Carbohydrates and Glycobiology: Structure, Function, and Stereochemistry

Carbohydrates and Glycobiology

Introduction

Carbohydrates, also known as saccharides or sugars, are the most abundant biomolecules on Earth. They are primarily derived from CO2 and H2O through the process of photosynthesis, which produces vast quantities of carbohydrates such as cellulose and other plant products. The general formula for carbohydrates is (CH2O)n, indicating their composition of carbon, hydrogen, and oxygen.

General Properties of Carbohydrates

• Do not catalyze complex chemical reactions as proteins do.

• Do not replicate themselves like nucleic acids (DNA, RNA).

• Not built from a genetic blueprint as nucleic acids and proteins are.

• Highly heterogeneous in size and composition compared to other biomolecules.

• Structural variation is fundamental to their biological activity, especially in recognition events between proteins and cells.

Functions of Carbohydrates

Nutritional	Structural	Information
Energy storage: glycogen, starch Fuels: glucose Metabolic intermediates	Cell walls: bacteria, plants Exoskeleton: arthropods Connective tissues: cartilage, bone Components of nucleotides	Molecular recognition Cell-cell communication

Classification of Carbohydrates

By Number of Sugar Units

• Monosaccharides: Single sugar unit (e.g., glucose, fructose)

• Disaccharides: Two sugar units joined by a glycosidic bond (e.g., maltose, sucrose)

• Oligosaccharides: 3–20 sugar units, often found attached to proteins (glycoproteins) or lipids (glycolipids)

• Polysaccharides: More than 20 sugar units (e.g., starch, cellulose, glycogen)

Monosaccharide Structure and Nomenclature

Monosaccharides are either aldehyde (aldose) or ketone (ketose) derivatives of straight-chain polyhydroxy alcohols. They are classified by:

• Number of carbon atoms (triose, tetrose, pentose, hexose, etc.)

• Location of the carbonyl group (aldose: at the end; ketose: within the chain)

• Stereochemistry (D- and L- isomers, based on the chiral carbon furthest from the carbonyl group)

General Formula

For monosaccharides: $C_n(H_2O)_n$

Stereochemistry of Monosaccharides

Stereoisomers have the same order and types of bonds but differ in the spatial arrangement of atoms. The number of possible stereoisomers is $2^n$, where $n$ is the number of chiral carbons.

• Enantiomers: Mirror-image isomers (e.g., D- and L-glyceraldehyde)

• Epimers: Isomers that differ at only one chiral carbon (e.g., D-glucose and D-galactose are epimers at C4)

Cyclization of Monosaccharides

Monosaccharides with five or more carbons can cyclize to form ring structures. Cyclization involves the nucleophilic attack of a hydroxyl group on the carbonyl carbon, forming a hemiacetal (from an aldehyde) or hemiketal (from a ketone).

• Pyranose: Six-membered ring (e.g., glucose)

• Furanose: Five-membered ring (e.g., fructose)

General Cyclization Reaction

For an aldehyde:

$ext{Aldehyde} + ext{Alcohol} ightarrow ext{Hemiacetal}$

For a ketone:

$ext{Ketone} + ext{Alcohol} ightarrow ext{Hemiketal}$

Anomers

Cyclization creates a new chiral center at the carbonyl carbon, called the anomeric carbon. Two possible isomers (anomers) can form:

• α-anomer: The OH group on the anomeric carbon is trans to the CH2OH group.

• β-anomer: The OH group on the anomeric carbon is cis to the CH2OH group.

Interconversion between α and β forms occurs via the linear form.

Glycosidic Bonds

Monosaccharides are linked by glycosidic bonds, formed by condensation between the anomeric carbon of one sugar and a hydroxyl group of another. The bond can be hydrolyzed by enzymes.

General Glycosidic Bond Formation

$ext{Monosaccharide}_1 + ext{Monosaccharide}_2 ightarrow ext{Disaccharide} + H_2O$

Common Disaccharides

Name	Constituent Sugars	Linkage
Maltose	Glucose + Glucose	α(1→4)
Sucrose	Glucose + Fructose	α(1→2)β
Lactose	Galactose + Glucose	β(1→4)
Trehalose	Glucose + Glucose	α(1→1)α

Polysaccharides (Glycans)

Polysaccharides are long chains of monosaccharide units. They can be:

• Homopolysaccharides: Composed of one type of monosaccharide (e.g., cellulose, chitin)

• Heteropolysaccharides: Composed of more than one type of monosaccharide

• Branched or unbranched

Structural Polysaccharides

• Cellulose: Homopolymer of β(1→4) linked glucose; unbranched; provides structural support in plant cell walls

• Chitin: Homopolymer of β(1→4) linked N-acetylglucosamine; found in exoskeletons of arthropods; forms strong hydrogen-bonded networks

Energy Storage Polysaccharides

• Starch: Synthesized by plants; consists of amylose (linear, α(1→4) linked glucose) and amylopectin (branched, α(1→4) and α(1→6) linkages)

• Glycogen: Storage polysaccharide in animals; highly branched (α(1→4) and α(1→6) linkages); more compact and suited for rapid mobilization of glucose

Amylose Structure

• Linear polymer of α(1→4) linked D-glucose units

• Has a reducing end (free anomeric carbon) and a non-reducing end

• Stable conformation is a helical structure

Biological Implications of Carbohydrate Storage

• Storing glucose as glycogen prevents high cytosolic osmolarity, which could lead to cell rupture.

• Glycogen is insoluble and does not contribute significantly to osmolarity, unlike free glucose.

• High intracellular glucose concentration would require large energy expenditure to maintain against extracellular levels.

Example: Carbohydrates in Viral Recognition

Carbohydrates (glycans) on the surfaces of viruses, such as SARS-CoV-2, play crucial roles in molecular recognition and immune evasion.

Additional info:

• Carbohydrate stereochemistry is essential for enzyme specificity and biological function.

• Glycosidic bond nomenclature specifies the configuration (α or β) and the carbon atoms involved in the linkage.