Polysaccharides

N-Linked Glycans

Attached to the nitrogen atom in the side-chain amide group of an asparagine residue Sequence for attachment is –Asn–X–Ser/Thr–, where X can be any amino acid. Often have complex, branched structures and play roles in protein targeting and recognition, such as in immunoglobulins.

O-Linked Glycans

Attached via an O-glycosidic bond to the hydroxyl group of serine or threonine residues

O proteins and blood types

There are O-linked glycans on the surfaces of every cell in any blood type except O,

which has no glycan. These glycans are recognized by other cells in the body so they

know that it’s not a foreign entity in the body

How does influenza use glycoproteins to destroy its host?

One type of glycoprotein on influenza binds to the host receptor allowing the virus to

invade the cell. The other type cleaves the sialic acid molecules on the surface to allow

the virus to replicate and mature inside the cel

N vs O linked Glycans and biological roles 

N-linked glycans-  protein recognition, intracellular targeting, and ensuring proteins reach their correct cellular destinations.

O-Linked Glycans: increasing fluid viscosity, cellular identification, such as blood group antigens

Peptidoglycan

alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM); tetrapeptide sequence: (L-Ala)-(D-Glu)-(L-Lys)-(D-Ala)

Glycosaminoglycans (GAGs)

in vertebrates, also known as mucopolysaccharides. They are composed of repeating disaccharide units, where one sugar is either N-acetylgalactosamine or N-acetylglucosamine. These units often carry carboxyl and sulfate groups, giving GAGs a high density of negative charges.

Chitin structure

structural polysaccharide that is a homopolymer of N-acetyl-β-D-glucosamine. It resembles cellulose but differs by having an acetylated amino group replacing the hydroxyl group on carbon 2 of each residue. Key component in the exoskeletons of arthropods and mollusks. Like collagen in vertebrates, chitin provides a matrix for mineralization, contributing to the rigidity and durability of these exoskeletons. Chitin's structure is characterized by β(1→4) glycosidic linkages, similar to cellulose, which contributes to its strength and stability.

Xylans

polymers made of β(1→4)-linked d-xylopyranose units, often with additional substituents.

Glucomannans

consist of alternating β-D-glucopyranose and β-D-mannopyranose units, also linked by β(1→4) bonds.

Amylose

A linear polymer with α(1→4) glycosidic bonds.

Amylose Secondary Structure

has a unique secondary structure due to its linear chain of glucose units connected by α(1→4) glycosidic bonds. This arrangement allows amylose to form a helical structure, which is stabilized by hydrogen bonds between the glucose residues. The helix has a large interior core, which is a characteristic feature of amylose. The helical conformation is favored because each glucose residue is angled relative to the previous one, promoting a regular, spiral shape.

Amylopectin

A branched polymer with both α(1→4) linear and α(1→6) branched linkages, creating branch points.

The branched structure of amylopectin and glycogen

allows for efficient glucose mobilization, as enzymes can simultaneously attack multiple nonreducing ends. This is crucial for quick energy release when needed.

Cellulose

major structural polysaccharide found in plants, particularly in woody and fibrous varieties like trees and grasses. It is the most abundant polymer in the biosphere. Structurally, cellulose is a linear polymer composed of d-glucose units linked by β(1→4) glycosidic bonds. This configuration allows cellulose chains to form extended structures where each glucose unit is rotated 180° relative to its neighbor. These chains align side by side, creating ribbons that are stabilized by hydrogen bonds, similar to the β-sheet structure in silk fibroin. This arrangement gives cellulose its remarkable mechanical strength and limited extensibility.

Cellulose vs. starch

Unlike starch, which has α(1→4) linkages, the β(1→4) linkages in cellulose make it indigestible by human enzymes.

Amylopectin’s branching allows

faster mobilization of glucose for plant metabolism.

biological functions of carbohydrates

• Metabolism, storage, and generation of energy (glucose, glycogen, and starch)

• Molecular recognition (immune system)

• Cell protection (bacteria and plant cell walls) 

• Cell adhesion (glycoproteins) 

• Biological lubrication (glucoaminoglycans)

• Maitenance of biological structure (in cellulose and chitin)

Metabolism, storage, and generation of energy

glucose, glycogen, and starch

Molecular recognition

Cell protection

bacteria and plant cell walls

Cell adhesion

glycoproteins

Biological lubrication

glucoaminoglycans

Maitenance of biological structure

cellulose and chitin