Lecture 13: Polysaccharides

• Structural elements of homopolysaccharides

  • Plant cell walls and animal exoskeletons
• Homopolysaccharides

  • Only has one kind of monosaccharide unit
  • Serve as storage forms and structural elements
• Polysaccharides generally do not have defined lengths or molecular weights
• No template/instructions for formation of polysaccharides
• Polysaccharides


  Many monosaccharides linked together
• Starch and glycogen have a very high water content within them due to exposed hydroxyl groups that can hydrogen bond
• Mechanism for formation of polysaccharide

  Built into the enzymes that catalyze the polymerization of the monomer units within polysaccharides
• Amylopectin


  • Larger than amylose with α1→4 linkages between glucose residues and highly branched due to α1→6 linkages
• Starch

  • Contains two types of glucose polymer, amylose and amylopectin
• Structure of starch and glycogen

  • Starch (amylose) contains one non-reducing end and one reducing end because it's linear with α1→4 linkages. Amylopectin or Glycogen has α1→6 linkages in addition to α1→4 linkages
• Storage of Glucose as Polymers Avoids High Osmolarity
• Polysaccharide Storage forms

  • Used as fuel: Starch and glycogen
• Heteropolysaccharides


  • More than two kinds of monosaccharide units
  • Provide extracellular support
• Proteins and polysaccharides


  • Put together in a different way
  • Differ in mechanisms of assembly
• Termination point in the formation of polysaccharides is random
• Polysaccharides


  • Most of the carbohydrates in nature occur as these
  • Another name is glycans
  • Differ in identity of recurring monosaccharide units, degree of branching, and types of bonds linking the units
• Amylose


  • Long, unbranched chains of D-glucose connected by α1→4 linkages
• Energy storage

  • Starch and Glycogen: two main types of energy storage polysaccharides
• Storage polysaccharides

  • Starch in plant cells
  • Glycogen in animal cells
• Glycogen


  • Polymer of α1→4 linked glucose subunits, with α1→6 linked branches, more branched and compact than starch
• Starch as a whole


  • α1→6 branch points are from amylopectin
• Chitin is used to fill exoskeletons and is water-resistant due to the acetylated amino group
• Minimizing energy maximizes H-bonding, leading to energetically favorable conformations
• Homopolysaccharides

  • Cellulose: tough, fibrous, water-insoluble substance
  • Chitin: linear homopolysaccharide composed of N-acetylglucosamine residues in β1→4 linkages
• Osmolarity: water moving from one location to another to make the concentration on both sides equal
• Glycogen


  Due to the α1→6 linkages aside from the α1→4 linkages
• Free rotation about C-O bonds linking the residues is limited by steric hindrance, affecting the folding of homopolysaccharides
• Starch


  • The α1→6 branch points are from amylopectin
  • Storage of Glucose as Polymers Avoids High Osmolarity
• Glucose storage as a monomer barely contributes to the osmolarity, leading to rupture of the cell due to osmotic pressure
• Steric Factors and Hydrogen Bonding

  • The 3D structure of homopolysaccharides is stabilized by weak interactions within the molecule itself or between other molecules
  • Hydrogen bonding is important due to the hydroxyl groups forming H-bonds
• Animals do not have the enzyme to hydrolyze β1→4 glycosidic bonds, so they do not digest cellulose
• 3D structures of disaccharides are described in terms of ɸ and Ѱ around the glycosidic bond, with certain conformations being more stable than others
• Repeating Units of Glycosaminoglycans of ECM

  Glycosaminoglycans are linear polymers composed of repeating disaccharide units, one monosaccharide is always a modified sugar and the other is usually a uronic acid
• Peptidoglycan Reinforces the Bacterial Cell Wall


  Peptidoglycan is a rigid component of bacterial cell walls, contains sugar and amino acids, heteropolymer of alternating β1→4 linked with N-acetylglucosamine and N-acetylmuramic acid residues, cross-linked by short peptides
• Conformations

  • Certain conformations are more stable than others
  • Low energy confirmation: most disaccharides are in this conformation, minimizing energy and maximizing H-bonding, energetically favorable
  • High energy conformation: sterically hindered, atoms are too close to each other
• Linear Structure of Cellulose


  Cellulose contains β1→4 linkages, most energetically favorable configuration is a fully stretched out linear structure, each chain is turned 180 degrees relative to its neighbors, all -OH groups are available for H-bonding with neighboring chains
• Glycosaminoglycans are unique to animals and bacteria, some contain esterified sulfate groups, the overall structure is highly negatively charged to avoid unfavorable electrostatic interactions
• Helical Structure of Starch and Glycogen

  Tightly coiled helix, most stable 3D structure of the α1→4 linkages chains of starch and glycogen, six residues per turn, core of the helix is precisely to the right to accommodate complex ions
• Glycosaminoglycans in the Extracellular Matrix

  Extracellular matrix (ECM) is composed of interlocking meshwork of heteropolysaccharides and fibrous proteins, provides a porous pathway for nutrient and O2 diffusion, contains both protein and saccharide material
• Wood and cell wall are strong and tough due to extensive interchain H bonding