CARBOHYDRATES

Carbohydrates: Nomenclature, Isomerism, and Major Polysaccharides

  • General concept

    • Carbohydrates are polyhydroxy aldehydes (aldose) or polyhydroxy ketones (ketose), or compounds that yield these derivatives on hydrolysis.
    • Their general formula is often represented as [C(H<em>2O)]</em>n[C(H<em>2O)]</em>n, historically viewed as “hydrates of carbon.”
    • The terms carbohydrate and saccharide are closely related.
    • Major energy roles:
    • Monosaccharides provide quick energy via glycolysis (e.g., glucose → glycolysis → ATP).
    • Some polysaccharides act as long-term energy storage (e.g., glycogen in animals).
    • Important symbols to remember:
    • Glucose: the most important carbohydrate.
    • Glycogen: storage form of glucose.
    • Glucose → Glycolysis → ATP → energy; Macromolecules as long-term energy sources.
    • Naming basis:
    • Functional group: aldehyde vs ketone (aldose vs ketose).
    • Carbon count prefixes: 3 = Tri (Trioses), 4 = Tetra (Tetroses), 5 = Penta (Pentoses), 6 = Hexa (Hexoses), 7 = Hepta (Heptoses), 8 = Octa (Octoses).

  • Page 1: Key points to remember

    • Aldose vs ketose are structural isomeric forms.
    • “Aldo-keto isomers” differ by functional group.
    • Basic naming and carbon-counting rules as listed above.

  • Page 2: Classification of monosaccharides

    • Type of sugar: Aldoses and Ketoses.
    • 3 (Trioses): Glyceraldehyde; Dihydroxyacetone.
    • 4 (Tetroses): Erythrose; Erythrulose.
    • 5 (Pentoses): Ribose, Xylose.
    • 6 (Hexoses): Glucose, Galactose, Fructose.
    • 7 (Heptoses): Glucoheptose; Sedoheptulose.
    • 3. Number of sugar units (classifications by chain length):
    • 1 → Monosaccharide
    • 2 → Disaccharide
    • 3 → Trisaccharide
    • 4 → Tetrasaccharide
    • 5 → Pentasaccharide
    • 6 → Hexasaccharide
    • 7 → Heptasaccharide
    • 2–10 → Oligosaccharide
    • >10 → Polysaccharides

  • Page 3: Structure and isomerism of monosaccharides

    • Straight chain form (Fischer projection): carbon atoms intersecting at chiral centers define configuration.
    • Haworth projection: applies to five- or six-membered rings; more stable for cyclic forms.
    • Chair form: even more stable conformation for six-membered rings.
    • Isomers share the same chemical formula but differ in arrangement; structural forms may be interconverted under certain conditions.

  • Page 4: Stereoisomers, enantiomers, epimers, and optical activity

    • Enantiomers: non-superimposable mirror images; same formula and same connectivity but different spatial arrangement.
    • Chiral molecules are necessary for optical activity (rotation of plane-polarized light).
    • Optical activity:
    • Dextrorotatory (D or +): rotates plane-polarized light to the right.
    • Levorotatory (L or −): rotates plane-polarized light to the left.
    • D and L correspond to configuration relative to the penultimate (next-to-last) carbon in the Fischer projection (D = right, L = left).
    • Epimers: diastereomers that differ in configuration at exactly one chiral center.
    • Diastereomers: stereoisomers that are neither mirror images nor superimposable.
    • Note: The D/L designation refers to optical activity reference, not to absolute R/S configuration.
    • A line: D —> right; L —> left; Penultimate carbon is the reference for D/L.

  • Page 5: Cyclization and mutarotation

    • In solution, monosaccharides with 5 or 6 carbon atoms tend to cyclize via intramolecular attack.
    • Mechanisms of cyclization:
    • Aldoses: C1 carbonyl reacts with C5 hydroxyl to form a hemiacetal.
    • Ketoses: C2 carbonyl reacts with C5 hydroxyl to form a hemiketal.
    • The carbonyl carbon becomes chiral in cyclization (anomeric carbon).
    • Anomers:
    • Alpha (α): OH at the anomeric carbon is trans to the CH2OH group.
    • Beta (β): OH at the anomeric carbon is cis to the CH2OH group.
    • Mutarotation: interconversion between α- and β-forms in solution until equilibrium is reached; forms are interchangeable over time.
    • Practical note: orientation references (α vs β) are location-based; alpha often described as below/under, beta as above in certain instructional contexts.

  • Page 6: Monosaccharide reactions

    • 1. Furfural formation and Molisch’s test: detects carbohydrates in a sample.
    • 2. Enolization/Tautomerization: shifts of hydrogen between carbon atoms to form enediols; important for rearrangements of aldoses/ketoses.
    • 3. Oxidation / sugar acid formation: oxidation of aldoses/ketoses to sugar acids.
    • 4. Reduction (to polyhydroxy alcohols) and dehydration under strong mineral acids:
    • Strong acids (e.g., conc. H2SO4, HCl, HNO3) cause dehydration of sugars, yielding furfural derivatives.
    • Seliwanoff’s test: differentiates ketoses from aldoses based on rate of dehydration to furfural derivatives.
    • Alkaline conditions can promote tautomerization (aldose ↔ ketose) and oxidation of the terminal aldehyde or keto groups depending on oxidant.

