Carbohydrates: Polysaccharides

Question 1

Describe the structures of the polysaccharides that make up cellulose and chitin, including the
- types of monosaccharide residues
- types of linkages
- branching.

Answer 1

Cellulose:
- Linear (unbranched) chain of D-glucose residues joined by (β1→4) linkages
- Up to 15,000 units long
- Structure is Rigid and Insoluble
- 180 degree rotation between sequential monosaccharides
- H-bonds form between sequential units (and between adjacent chains)
- Can exclude water and form fibrous structures
- Animals lack enzyme to catabolize (β1→4) linkages (cellulases)

Cellulose Chains will align side-by-side to form extensive inter-molecular interactions

Question 2

Describe the structures of the polysaccharides that make up chitin and cellulose, including the
- types of monosaccharide residues
- types of linkages
- branching.

Answer 2

Chitin
- Linear (unbranched)
- Homopolysaccharide
- Residues: N-Acetylglucosamine (GlcNAc)
- Linkage: (β1→4) glycosidic linkage
- 180 degree rotation of sequential units
- forms exoskeleton
- Chitin contains calcium carbonate - calcification occurs

Cellulose:
- Linear (unbranched) chain of D-glucose residues joined by (β1→4) linkages
- Up to 15,000 units long
- Structure is Rigid and Insoluble
- 180 degree rotation between sequential monosaccharides
- H-bonds form between sequential units (and between adjacent chains)
- Can exclude water and form fibrous structures
- Animals lack enzyme to catabolize (β1→4) linkages (cellulases)
- Cellulose Chains will align side-by-side to form extensive inter-molecular interactions

Question 3

Describe the structures of the polysaccharides that make up starch and glycogen, including the types of monosaccharide residues, types of linkages and branching.

Answer 3

Both Starch and Glycogen are homopolysaccharides of Glucose units, stored as granules inside cells

Starch:
Made from 2 polysaccharides:
(1)Amylose: Unbranched homopolysaccharide
- Linear unbranched chain of D-glucose joined by (α1→4) linkages
- Contains Reducing end (long unbranched polysaccharide will have one reducing end)
- Parallel to cellulose but ALPHA rather than Beta
- Helical Structure

(2)Amylopectin: Branched homopolysaccharide
- Highly-branched homopolysaccharide (D-glucose)
- Primarily (α1→4) linkages with (α1→6) linkages (At Branch Points) every 24-30 residues
- Helical Structure

STARCH:
- Amylose and Amylopectin associate together in double helices
- Amylopectin branches also wind together into double helices
- Degradation occurs at non-reducing ends - multiple sites of catalysis

Relatively unsoluble
“Wet” - unfolds and refolds -> thickening

BREAKDOWN AND SYNTHESIS BOTH OCCUR AT NON-REDUCING ENDS

Starch is a made of two polysaccharides: amylose and amylopectin
- Amylose is a linear polymer of glucose monomers linked by α(1→4) glycosidic bonds, which forms a helical structure due to intramolecular hydrogen bonding
- Amylopectin is a branched polymer of glucose monomers, with α(1→4) glycosidic bonds forming the backbone and α(1→6) glycosidic bonds forming the branches. The branching of amylopectin disrupts the regularity of the helix, but the overall structure is still helical.

Glycogen is a highly branched polysaccharide that is structurally similar to amylopectin, but with more frequent branching:
- Primarily α(1→4) linkages with α(1→6) every 8-14 residues
- The branching of glycogen creates a more compact structure, allowing for efficient storage of glucose in cells.

Cellulose is a linear & extended polymer of glucose linked by β(1→4) glycosidic bonds.
- Due to the β linkage, the glucose monomers cannot rotate around the bond, resulting in a straight, linear structure.
- Multiple chains of cellulose are linked together by hydrogen bonds, forming a rigid and strong structure that provides strength and rigidity to plant cell walls.

Question 4

Describe the structures of the polysaccharides that make up starch and glycogen, including the types of monosaccharide residues, types of linkages and branching.

Answer 4

Both Starch and Glycogen are homopolysaccharides of Glucose units, stored as granules inside cells

Glycogen:
- Highly-branched homopolysaccharide (D-glucose)
- Primarily (α1→4) with (α1→6) linkages every 8-14 Residues (more frequent than in amylopectin = decrease ability to form helix)
- Multiple non-reducing ends (single Reducing end)
- Form spherical structure with Reducing End making up the Core
- Reducing end (core) is glycosidically bound to protein = no hemiacetal but still considered Reducing
- structurally similar to amylopectin, but with more frequent branching

Relatively unsoluble
“Wet” - unfolds and refolds -> thickening

BREAKDOWN AND SYNTHESIS BOTH OCCUR AT NON-REDUCING ENDS

Starch:
Made from 2 polysaccharides:
(1)Amylose: Unbranched homopolysaccharide
- Linear unbranched chain of D-glucose joined by (α1→4) linkages
- Contains Reducing end (long unbranched polysaccharide will have one reducing end)
- Parallel to cellulose but ALPHA rather than Beta
- Helical Structure

(2)Amylopectin: Branched homopolysaccharide
- Highly-branched homopolysaccharide (D-glucose)
- Primarily (α1→4) linkages with (α1→6) linkages (At Branch Points) every 24-30 residues
- Helical Structure

STARCH:
- Amylose and Amylopectin associate together in double helices
- Amylopectin branches also wind together into double helices
- Degradation occurs at non-reducing ends - multiple sites of catalysis

