BIOCHEM Chap. 7: Carbohydrates

Carbohydrates: Structure and Classification

Monosaccharides and Disaccharides

Carbohydrates are polyhydroxy aldehydes or ketones, commonly represented by the empirical formula (CH2O)n. They are classified based on the number of sugar units and the type of functional group present.

• Monosaccharides: Simple sugars composed of a single unit, such as glucose, fructose, and ribose.

• Oligosaccharides: Composed of a few (typically 2–10) monosaccharide units.

• Polysaccharides: Large polymers consisting of many monosaccharide units, e.g., starch, glycogen, cellulose.

Monosaccharides can be classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group). Examples include:

• Aldose: Glucose, ribose, glyceraldehyde

• Ketose: Fructose, dihydroxyacetone

Example: Glyceraldehyde is an aldose; dihydroxyacetone is a ketose.

Stereochemistry of Monosaccharides

Monosaccharides possess multiple chiral centers, leading to various stereoisomers. The number of stereoisomers for an aldohexose is calculated as:

• Formula: $2^n$ where n is the number of chiral centers.

• For aldohexoses, $n = 4$, so there are $2^4 = 16$ stereoisomers.

• These include pairs of enantiomers (mirror images) and diastereomers.

• D- and L- Isomers: The D/L system refers to the configuration relative to glyceraldehyde, not absolute configuration.

Cyclic Forms and Anomers

Monosaccharides can cyclize to form cyclic hemiacetals (aldoses) or hemiketals (ketoses). Cyclization creates a new chiral center at the anomeric carbon, resulting in two anomers:

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

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

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

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

Example: The cyclic form of glucose is called a pyranose.

Formation of Glycosidic Bonds

Monosaccharides can react with alcohols to form acetals (glycosidic bonds). The bond between the sugar and the alcohol is called an O-glycosidic bond. Glycosidic bonds link monosaccharides to form disaccharides and polysaccharides.

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

• Reducing sugar: Has a free anomeric carbon that can be oxidized.

• Non-reducing sugar: Both anomeric carbons are involved in glycosidic bonds (e.g., sucrose).

Examples of Glycosidic Linkages

Disaccharide	Glycosidic Bond
Lactose	β(1 → 4)
Sucrose	α(1 → 2)
Maltose	α(1 → 4)

Polysaccharides

Polysaccharides are classified as homopolysaccharides (one type of sugar) or heteropolysaccharides (multiple types of sugars).

• Homopolysaccharides: Starch, glycogen, cellulose

• Heteropolysaccharides: Glycosaminoglycans, proteoglycans

Example: Starch is a homopolysaccharide of glucose; glycosaminoglycans are heteropolysaccharides.

Biological Functions of Polysaccharides

• Energy storage: Starch (plants), glycogen (animals)

• Structural: Cellulose (plants), chitin (arthropods)

• Recognition and signaling: Glycoproteins, glycolipids

Glycogen and Starch

Glycogen is a highly branched polymer of glucose, similar to amylopectin but more extensively branched. This structure allows rapid mobilization of glucose units.

• Glycogen: α(1 → 4) glycosidic bonds with α(1 → 6) branches

• Starch: Amylose (linear, α(1 → 4)), Amylopectin (branched, α(1 → 6) at branch points)

Cellulose

Cellulose is a linear polymer of glucose with β(1 → 4) glycosidic bonds. Its structure is stabilized by extensive hydrogen bonding, making it insoluble and resistant to enzymatic hydrolysis by most animals.

• Cellulose: β(1 → 4) glycosidic bonds

• Enzymatic hydrolysis: Only certain microorganisms possess enzymes to break β(1 → 4) bonds.

Glycoproteins, Proteoglycans, and Glycosylation

Carbohydrates can be covalently attached to proteins and lipids, forming glycoproteins and proteoglycans. Glycosylation affects protein folding, stability, and cell signaling.

• O-linked glycosylation: Carbohydrate attached to the hydroxyl group of serine or threonine.

• N-linked glycosylation: Carbohydrate attached to the amide nitrogen of asparagine.

Nucleotides and N-Glycosidic Bonds

The anomeric carbon of a monosaccharide can react with an amine to form an N-glycosidic bond, as seen in nucleotides (DNA and RNA).

Component	Example
Sugar	Deoxyribose, ribose
Base	Adenine, guanine
Phosphoryl group	Phosphate

Biological Importance and Applications

• Antibiotics: Some antibiotics inhibit cell wall biosynthesis by targeting glycosidic bond formation in bacteria.

• Enzyme specificity: Enzymes such as ribonuclease are specific for certain sugar substrates due to their active site structure.

Summary Table: Key Carbohydrate Types and Bonds

Type	Bond	Function
Starch (amylose)	α(1 → 4)	Energy storage in plants
Starch (amylopectin)	α(1 → 4), α(1 → 6)	Branched energy storage in plants
Glycogen	α(1 → 4), α(1 → 6)	Branched energy storage in animals
Cellulose	β(1 → 4)	Structural support in plants
Sucrose	α(1 → 2)	Transport sugar in plants

Additional info:

• 2-deoxyribose differs from ribose by having a hydrogen instead of a hydroxyl group at carbon 2.

• Enzymes are required to synthesize nucleotides from ribose in cells.

• Water is required to hydrolyze glycosidic bonds during digestion.