7.2 Polysaccharides · Biochemistry 2027

Some homopolysaccharides store fuel

A polysaccharide is a polymer of monosaccharide residues joined by glycosidic bonds, homopolysaccharide with one residue type and heteropolysaccharide with several
Storage polysaccharides pack sugar into many bonds, with plant starch and animal or fungal glycogen as the main examples
The polymer form limits the rise in free solute particles and so reduces osmotic load
Starch is amylose with mainly alpha(1-4) and little branching plus amylopectin with an alpha(1-4) backbone and alpha(1-6) branches
The blue iodine colour involves iodine complexes inside the amylose helix, though it varies with chain length and temperature
Glycogen is more branched than amylopectin, giving more non-reducing ends, and serves blood glucose in liver and local ATP supply in muscle
Glycogen phosphorylase cleaves non-reducing-end alpha(1-4) bonds with Pi to give glucose 1-phosphate, while alpha(1-6) branches need a debranching enzyme
An unbranched chain of n residues has n-1 glycosidic bonds and needs n-1 waters for complete hydrolysis
Example: a 1,000-residue glucan has 999 bonds and a molar mass near 162,158 g/mol

Cellulose is unbranched beta-D-glucose joined beta(1-4), where the bond configuration changes chain shape and assembly
Cellulose chains align into microfibrils held by hydrogen bonds
Humans lack an efficient endogenous beta(1-4) hydrolase, and ruminants and termites rely on microbial cellulases
Chitin is beta(1-4) N-acetyl-D-glucosamine and forms arthropod exoskeletons and fungal walls
The chitin backbone resembles cellulose but carries an acetamido group at C2
Mechanical behaviour of an exoskeleton also depends on the proportion and arrangement of proteins and minerals
Chitosan from partial deacetylation does not have the same repeat unit as chitin

Three-dimensional shape depends on rotation about glycosidic bonds and the geometry of the sugar rings
Torsion angles phi and psi describe the allowed rotations, and not every combination is accessible
The labels alpha(1-4) and beta(1-4) alone cannot establish a helix or a straight chain
Real shape depends on intra- and intermolecular hydrogen bonds, solvent, temperature, ions, chain length and crystallinity
In structural polysaccharides cooperative hydrogen bonds between neighbouring chains stabilise fibres and layers
Water competes for hydroxyl groups, so hydrogen bonding alone does not settle stability
Crystalline and disordered regions coexist, and the degree of order governs water access, enzymic digestion and mechanical behaviour

Peptidoglycan is a chain of alternating GlcNAc and MurNAc joined beta(1-4) with short peptide cross-links on MurNAc
Because two different sugars repeat, the glycan is a heteropolysaccharide, and the cross-links create a network with mechanical strength
Gram-positive bacteria have a thick peptidoglycan layer while Gram-negative bacteria have a thin layer in the periplasm plus an outer membrane
Archaea lack typical bacterial peptidoglycan and cannot be described in the same way
Lysozyme cleaves the NAG-NAM beta(1-4) bond while beta-lactams inhibit the PBPs that form peptide cross-links, so the targets differ
Brown algal alginate is a heteropolysaccharide of mannuronate and guluronate, and G-rich blocks form gels through Ca2+ interactions
Red algal agarose repeats galactose and 3,6-anhydrogalactose, and carrageenans differ in sulfate content
Calcium alginate beads are a model system and not equivalent in organisation to a natural brown algal wall

Glycosaminoglycans are long unbranched heteropolysaccharides built mostly from repeating disaccharide units
Amino sugars usually alternate with uronic acids, though keratan sulfate contains galactose instead
Carboxylate dissociation and sulfate substitution give a negative charge that shapes water and cation distribution
This contributes to matrix hydration, viscoelasticity, resistance to compression, protein binding and signalling
Hyaluronan repeats GlcA and GlcNAc, is generally not sulfated and is not covalently attached to a core protein
Its viscosity and water retention are not explained by charge alone, since chain occupancy, hydration, ions, molecular mass and other matrix components all contribute
Some glycosaminoglycans are covalently attached to core proteins as proteoglycans, while hyaluronan forms large non-covalent assemblies

The eight polysaccharides fall into storage forms, structural forms and one matrix polymer when arranged by residue, bond and function
With N = 500 residues there are 499 bonds and 499 waters are needed in theory for complete hydrolysis
For any acyclic tree structure the bond count equals residues minus one, whatever the branching
Twelve alpha(1-6) branch points that each create a new end give 1 + 12 = 13 non-reducing ends
The calculation is a simple chain model and does not capture real branching topology or enzyme access

References

**표 7.2-1**Bonds and branching in storage polysaccharides
Storage polysaccharide	Main bonds	Branching	Main role
Amylose	alpha(1-4)	Largely unbranched	Component of plant starch
Amylopectin	alpha(1-4) and alpha(1-6)	Branched	Carbohydrate storage in plants
Glycogen	alpha(1-4) and alpha(1-6)	Rather highly branched	Carbohydrate storage in animals and fungi

**표 7.2-2**Cellulose and chitin compared
Feature	Cellulose	Chitin
Repeat unit	D-Glc	GlcNAc
Glycosidic bond	beta(1-4)	beta(1-4)
Main location	Plant cell wall	Arthropod exoskeleton and fungal wall
Structural effect	Aligned microfibrils	Fibres combined with protein and mineral
Human digestion	No endogenous cellulase	Not a usual dietary carbohydrate

**표 7.2-3**Repeat composition of glycosaminoglycans
Glycosaminoglycan	Typical repeat	Usually sulfated	Main biological context
Hyaluronan	GlcA + GlcNAc	No	Hydrated matrix and synovial fluid
Chondroitin sulfate	GlcA + GalNAc	Yes	Cartilage and matrix
Heparan sulfate	GlcA or IdoA with GlcNAc derivatives	Variable	Cell surface, matrix and signalling
Keratan sulfate	Gal + GlcNAc	Yes	Cornea, cartilage and others