Question 5

Explain the benefits of storing glucose in branched structures, and in polymeric form

Answer 5

Glycogen vs Glucose
(1) Glycogen is less reactive than glucose (aldehyde aspect drives spontaneous Rxn in glucose)
- Glycogen has only one reducing end (attached to a protein)
- Multiple Non-Reducing ends

(2) Impact on Osmosis: Lower Concentration
- Glycogen single mol and precipitates = Contribute less to osmolarity (Decreases osmotic effect
- Doesn’t draw in water to the extent that glucose would
- 0.01uM glycogen (insoluble) ≈ 0.4M glucose
- [gluc]bloodstreem ~5mM

Question 6

List differences in polysaccharide composition that account for structural differences and list the forces involved in stabilizing folded conformations.

Answer 6

Composition:
-Homopolysaccharides - one repeated monosaccharide unit
-Heteropolysaccharides: two or more monosaccharide units (mixture) (more complex)

Structures adopted are determined by
- monosaccharide units (glucose, fructose, etc)
- nature of glycosidic bond (α(1→4) or β(1→4) linkages)
- Branching may occur affecting overall structure
- Degree of polymerization (# of monosaccharide units/Length)

Stabilizing Forces:
- Van der Waals interactions: between non-polar groups in the polysaccharide chain
- H-Bonding
- Hydrophobic Interactions
- Maximize H-bonding
- Minimize steric interactions
- May associate with or exclude WATER (may depend on polysaccharide: Starch=water // Cellulose=/=water)

Structural Polysaccharides:
- Peptidoglycans (bacterial cell walls)
- Agarose (algae)
- Glycosaminoglycans (ECM in animals)

Structural HOMOPolysaccharides:
- Cellulose (plants)
- Chitin (insects)

Storage Homopolysaccharides:
- Starch (plants)
- Glycogen (animals)

Question 7

Starch/Glycogen/Cellulose: Describe the most stable folded structures for starch and glycogen compared to cellulose.

Answer 7

Starch and glycogen are both polysaccharides composed of glucose monomers
- Both are stored as granules inside cells
- The most stable folded structures for starch and glycogen are helical structures

Cellulose is a polysaccharide composed of glucose monomers linked by β(1→4) glycosidic bonds.
- most stable folded structure for cellulose is a straight, linear structure

Starch is a made of two polysaccharides: amylose and amylopectin
- Amylose is a linear polymer of glucose monomers linked by α(1→4) glycosidic bonds, which forms a helical structure due to intramolecular hydrogen bonding
- Amylopectin is a branched polymer of glucose monomers, with α(1→4) glycosidic bonds forming the backbone and α(1→6) glycosidic bonds forming the branches. The branching of amylopectin disrupts the regularity of the helix, but the overall structure is still helical.

Glycogen is a highly branched polysaccharide that is structurally similar to amylopectin, but with more frequent branching:
- Primarily α(1→4) linkages with α(1→6) every 8-14 residues
- The branching of glycogen creates a more compact structure, allowing for efficient storage of glucose in cells.

Question 8

PEPTIDOGLYCAN: Describe the structures of the polysaccharides and peptides that make up peptidoglycan including the types of linkages and branching.

Answer 8

Peptidoglycans:
- Mixture of polysaccharide & peptide-like structures
- Alternating N-acetylglucosamine (GlcNAc) (NAG) and N-acetylmuramic acid (Mur2Ac) (NAM)
- Joined via (β1→4) glycosidic bonds
- Linear Chain
- Rigid structure to reinforce bacterial cell walls

Adjacent chains are crosslinked by peptide-based structures
- Peptide cross-links contain both L- and* D- amino acids*
- Joined via the Lys side chain to adjacent pentaglycine (5x glycine) groups
- Varies depending on Gram+ or Gram-

Lysozyme hydrolyzes glycosidic bonds // breaks bonds between adjacent monosaccharide units

Penicillin inhibits formation/disrupts of peptide cross-links

Question 9

Compare and contrast peptide structures in peptidoglycan with polypeptide structures.

Answer 9

Peptidoglycan:
Complex polymer composed of:
- repeating disaccharide units composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
- Peptide chains attached at NAM residues
- Peptide chains contain both L- and D- amino acids linked by peptide bonds - form “mesh”
- Cross-linking gives strength and rigidity
- Repeating units of sugars and AA’s
- unique to bacteria

Polypeptides:
- linear polymers of solely AA’s building proteins
- peptide bond
- Sequence determines structure

Question 10

Glycosaminoglycans:
Describe structural features of glycosaminoglycans.

Answer 10

Glycosaminoglycans (GAG)
Unbranched Heteropolysaccharides
- Repeating disaccharides in linear chains
- Mixture of Uronic Acid derivatives and Glucosamine/galactosamine derivatives
- Often Sulfated groups or Carboxylated dervatives
- Neg Charge (Adopt extended structures to reduce charge repulsion)
- Involved in intermolecular interactions

Components of ECM in animals
- Porous network that supports cells
- Associated with fibrous proteins (like collagen)
- May be part of a conjugated protein structure (proteoglycans)

Question 11

Explain the reason for the extended conformations of some glycosaminoglycans.

Answer 11

Extended structures are adopted to reduce charge repulsion (Glycosaminoglycans are Negatively charged)
- because often contains sulfated groups or carboxylated derivatives (SO3- / COO-)