Carbohydrates and Structural Polysaccharides

Four classes of large biological molecules

• Carbohydrates, proteins, lipids, nucleic acids

Monosaccharides: building blocks

• Monosaccharides are simple sugars; fuel and raw materials

• General formula: (CH{2}O)n

• Glucose: C{6}H{12}O_{6}

• Classification by:

  • Number of carbons: trioses $(C{3}H{6}O{3})$, pentoses $(C{5}H{10}O{5})$, hexoses $(C{6}H{12}O_{6})$

  • Location of carbonyl group: aldose or ketose

  • Arrangement of hydroxyl groups; linear or ring forms

Aldose vs. ketose

• Aldose: carbonyl group at end of carbon chain

• Ketose: carbonyl group in middle of carbon chain

Ring vs linear forms

• Sugars exist in linear and ring forms; ring formation involves reaction of carbonyl with a hydroxyl group

Disaccharides and glycosidic linkages

• A disaccharide forms when a dehydration reaction joins two monosaccharides

• Covalent bond: glycosidic linkage

• Example: maltose (glucose–glucose) with 1–4 glycosidic linkage

Glycosidic linkages

• Linkages can form between any two hydroxyl groups; location and geometry vary among polysaccharides

• α-1,4-glycosidic linkage and β-1,4-glycosidic linkage

  • α and β refer to the orientation of the C-1 hydroxyl relative to the plane of the glucose rings

Polysaccharides

• Polysaccharides = polymers of sugars

• Determined by: sugar monomers and glycosidic linkages

• Functions: store chemical energy; provide fibrous structural materials; indicate cell identity

Storage polysaccharides of plants

• Starch: storage polysaccharide of plants; glucose polymer

  • mixture of branched amylopectin and unbranched amylose; all α-glucose polymer

• Plants store surplus starch as granules in chloroplasts and plastids

Glycogen (animals)

• Storage polysaccharide in animals; highly branched α-glucose polymer

• Stored mainly in liver and muscle cells

Carbohydrates and energy storage

• Store chemical energy; provide energy in cells

• In photosynthesis: light energy is stored in chemical bonds of CH2O

• Carbohydrates have more free energy than CO2 because C–H and C–C bonds are less polar than C–O bonds

Structural polysaccharides

• Cellulose: major component of plant cell walls; polymer of glucose with β-1,4 linkages

• Chitin: structural polysaccharide of β-glucoses with N-acetylglucosamine; in arthropod exoskeletons and fungal cell walls; β-1,4-glycosidic linkages; hydrogen bonds between strands

• Peptidoglycan: structural support for bacterial cell walls; backbones of alternating monosaccharides; β glycosidic linkages

How carbohydrates provide structure

• Form long strands with inter-strand bonds; fibers or sheets

• β-1,4-glycosidic linkages are difficult to hydrolyze; few enzymes accommodate this geometry

• Absence of water in fibers also limits hydrolysis; structural polysaccharides are resistant to degradation

Microbial digestion and herbivores

• Some microbes produce enzymes to digest cellulose

• Many herbivores rely on symbiotic microbes (e.g., cows, termites)

Carbohydrates: cell identity

• Carbohydrates on cell surfaces indicate cell identity

• Display information as glycoproteins (proteins covalently bonded to carbohydrates)

• Roles: cell–cell recognition and cell–cell signaling

Research example: sperm–egg glycoprotein interaction

• Hypothesis: sperm attach to the carbohydrate component of egg-surface glycoproteins

• Experimental setup: isolate glycoproteins; separate protein and carbohydrate; test attachment after treatments

• Results: carbohydrate component blocks sperm attachment as effectively as intact glycoprotein; pure protein blocks few attachments

• Conclusion: Sperm recognize and bind to the carbohydrate component of egg-surface glycoproteins