Monosaccharides and Glucose Hemiacetals Quiz

Carbohydrates: Structure and Classification

• Carbohydrates are hydrates of carbon (H₂O of C).
• They are the most abundant organic molecules in nature.
• Their empirical formula is (CH₂O)ₙ.
• They are classified as monosaccharides, disaccharides, and polysaccharides.

Definition

• Carbohydrates are defined as polyhydroxyaldehydes or ketones, or compounds that produce them on hydrolysis.

Importance of Carbohydrates

• They are the most abundant dietary source of energy (4 Cal/g) for organisms.
• They are precursors for many organic compounds (e.g., fats, amino acids, ketone bodies).
• Carbohydrates (e.g., glycoproteins and glycolipids) participate in cell structure, cell growth, adhesion, and fertilization.
• They are structural components of many organisms (e.g., plant fiber (cellulose), insect exoskeletons, and microorganism cell walls).
• They serve as the storage form of energy (e.g., glycogen) to meet immediate energy needs.

Structure of Carbohydrates

• Examples include D-glyceraldehyde, D-glucose, and D-fructose.
• Molecular structures are shown in the slides.

Classification of Carbohydrates

• Classification is based on the number of carbon atoms.
• Monosaccharides are further categorized into aldoses and ketoses, with examples of trioses, tetroses, pentoses, hexoses, and heptoses.
• Disaccharides are formed from two monosaccharides linked by glycosidic bonds. Examples include maltose, sucrose, lactose, and trehalose.
• Polysaccharides are long chains of monosaccharides. They are categorized into homopolysaccharides (e.g., starch, glycogen, cellulose, inulin, dextrans, chitin), and heteropolysaccharides (e.g., hyaluronic acid, heparin, chondroitin sulphate, dermatan sulphate, keratan sulphate).

Importance of Monosaccharides

• Key monosaccharides include glyceraldehyde, erythrose, ribose, glucose, and galactose.
• Glyceraldehyde and erythrose are intermediates in glycolysis and HMP shunt.
• Ribose is an essential component of nucleic acids.
• Glucose provides energy and produces amino acids and fatty acids.
• Galactose is an essential component of milk, used to produce lactate.

Stereoisomers of Glyceraldehyde

• D-glyceraldehyde and L-glyceraldehyde.

D and L Isomerism of Glucose

• Naturally occurring sugars are D-sugars.
• The body can metabolize only D-sugars.

Optical Activity

• Dextrorotatory (d) (+) : sugar solution rotates plane-polarized light to the right.
• Levorotatory (l) (-) : sugar solution rotates plane-polarized light to the left.

Different Representations of Glucose Structure

• Open chain projection.
• Fischer's formula.
• Haworth formula.

Formation of Ring Structure

• Ring structures form via hemiacetal reactions.
• D-glucose exists as an intramolecular hemiacetal.

Anomers/Anomeric Carbon

• Isomers that differ only in their configuration about the anomeric carbon are called anomers.
• The anomeric carbon is the hemiacetal carbonyl carbon.
• For aldoses (e.g., glucose), the 1st carbon is the anomeric carbon; for ketoses (e.g., fructose), the 2nd carbon is the anomeric carbon.

Glucose in Pyranose Form

• α-D-glucose and α-D-glucopyranose.

α and β Forms of D-Glucose

• α-D-glucose and β-D-glucose are shown in different representations.

Fructose in Furanose Form

• α-D-fructose furanose.

α and β Anomers of D-Fructose

• α-D-fructose and β-D-fructose are shown in furanose form.

Epimerism

• Sugars differ only in the configuration around one carbon atom.
• Examples include D-glucose and D-galactose (differ at the 4th carbon), and D-glucose and D-mannose (differ at the 2nd carbon).

Examples for Epimers

• Structures of D-mannose, D-glucose, and D-galactose are shown.

Mutarotation

• A change in the specific rotation of an optically active sugar in solution over time.

Mutarotation (Example)

• A fresh solution of a-D-glucose rotates plane-polarized light at + 112° and changes to +52.5° after 12-18 hours.

Reactions of Monosaccharides

• Oxidation, reduction, dehydration, formation of esters, and glycoside formation are discussed

Reducing Properties

• Sugars with a free aldehyde or ketone group at the anomeric carbon are reducing sugars; their reduction can be tested via Benedict's test, Fehling's test, and Barfoed's test.
• Osazone test is a confirmatory test for monosaccharides.
• Reduction occurs more efficiently in alkaline than acidic solutions.

