Carbohydrates — Comprehensive Study Notes

Outline and Core Concepts

  • Carbohydrates overview: polyhydroxy aldehydes or polyhydroxy ketones with diverse roles in biology.
  • Major functions include structural components, energy storage, and participation in recognition processes between cells and molecules.
  • Carbohydrate features common to most molecules:
    • The existence of at least one asymmetric center; often two or more.
    • Ability to exist in linear or cyclic structures.
    • Ability to form polymeric structures via glycosidic bonds.
    • Capacity to form multiple hydrogen bonds with water or other surroundings.
  • Structural and functional relationships to other biomolecules: occurrence as glycolipids (lipids with carbohydrate components) and glycoproteins (proteins with carbohydrate moieties).
  • Classification framework (three groups):
    • Monosaccharides – simple sugars with formula (CH<em>2O)</em>n(CH<em>2O)</em>n (n = number of carbons), cannot be hydrolyzed to simpler sugars under mild conditions.
    • Oligosaccharides – 2 to 20 monosaccharides linked by glycosidic bonds, with water removed on each linkage formation; disaccharides are the most common oligosaccharides.
    • Polysaccharides – polymers of simple sugars and derivatives.
  • Important terminology: aldoses vs. ketoses; aldose/ketose naming follows the carbonyl group position; examples include aldohexose (e.g., glucose) and ketohexose (e.g., fructose).

Monosaccharides: Structure, Nomenclature, and Basic Properties

  • General monosaccharide features:
    • I.e., 3–7 carbon atoms commonly in sugars; sweet and water-soluble.
    • Classification by carbonyl chemistry: aldoses (aldehyde) and ketoses (ketone).
    • Carbon-number naming: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C).
  • Nomenclature examples:
    • Trioses: aldotriose, ketotriose.
    • Hexoses: aldohexose, ketohexose (e.g., D-glucose is an aldohexose).
  • Simplest sugars: the basic building blocks for more complex carbohydrates.
  • Stereochemical implications: chiral carbons create distinct enantiomers and diastereomers (see stereochemistry section).

Stereochemistry: Enantiomers, Diastereomers, Epimers, and Related Concepts

  • Enantiomers: non-superimposable mirror images; identical physical properties except for direction of plane-polarized light.
  • Diastereomers: stereochemical isomers that differ at one or more (but not all) chiral centers.
  • Epimers: diastereomers that differ at exactly one chiral center.
  • D- and L-designations: configuration relative to glyceraldehyde; D- and L-forms are mirror images.
  • Example relationships:
    • D-erythrose and L-erythrose are enantiomers.
    • D-erythrose and D-threose are diastereomers.
  • Practical relevance: stereochemistry governs recognition, binding, and enzymatic processing in carbohydrate metabolism.

Cyclic Structures, Anomers, and Mutarotation

  • A prominent chemical feature: ability to form cyclic structures via attack of alcohols on carbonyl groups to form hemiacetals (aldoses) or hemiketals (ketoses).
  • Cyclization yields two anomeric forms (anomers): α and β.
  • Haworth projections and chair/boat representations are used to depict cyclic forms (pyranose and furanose rings).
  • Anomers: different configuration at the anomeric carbon (C-1 in aldoses; C-2 in ketoses after cyclization).
  • Mutarotation: interconversion between α- and β-anomers in solution, changing optical rotation over time.

Derivatives of Monosaccharides

  • 1) Oxidation – formation of aldonic, uronic, and aldaric acids:
    • Reducing sugars can be oxidized by mild oxidants (e.g., Fehling’s, Benedict’s, Barfoed’s) when the anomeric OH is free.
    • Aldonic acids: aldehyde group (C-1) oxidized to carboxylic acid (e.g., glucose to gluconic acid).
    • Uronic acids: primary alcohol at C-6 oxidized to carboxylate (e.g., glucose to glucuronic acid).
    • Aldaric acids: oxidation at both C-1 and C-6 (e.g., glucose to glucaric acid).
  • 2) Mild reduction to sugar alcohols (alditols): sorbitol, mannitol, xylitol; sweeteners used in sugarless products.
  • 3) Oxidation at C-6 position for aldohexoses can yield uronic acids; examples shown include glucuronic acid and related lactones.
  • 4) Amino sugars: amino-substituted sugars such as glucosamine and galactosamine; N-acetylated derivatives common in nature (e.g., N-acetylglucosamine in chitin).
  • 5) Deoxy sugars: lack one or more hydroxyl groups (e.g., deoxyribose).
  • 6) Sugar phosphates: mono-, di-, and triphosphates of sugars (e.g., glucose-1-phosphate; fructose-1,6-bisphosphate); essential in metabolism and signaling; representative molecule shown: ATP (adenosine triphosphate) as a sugar phosphate-containing nucleotide.
  • 7) Amino sugars and derivatives: examples include N-acetylglucosamine, N-acetylmuramic acid, and neuraminic acid (sialic acid, NeuNAc).
  • 8) Acetals, ketals, and glycosides: glycosidic bonds formed by reaction of sugar hydroxyls (often the anomeric OH) with other hydroxyl-containing species, producing glycosides.

