Carbohydrate Structure and Function

4.1 Carb Classification

This section outlines the basic classification and nomenclature of carbohydrates, including monosaccharides, stereochemistry, and common structural variations.

• Nomenclature
  • Basic structural units are monosaccharides.
  • Simplest has 3 carbons: triose; 4: tetrose; 5: pentose; 6: hexose.
  • Aldoses: carbohydrates with aldehyde as most oxidized group.
  • Ketoses: carbohydrates with ketone as most oxidized group.
  • 6-carbon sugar with aldehyde = aldohexose.
  • Glyceraldehyde = aldotriose, polyhydroxylated aldehyde; carbonyl carbon most oxidized (lowest number, C1 in aldose).
  • Can participate in glycosidic linkages; sugars = glycosyl residues.
  • Simplest ketose = dihydroxyacetone; carbonyl carbon C2.
• Common Names
  • Mannose = C1 epimer of glucose.
  • Galactose = C4 epimer.
  • D-glucose: all OH on right except C3; aldohexose.
  • In aldoses, each nonterminal C is chiral; glucose has 4 chiral carbons, 2 terminals.
• Stereochemistry
  • Optical isomers = stereoisomers; same chemical formula.
  • Enantiomers = nonidentical, nonsuperimposable mirror images (D and L glyceraldehyde).
  • 2 enantiomers per chiral carbon; stereoisomers = 2^n (n = number of chiral centers).
  • D = positive rotation, L = negative; rotation determined experimentally.
• Fisher Projection
  • Horizontal lines = wedges (out), vertical = dash (into).
  • Chiral centers of D-glucose opposite L-glucose.
  • D-glucose (below)

  

• Types
  • Enantiomers: same sugar, different optical families.
  • Diastereomer: same family, not mirror images.
  • Epimer: diastereomer differing at only one chiral center.

4.2 Cyclic Sugar Molecules

Monosaccharides cyclize to form stable hemiacetals or hemiketals, producing alpha and beta anomers with specific stereochemistry and conformations.

• Monosaccharides have hydroxyl group, act as nucleophile.
• Hemiacetals from aldoses, hemiketals from ketoses.
• Stable cyclic forms: pyranose (6-membered) or furanose (5-membered).
• Carbonyl carbon becomes chiral → anomeric carbon.
• Two ring forms: alpha (-OH trans to CH2OH) and beta (-OH cis to CH2OH); anomers of each other.
• Hexose Conformations
  • Haworth projection: 3D cyclic structure.
  • Pyranose rings adopt chair conformation.
  • Right-side Fischer groups point down in Haworth.
• Mutarotation
  • Rings in water cycle between open and closed forms → alpha and beta anomers.
  • Spontaneous change catalyzed by acid/base.
  • Alpha less favored (axial OH) → strain; beta more stable (less e⁻ repulsion).

4.3 Monosaccharides

Monosaccharides undergo oxidation, reduction, esterification, and glycoside formation, with specific reagents used to detect reducing sugars.

• Oxidation and Reduction
  • Oxidation yields energy; aldoses oxidized to aldonic acids.
  • Aldoses act as reducing agents; ring form oxidation → lactone (e.g., vitamin C).
  • Reagents for reducing sugars
    • Tollens: AgNO3 + NaOH → Ag2O, dissolve in NH3 → [Ag(NH3)2]⁺; aldehyde → silver mirror.
    • Benedict’s: oxidizes aldehyde → red Cu2O precipitate.
    • Glucose oxidase: specific to glucose.
  • Ketose sugars also reducing: tautomerize under basic conditions → aldose (keto-enol shift).
  • Aldose reduced → alditol; deoxy sugar = H replaces OH (e.g., D-2-deoxyribose in DNA).
• Esterification
  • OH groups react with COOH/derivatives → esters.
  • Phosphorylation: phosphate from ATP → glucose (hexokinase).
• Glycoside Formation
  • Hemiacetals → acetals (alpha/beta) via dehydration; water leaves.
  • C-O bond = glycosidic bond.
  • Di/polysaccharides = glycosidic bonds between monosaccharides.
  • Furanose → furanosides; pyranose → pyranosides.
  • Hydrolysis required to break bonds.

4.4 Complex Carbohydrates

Complex carbohydrates include di-, oligo-, and polysaccharides, with structural diversity determined by glycosidic linkages and branching.

• Disaccharides
  • Glycosidic bonds: 1,2; 1,4; 1,6 linkages.
  • Linkage named for anomeric carbon of first sugar.
  • Alpha 1,4: maltose; beta 1,4: cellobiose.
  • Common examples:
    • Sucrose = glucose α1,2 fructose
    • Maltose = glucose α1,4 glucose
    • Lactose = galactose β1,4 glucose
• Polysaccharides
  • Long chains of monosaccharides; glucose most common.
  • Homopolysaccharide: one type of sugar; heteropolysaccharide: multiple types.
  • Linear vs. branched: branching occurs when monosaccharide forms 2 glycosidic bonds.
• Cellulose
  • Beta D-glucose linked β1,4; H-bonds stabilize.
  • Structural in plants; humans can't digest (fiber in diet).
  • Cellulase in termites, cows, goats.
• Starches
  • Alpha D-glucose monomers; digestible.
  • Amylose: linear α1,4; amylopectin: α1,4 + α1,6.
  • Iodine test fits in amylose helix.
  • Beta amylase cleaves nonreducing end → maltose; alpha amylase cleaves randomly.
  • Debranching enzymes for amylopectin.
• Glycogen
  • More α1,6 branching than starch (1/10 glucose vs. 1/25 in amylopectin).
  • Branching → solubility ↑, energy efficiency ↑.
  • Glycogen phosphorylase cleaves from nonreducing ends → glucose 1-phosphate.
  • Deficiency → GSD VI: glycogen accumulation, hypoglycemia.

  

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