Lecture 23: Polysaccharides, PROTACS and LYTACS

Polysaccharides

No set agreed upon differentiation

Most carbs in nature

+20,000 kDa

Also called glycans

Homopolysaccharides

Contain only a single monomeric sugar species

Serve as storage forms and structural elements

Heteropolysaccharides

Contain two or more kinds of monomers

Provide support for the extracellular matrix

Polysaccharide Synthesis

Distinct from protein synthesis

There is no template for polysaccharide synthesis

Synthesis is intrinsic to enzyme that catalyzes the polymerization of monomers

Reason why there is no set length for a polysaccharide

Storage Polysaccharides

Can be homopolysaccharides

Starch in plants and glycogen in animals

These molecules are heavily hydrated because they have many exposed hydroxyl groups available to hydrogen bond

Starch

Contains two types of glucose polymer (amylose and amylopectin)

Amylose

Long, unbranched chains of D glucose residues connected by alpha 1→4 linkages

Amylopectin

Larger than amylose with alpha 1 → 4 linkages between glucose residues and highly branched due to alpha 1 → 6 linkages

Glycogen

Polymer of alpha 1 → 4 linked glucose subunits with alpha 1 →6 linked branches

More extensively branched

More compact than starch

Draw Structure of Starch and Glycogen

Drawn

Why is glucose not stored in the cell

Glucose is highly water soluable, if it were to dissolve in the cell the osmotic pressure of the cell would change and the cell would be hypotonic causing water to rush into the cell and the cell would burst. Glycogen is not as soluble and this would not happen.

Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers

Hepatocytes and Storage of Glucose

In the fed state, store glycogen equivalent to a glucose concentration of 0.4 M

0.4 M glucose in the cytosol would elevate the osmotic entry of water might rupture the cell

Cellulose

Homopolysaccharide that serves structural role

Linear, unbranched, consists of 10,000 to 15,000 d-glucose units

Glucose residues have the beta configuration, beta 1 → 4 glycosidic linkage

Animals do not have the enzymes to digest beta 1,4 linkages

Chitin

Linear homopolysaccharide composed of N-acetylglucosamine residues in beta 1,4 linkage

Acetylated amino group makes chitin more hydrophobic and water resistant than cellulose

Homopolysaccharide Folding

Steric and hydrogen bonding influence folding

Three dimensional structures stabilize by weak interactions within or between molecules

Hydrogen bonding is especially important due to the high number of hydroxyl groups in polysaccharides

Free rotation about both C-O bonds linking the residues is limited by steric hindrance by substituents

Peptidoglycan

Reinforces the bacterial cell wall

Ridgid component of bacterial cell walls

Heteropolymer of alternating beta 1 → 4 linked N-acetylmuramic acid residues

Cross linked by short peptides

Glycoconjugate

Biologically active molecule consisting of an informational carbohydrate joined to a protein or lipid