Oligosaccharides

  • Usual monosaccharide components: glucose, fructose, mannose, galactose (hexoses); ribose and xylose (pentoses).
  • Disaccharides are the simplest oligosaccharides; they have no free anomeric hydroxyl group available for mutarotation.
  • Common disaccharides:
    • Maltose: glucose–glucose with an α-1,4 glycosidic bond; both carbohydrates are reducing sugars.
    • Cellobiose: glucose–glucose with a β-1,4 glycosidic bond; both reducing sugars.
    • Sucrose: glucose–fructose with an α-1,2 glycosidic bond; non-reducing because the anomeric carbons are involved in the linkage.
    • Trehalose: glucose–glucose with an α-1,1 glycosidic bond; non-reducing; found in organisms adapted to temperature variations; referred to as the blood sugar of insects.
  • Shorthand notation for linkages and complete names:
    • Glcα1-4Glc; complete name: O-α-D-glucopyranosyl-(1→4)-D-glucopyranose.
    • Glcβ1-4Glc; complete name: O-β-D-glucopyranosyl-(1→4)-D-glucopyranose.
  • Hydrolysis of sucrose:
    • Enzyme: invertase (due to change in optical rotation upon hydrolysis).
    • Products: α-D-glucose and α-D-fructose; net rotation change of approximately −39.5exto-39.5^ ext{o}.
    • Also hydrolyzed by dilute acid or by sucrase in the human intestine.
  • Higher oligosaccharides (examples and notes):
    • Melezitose (found in honey).
    • Amygdalin (in seeds of Rosaceae; glycoside of bitter almonds, kernels of cherries, peaches, apricots).
    • Cycloheptaamylose (a starch-derived polyglucoside used in chromatography).
    • Laetrile (pseudoscientific cancer treatment; no robust evidence).
    • Stachyose (present in many plants; can cause flatulence since humans cannot digest it).
    • Dextrantriose (component of sake and honeydew).
  • Oligosaccharides as antibiotics (examples): Bleomycin A, Bleomycin A2, Aburamycin C, Sulfurmycin B, Stretomycin; used clinically as anti-tumor or antibacterial agents in various contexts.

Polysaccharides (Glycans)

  • Polysaccharides are long polymers of monosaccharides; they include:
    • Homopolysaccharides (homoglycans): cellulose, starch, glucans, mannans, etc.
    • Heteropolysaccharides (heteroglycans): pectins (composed of arabinose, galactose, and galacturonic acid).
  • Functions:
    • Storage materials: starch, glycogen.
    • Structural components: cellulose, chitin.
    • Protective coatings: mucopolysaccharides like hyaluronic acid.

Storage Polysaccharides

  • Starch: storage polysaccharide in plants, consists of two components:
    • Amylose: linear polymer of glucose; length ~$100$–$1000$ residues.
    • Amylopectin: highly branched polymer; length ~$300$–$6000$ residues; branching occurs every 12–20 units via α-(1→6) linkages.
  • Location and context:
    • Stored in chloroplasts (photosynthetic sites) and amyloplasts (storage organelles).
  • Structural shorthand: a typical representation includes both α-(1→4) and α-(1→6) linkages forming branches.
  • Glycogen: storage polysaccharide in animals; similar to amylopectin but more highly branched (branch points roughly every 8–12 glucose units).
  • Dextran: mainly α-(1→6) linkages, with occasional 1→2, 1→3, or 1→4 branches.

Structural Polysaccharides

  • Cellulose: β-(1→4) glucose linkage; the most abundant carbohydrate polymer on Earth.
    • Structural rationale: alternating 180° flips of glucose units maximize inter- and intrachain hydrogen bonding, leading to very high strength.
    • Human digestion: not digested by humans; certain ruminants and termites harbor bacteria with enzymes capable of cellulose degradation.
    • Figure-related description: intrachain H-bonds and interchain H-bonds stabilize the extended structure.
  • Chitin: similar to cellulose but contains acetylated amino groups; three packing variations exist: α-chitin (parallel), β-chitin (antiparallel), and δ-chitin (mixed parallel and antiparallel sheets).
  • Alginates: composed of β-D-mannuronate and α-L-guluronate units; form gels via calcium ion cross-linking.
    • Calcium alginate gel formation: Ca^{2+} ions cross-link segments rich in guluronate units, creating an “egg-carton” network.
  • Mannan and related polymers: includes poly(mannuronate) derivatives; these polymers form extended hydrogen-bond networks with water.
  • Carbohydrate gels with cations and extended networks: important for gel formation in biotechnological and food applications.

