optical activity.

Enantiomers and Epimers

• Enantiomers are nonsuperimposable mirror images.
• Epimers differ at only one asymmetric carbon atom.

Hemiacetal and Hemiketal Formation

• Aldehydes react with alcohols to form hemiacetals.
• Ketones react with alcohols to form hemiketals.
• Monosaccharides can cyclize via hemiacetal or hemiketal formation.
• Cyclization produces ring structures (e.g., pyranose, furanose) which create new asymmetric (chiral) centers, the anomeric carbon.
• Haworth projections depict the cyclic structure of sugars.

Mutarotation

• α and β anomers readily interconvert in solution through an open-chain intermediate.

Reactions of Monosaccharides: Action of Base on Sugars

• Sugars are weak acids forming salts in high pH.
• Formation of a 1,2-enediol intermediate.
• The Lobry de Bruyn-Alberta von Eckenstein reaction is a reversible isomerization of monosaccharides in alkaline solution.
• Enediols are highly reactive, making sugars powerful reducing agents in alkaline environments.

Reactions of Monosaccharides: Action of Base on Sugars

• Reducing sugars react with oxidizing agents (e.g., Cu²⁺) and form a precipitate.
• Copper sulfate is a common oxidizing agent.
• The reduction of the copper ion indicates an aldehyde group oxidation, signifying a reducing sugar.
• Examples include Fehling's solution and Benedict's solution, used to test for reducing sugars.

Oxidation Reactions

• Aldoses can oxidize into 3 types of acids: aldonic acids, uronic acids, and saccharic acids.
• The aldehyde group can be oxidized to a carboxylic acid group (producing aldonic acid).
• The primary alcohol group can be oxidized whilst preserving the aldehyde group, forming uronic acids.
• Oxidizing both ends creates saccharic acids, also known as glucosaccharic acid when glucose is the reacting monosaccharide.

Reduction Reactions

• Reduction converts the carbonyl group (-CHO or >C=O) to an alcohol group (-CH₂OH).
• The result is an alditol or a sugar alcohol. Examples of alditols include sorbitol and mannitol.

Dehydration: Action of Strong Acids on Monosaccharides

• Strong acids dehydrate monosaccharides.
• D-ribose, under heat and concentrated sulfuric acid, gives furfural.
• D-Glucose forms 5-hydroxymethyl furfural under similar conditions.
• The furfurals react with reagents like -naphthol, giving a characteristic purple color used for carbohydrate detection via the Molisch test.

Formation of Osazones

• Carbohydrates (monosaccharides) react with phenylhydrazine to create osazones.
• This reaction involves the aldehyde group.
• The product is a crystalline compound.
• This method was once used for identifying different sugars. Now, it is seldom utilized, replaced by more sensitive methods like HPLC.

Formation of Esters

• Several important sugar esters are involved in cellular metabolism.
• The alcohol groups of monosaccharides can be esterified through enzymatic and non-enzymatic reactions.
• The phosphate moiety is often donated by ATP.

Fermentation

• Fermentation is an energy-yielding metabolic pathway that doesn't change the oxidation state.
• Yeast uses glucose, fructose, or maltose to generate pyruvate which is converted to ethanol and CO₂.

Derivatives of Monosaccharides

• Sugar acids: Formed by oxidizing the aldehyde or primary alcohol group in a monosaccharide (e.g., gluconic acid).
• Sugar alcohols (alditols): Formed by reducing aldoses or ketoses, including examples like sorbitol and mannitol.
• Amino sugars: Hydroxyl groups in monosaccharides are replaced by amino groups.
• Deoxysugars: Sugars with one less oxygen atom than the parent molecule, such as deoxyribose.
• Vitamin C (ascorbic acid): A water-soluble vitamin.

Glycosidic Bonds

• Anomeric hydroxyl groups join two sugars by removing a water molecule to form a glycosidic bond.
• The link forms a disaccharide.
• Examples include maltose, formed by two glucose molecules.

Disaccharides

• Formed when two monosaccharides link via a glycosidic bond.
• The specific glycosidic bond distinguishes different disaccharides (e.g., maltose has an α(1→4) glycosidic bond).
• When hydrolyzed (e.g. with acids or enzymes), they yield their component monosaccharides.