Benedict's Test: Principle

• The principle of Benedict's test.

Benedict's Test

• A simple method for identifying reducing substances (like glucose) in urine.

Benedict's Test Procedure

• Add the Benedict's reagent followed by the urine sample, heat for a few seconds.
• The color change from green to brick red indicates glucose presence, with color intensity correlating with glucose concentration.

Clinical Applications of Benedict's Test

• Semi-quantitative test to determine sugar concentration in urine.
• Color changes reflect glucose levels:
• 0% sugar – blue
• Mild (0.5-1%) – green
• Moderate (1-1.5%) – yellow
• Severe (1.5-2%) – brick red precipitate
• Used to diagnose diabetes mellitus.

Importance of Disaccharides

• Maltose, lactose, sucrose, and trehalose are highlighted

Maltose

• A malt sugar formed from two α-D-glucose units linked by an α(1→4) glycosidic bond.
• Produced during starch digestion by pancreatic alpha-amylase.

Lactose

• Found in milk.
• Composed of β-D-galactose and α-D-glucose linked by a β(1→4) glycosidic bond.
• Hydrolyzed by the enzyme lactase to glucose and galactose.
• Lactose intolerance is due to insufficient lactase activity.

Sucrose

• A cane sugar formed from an α-D-glucose and a β-D-fructose linked by an α(1→2) glycosidic bond.
• Non-reducing sugar.

Trehalose

• A disaccharide with two glucose units linked by an α(1→1) glycosidic bond.
• Found in some plants, fungi, and insects.
• Non-reducing sugar

Polysaccharide: Homopolysaccharides

• Starch: Stored carbohydrate in plants (e.g., cereals, roots, tubers, vegetables).
• Glycogen: Storage carbohydrate in animals (liver, muscle).
• Cellulose: Plant structural component, dietary fiber.
• Inulin: Found in onion, garlic, etc.
• Dextrans: A polymer of glucose commonly found in microorganisms.
• Chitin: Exoskeleton component in various insects and crustacean.

Starch

• A carbohydrate reserve in plants; found in cereals, roots, tubers, and vegetables.
• Consists of amylose (water-soluble) and amylopectin (water-insoluble) components.

Amylose

• A linear, unbranched chain of glucose units linked by α(1-4) glycosidic bonds -

Amylopectin

• A branched chain of glucose units, with α(1-6) linkages at the branch points.

Test for Starch

• Iodine test: Starch reacts with iodine solution to form a blue-colored complex.

Starch Digestion in the Body

• Starch is digested to dextrins by salivary alpha-amylase and further hydrolyzed into maltose by pancreatic alpha-amylase and glucose units.

Glycogen

• Animal storage carbohydrate in liver and muscles.
• More branched than amylopectin; every 11th glucose unit is branched .
• Synthesized in the body through the process of glycogenesis..

Cellulose

• Plant structural component; made of glucose units linked by β(1-4) glycosidic bonds. - Cannot be digested by humans due to lack of cellulase enzyme.

Inulin

• Found in onion, garlic, etc.
• Composed of fructose units linked by β(2-1) linkages
• Clinical purpose: used to monitor kidney function.

Chitin

• Structural component of exoskeletons of crustaceans (e.g., crabs, lobsters) and insects.
• Composed of N-acetyl glucosamine units joined by β(1→4) glycosidic linkages.

Importance of Heteropolysaccharides (GAGs)

• Hyaluronic acid: lubricant, shock absorber..
• Heparin: anticoagulant.
• Chondroitin sulfate: maintains tissue structure.
• Dermatan sulfate: maintains tissue structure.
• Keratan sulfate: important in corneal transparency.

Proteoglycans and Glycoproteins

• Proteoglycans: Heavily glycosylated proteins, found in extracellular matrix (e.g., decorin).
• Glycoproteins: Less glycosylated proteins, found on cell membranes or in body fluids (e.g., mucins).

Structure of a Proteoglycan

Glycoprotein

• A typical membrane glycoprotein.

Clinical Aspects of Mucopolysaccharidoses

• Inborn errors in metabolism; excess accumulation of GAGs in the body tissues.
• Deficiency of lysosomal enzymes that degrade GAGs.