Agarose, Agar, and Related Polysaccharides

  • Agar: mixture of agarose (neutral, uncharged) and agaropectin (charged due to sulfation and carboxylate groups).
  • Agarose: linear polymer that forms gels and double helices, widely used in chromatography and electrophoresis due to gel-forming properties.
  • Agarose vs agaropectin: agarose is the neutral, gel-forming component; agaropectin carries charges that affect gel properties.

Carrageenan

  • Carrageenan is a sulfated polysaccharide similar to agarose but based on D-galactose units rather than L-galactose.
  • It is highly sulfated and used mainly as a thickener, binder, and gelling agent in foods and consumer products (e.g., meat products, sausages, toothpaste, pudding).

Glycosaminoglycans (GAGs)

  • GAGs are repeating disaccharides, one unit typically an amino sugar and the other negatively charged due to sulfate or carboxylate groups; they occur as components of proteoglycans.
  • Examples and roles:
    • Dermatan sulfate: extracellular matrix of skin.
    • Chondroitins and keratan sulfate: tendons, cartilage, and connective tissues.
    • Heparin: natural anticoagulant.
    • Hyaluronate (hyaluronic acid): vitreous humor of the eye and synovial fluid in joints.

Glycolipids and Glycoproteins

  • Carbohydrates linked to lipids (glycolipids) and to proteins (glycoproteins) mediate cellular recognition, signaling, and interactions.
  • Key biological roles include:
    • Components of cell walls and extracellular matrices in plants, animals, and bacteria.
    • Involvement in recognition between cell types and recognition of cellular structures by other molecules.

Notation and Notable Examples

  • Shorthand and structural notations:
    • Glcα1-4Glc (O-α-D-glucopyranosyl-(1→4)-D-glucopyranose).
    • Glcβ1-4Glc (O-β-D-glucopyranosyl-(1→4)-D-glucopyranose).
  • Important reactions and measures:
    • Reducing vs non-reducing sugars depend on whether the anomeric carbon is free (reducing) or involved in a glycosidic bond (non-reducing).
    • Mutarotation leads to changes in optical rotation as α and β anomers interconvert in solution.

Practical and Contextual Notes

  • Blood sugar terminology: trehalose is noted as the “blood sugar” of insects, reflecting its role in insect metabolism and stress response.
  • Melezitose is a constituent of honey.
  • Amygdals and related glycosides such as amygdalin occur in seeds and kernels of several Rosaceae plants.
  • Dextrantriose, cycloheptaamylose, laetrile, stachyose appear as diverse oligosaccharides with various biological or industrial implications.
  • Oligosaccharides can function as antibiotics: Bleomycin A/A2, Aburamycin C, Sulfurmycin B, Streptomycin, among others, illustrate non-sugar biological activities for carbohydrate-containing molecules.
  • The structural integrity and functional versatility of polysaccharides derive from alternative linkage patterns (e.g., α- vs β-linkages) and branching, allowing diverse properties from gels to structural scaffolds.

Selected Formulas and Numerical Details (LaTeX)

  • Overall monosaccharide formula: ({
    m CH2O})n where n is the number of carbons.
  • Amylose length approximation: ext{Amylose}
    oughly 10^2 ext{ to } 10^3 ext{ residues}
  • Amylopectin length approximation: ext{Amylopectin}
    oughly 3 imes 10^2 ext{ to } 6 imes 10^3 ext{ residues}
  • Starch branching: α-(1→6) every 12–20 units in amylopectin; α-(1→4) linkages predominate elsewhere.
  • Egg-carton gel concept for alginates: cross-linking between poly(α-L-guluronate) blocks with Ca^{2+} ions forms a three-dimensional network.
  • Mutarotation (illustrative values for glucose):
    • α-D-Glucopyranose: [{
      m 8}]_{20}^{D} = +112.2^{\circ}
    • Equilibrium mixture (mutarotation): approximately +52.5∘+52.5^{\circ}
    • β-D-Glucopyranose (typical lower rotation): approximately a smaller positive value (e.g., around +18.7^{\circ} depending on conditions).
  • Sucrose hydrolysis products and net rotation change: α-D-glucose + α-D-fructose; net rotation change approximately −39.5∘-39.5^{\circ}; enzyme: invertase; alternative acid hydrolysis route.

Connections to Foundational Principles and Real-World Relevance

  • Carbohydrate metabolism relies on monosaccharide derivatives (e.g., glucose-1-phosphate) and phosphorylated sugars (e.g., ATP involvement in energy transfer and signaling).
  • Regulation of carbohydrate structures influences cell–cell recognition, immunity, and tissue organization (glycoproteins, proteoglycans, glycosaminoglycans).
  • Industrial and medical relevance: enzymatic and chemical modifications of carbohydrates yield products used in pharmacology (antibiotics, anticancer candidates), food technology (thickening/texture agents like carrageenan and agarose gels), and materials science (alginate gels for tissue engineering).
  • Evolutionary and ecological implications: diverse sugar structures underpin adaptation to environmental stresses (e.g., trehalose’s role in insect stress response).