  • Page 7: Osazone formation, glycosidic bonds, and reducing properties

    • Osazone formation: reaction of reducing sugars with phenylhydrazine to yield characteristic crystals (glucosazone, maltosazone, lactosazone).
    • Glucosazone (needle-shaped), Maltosazone (sunflower-petal-shaped), Lactosazone (powder-puff/tennis-ball-like).
    • Glycosidic bonds (linkage between sugars):
    • O-glycosidic bonds (glycosides) vs N-glycosidic bonds (aglycone): oxygen- or nitrogen-containing linkages.
    • Reducing vs non-reducing sugars:
      • Reducing sugars have a free carbonyl group (aldehyde or ketone) available for redox; Benedict’s test detects reducing sugars.
      • Non-reducing sugars have both anomeric carbons involved in glycosidic linkage (no free carbonyl).
    • Enzymes involved in glycosidic bond formation and transfer:
    • Glycosyltransferases (enzymes that transfer sugar moieties).
    • Maltase hydrolyzes α(1→4) linkages (glucose-glucose) as in maltose; lactase hydrolyzes β(1→4) linkages (glucose-galactose) as in lactose; sucrase hydrolyzes α(1→2) linkages (glucose-fructose) as in sucrose.
    • Common disaccharides and their constituents:
    • Maltose: glucose + glucose (α-1,4 linkage).
    • Lactose: glucose + galactose (β-1,4 linkage).
    • Sucrose: glucose + fructose (α-1,2 linkage); non-reducing because both anomeric carbons are involved.
    • Important note: The most used glycosidic bonds and their enzyme associations underpin many carbohydrate digestion pathways.

  • Page 8: Disaccharides and esterification; special disaccharides

    • Disaccharides (examples):
    • Sucrose (table sugar): composed of glucose + fructose.
      • Systemic name: O ext{-}oldsymbol{α}- ext{D-glucopyranosyl}-(1
        ightarrow2)-oldsymbol{β}- ext{D-fructofuranose}
      • Non-reducing sugar (both anomeric carbons are involved in the glycosidic bond).
      • Most abundant disaccharide in nature.
      • Hydrolyzed by sucrase to glucose + fructose.
    • Lactose (milk sugar): glucose + galactose.
      • Systemic name: O ext{-}oldsymbol{β}- ext{D-galactopyranosyl}-(1
        ightarrow4)-oldsymbol{β}- ext{D-glucopyranose}
      • Reducing sugar; hydrolyzed by lactase to glucose + galactose.
    • Isomaltose: glucose + glucose with an α(1→6) linkage (branched form in starch digestion contexts).
      • Reducing sugar; formed during starch digestion by isomaltose-producing pathways.
    • Ester formation and functionalization of hydroxyl groups (esterification): hydroxyl groups of sugars can be esterified to form acetates, propionates, benzoates, phosphates, etc.
    • Amino sugars and deoxy sugars:
    • Amino sugars = glucose linked to amino acids (e.g., N-acetylglucosamine in some GAGs).
    • Deoxy sugars: important in DNA/RNA (e.g., deoxyribose).
    • Pentoses and systemic naming: general idea that pentose systems play key roles in nucleic acids and metabolism.

  • Page 9: Polysaccharides; isomaltose; reducing vs non-reducing sugars; starch

    • Reducing vs non-reducing sugars (recap):
    • Reducing sugars possess a free aldehyde or ketone group; examples include maltose, lactose, glucose, and galactose.
    • Non-reducing sugars lack free aldehyde/ketone groups (e.g., sucrose, trehalose).
    • Polysaccharide structure concepts:
    • Linear vs branched polymers.
    • Homopolysaccharides: all units are the same monosaccharide.
    • Heteropolysaccharides: more than one type of sugar unit.
    • Major polysaccharides and examples:
    • Starch (in plants) and glycogen (in animals): storage forms of glucose.
    • Glycogen is highly branched, with α(1→4) linkages in the chains and α(1→6) linkages at branch points (every ~20–30 glucose units).
    • Amylose (linear) and Amylopectin (branched) are two principal parts of starch.
    • Agar and other polysaccharides contribute to structural or gel-forming properties.
    • Isomaltose introduction and linkages relate to starch digestion (isomaltase target).