Maltose

• Formed from two α-D-glucose molecules linked by an α(1→4) glycosidic bond—known as malt sugar.
• Produced during the partial hydrolysis of starch.
• Used as a nutrient and fermenting reagent.
• Related to isomaltose.

Lactose

• Composed of a β-D-galactose and an α-D-glucose molecule, linked by a β(1→4) glycosidic bond.
• Important for infants.
• Known as milk sugar.

Sucrose

• Formed from α-D-glucose and β-D-fructose linked by an α(1→2)β glycosidic bond.
• Called table sugar.
• Commercial sources are sugar cane and sugar beets.

Cellobiose

• Two glucose molecules joined by a β(1→4) glycosidic bond.
• Produced during partial hydrolysis of cellulose.

Polysaccharides

• Large polymers of monosaccharides.
• Homoglycans: Starch, cellulose, glycogen, and inulin.
• Heteroglycans: Gums, mucopolysaccharides.

Homopolysaccharides: Starch

• The most common storage polysaccharide in plants.
• Composed of amylose (10-30%) and amylopectin (70-90%) depending on the plant source.
• The chains vary in size, having different molecular weights.
• Major sources include rice, corn, wheat, potatoes, and cassava.
• Used as a binder in pharmaceuticals and tablets, and for numerous industrial applications.

Homopolysaccharides: Amylose and Amylopectin

• Amylose: Linear structure with α(1-4) glycosidic bonds.
• Amylopectin: Highly branched structure with α(1-4) and α(1-6) glycosidic bonds.

Homopolysaccharides: Dextrans

• Products from the reaction of glucose and transglucosidase from Leuconostoc mesenteroides.
• Used as plasma extenders, molecular sieves, in infant formula, and in dental plaques.

Homopolysaccharides: Inulin

• β(1→2) linked fructofuranoses — a linear polymer.
• Lower molecular weight than starch.
• Yields fructose upon hydrolysis and is used in diagnostic tests.

Homopolysaccharides: Glycogen

• Animal starch, a highly branched storage polysaccharide found in animal liver and muscle tissue.
• Composed of many glucose monomers linked by α(1→4) and α(1→6) glycosidic bonds.
• Staining with iodine gives red-violet color.
• Hydrolyzed by various enzymes, including amylases and glycogen phosphorylase.

Homopolysaccharides: Cellulose

• Polymer of β-D-glucose molecules linked via β(1-4) linkages.
• Structural polysaccharide, most abundant in nature.
• Not digested easily in most animals, but serves a crucial structural function.
• Cellulose is found in cotton, flax, and wood.

Products Obtained from Cellulose

• Microcrystalline cellulose: binder in tablets.
• Methylcellulose: suspending agent and laxative.
• Oxidized cellulose: hemostat.
• Sodium carboxymethyl cellulose: laxative.
• Cellulose acetate: used in rayon & enteric coatings.
• Nitrocellulose: explosive.

Homopolysaccharides: Chitin

• Second most abundant carbohydrate polymer.
• Structural polymer found in fungi and in the exoskeletons of crustaceans and insects.
• A derivative binds to iron and can act to slow meat rancidity.

Heteropolysaccharides: Glycoproteins & Proteoglycans

• Proteins with carbohydrate molecules attached.
• Glycoproteins: Proteins with carbohydrates attached; most abundant by weight.
• Proteoglycans: Proteins with chains of glycosaminoglycans (large polysaccharides). Play structural roles, lubrication, etc.
• Mucins or mucoproteins: Predominantly carbohydrates; play lubricating roles in body fluids.

Glycoaminoglycans

• Involved in extracellular functions (e.g., cartilage).
• Heparin: Important anticoagulant used in blood banks.

Lectins

• Glycoprotein that binds to specific oligosaccharides on cell surfaces.
• High specificity and moderate to high affinity.
• Found in all organisms (plants, animals, bacteria and viruses).
• Roles include cell-cell recognition, signaling, adhesion, immune response.
• Examples include certain plant lectins (e.g., concanavalin A), and animal lectins (e.g., mannan-binding lectin(MBL)).