  • Page 10: Mucopolysaccharides and related polysaccharides

    • Glycogen: highly branched storage polysaccharide in liver and muscle; around 10^6 glucose units; structure includes both ext{α}(1
      ightarrow4) linkages along chains and ext{α}(1
      ightarrow6) linkages at branch points.
    • Cellulose: linear polysaccharide of D-glucose units linked by eta(1
      ightarrow 4) glycosidic bonds; forms fibrous, highly stable structures in plants; hydrogen bonding between adjacent chains stabilizes the overall structure.
    • Chitin: polysaccharide of N-acetylglucosamine; linked by eta(1
      ightarrow4); major component of invertebrate exoskeletons and fungal cell walls.
    • Inulin: a fructose polymer with eta(2
      ightarrow1) glycosidic bonds; used as a marker for glomerular filtration rate in older clinical methods.
    • Dextran: highly branched glucose polymer; used as a plasma volume expander in medical settings.
    • Glycosaminoglycans (GAGs): structural mucopolysaccharides with sulfate and carboxyl groups; acidic molecules used in connective tissues and ECM.
    • Specific examples listed:
    • Dermatan sulfate, Keratan sulfate, Chondroitin sulfate.
    • Hyaluronic acid (a non-sulfated GAG important in ECM).
    • D- and L-6 sugar residues can be present in various GAGs (e.g., D-glucuronic acid and L-iduronic acid).
    • General structural note: these molecules often contribute to hydration, resilience, and structural integrity of tissues.

  • Page 11: Peptidoglycan, hyaluronic acid, heparin, and related roles

    • Peptidoglycan:
    • Major component of bacterial cell walls.
    • Penicillin targets enzymes responsible for forming peptide cross-links between peptidoglycan strands, inhibiting cell wall synthesis.
    • Crystal Violet (Gram stain) interacts with peptidoglycan to help classify bacteria (Gram-positive vs Gram-negative).
    • Hyaluronic acid:
    • A prominent glycosaminoglycan in the extracellular matrix; functions include lubrication and structural support.
    • Heparin:
    • Natural anticoagulant; acts as a cofactor to antithrombin III, enhancing inhibition of thrombin (IIa) and factor Xa; used clinically as an anticoagulant.
    • General clinical and biological context: these molecules influence structural integrity, immune interactions, and coagulation pathways.

  • Page 12: (Summary/closing slide)

    • Carbohydrates are a broad class including simple sugars to complex polysaccharides and glycosaminoglycans.
    • Core concepts covered include nomenclature, isomerism (enantiomers, epimers), cyclization and mutarotation, reducing vs non-reducing sugars, glycosidic bonds, disaccharides, oligosaccharides, and major polysaccharides.
    • Foundational links to metabolism (glycolysis, energy storage), structure (cell walls, ECM), and physiology (kidney function tests, coagulation) were highlighted throughout.

  • Key formulas and notations to remember

    • General formula: [C(H<em>2O)]</em>n[C(H<em>2O)]</em>n
    • Anomeric forms (cyclization): extα−anomer<br/>≠extβ−anomerext{α-anomer} <br />\neq ext{β-anomer}
    • Common linkages:
    • ext{(}eta ext{)}: eta(1
      ightarrow4), eta(1
      ightarrow2), etc.
    • ext{(} ext{α)} ext{): α(1
      ightarrow4), α(1
      ightarrow6)} (as in amylopectin branches)
    • Reducing vs non-reducing sugars:
    • Reducing: free carbonyl group present.
    • Non-reducing: both anomeric carbons involved in glycosidic bonds (e.g., sucrose).
    • Common disaccharide linkages:
    • Maltose: ext{Glucose–Glucose with } ext{α}(1
      ightarrow4)
    • Lactose: ext{Glucose–Galactose with } ext{β}(1
      ightarrow4)
    • Sucrose: ext{Glucose–Fructose with } ext{α}(1
      ightarrow2) (non-reducing)

  • Connections to prior and real-world relevance

    • Enzyme specificity in glycoside hydrolysis (maltase, lactase, sucrase) underpins digestion and many dietary considerations.
    • Mutarotation and cyclic forms underpin carbohydrate chemistry in solution and in biological recognition (e.g., glucose transporters).
    • Glycosaminoglycans (GAGs) and proteoglycans: critical for tissue hydration and resilience; dysregulation linked to diseases.
    • Penicillin’s mechanism highlights the importance of peptidoglycan cross-linking for bacterial cell walls and antibiotic action.

  • Summary of terms to memorize

    • Aldose, Ketose; Aldo-keto isomers; Hexose, Pentose, Triose etc.
    • Enantiomers, Epimers, Diastereomers; D- and L- configurations; Penultimate carbon reference.
    • Mutarotation; Anomeric carbon; α- and β-anomers.
    • Reducing vs Non-Reducing sugars; Osazone types (glucosazone, maltosazone, lactose osazone).
    • Glycosidic bonds: O-glycosidic vs N-glycosidic; Glycosyltransferases; specific enzyme examples (maltase, lactase, sucrase).

  • Quick reference disaccharides

    • Sucrose: O{-}oldsymbol{α}{-} ext{D-glucopyranosyl}(1
      ightarrow2)oldsymbol{-}oldsymbol{β}{-} ext{D-fructofuranose}; non-reducing; most abundant in nature; hydrolyzed by sucrase.
    • Lactose: O{-}oldsymbol{β}{-} ext{D-galactopyranosyl}(1
      ightarrow4)oldsymbol{-}oldsymbol{β}{-} ext{D-glucopyranose}; reducing; hydrolyzed by lactase.
    • Maltose: glucose + glucose (α(1→4)); reducing; produced by digestion of starch/glycogen.
    • Isomaltose: glucose + glucose (α(1→6)); reducing; produced during digestion by isomaltase.