Carbohydrates in Biochemistry

General Functions of Carbohydrates

Carbohydrates are essential biomolecules that serve as energy sources, structural components, and participate in cell recognition and signaling processes.

• Energy Source: Carbohydrates such as glucose are primary fuels for cellular metabolism.

• Structural Role: Polysaccharides like cellulose and chitin provide structural integrity in plants and some animals.

• Cell Recognition: Glycoproteins and glycolipids on cell surfaces are involved in cell-cell recognition and signaling.

Carbohydrate Stereochemistry

Stereocenters and Isomerism

Carbohydrates often contain multiple stereocenters, leading to various isomeric forms.

• Stereocenter: A carbon atom bonded to four different groups, leading to chirality.

• Enantiomers: Non-superimposable mirror images (e.g., D- and L-glucose).

• Epimers: Diastereomers differing at only one chiral center (e.g., glucose and galactose).

• Anomers: Isomers differing at the new chiral center formed on ring closure (α and β forms).

Types of Isomers

• Constitutional Isomers: Same molecular formula, different connectivity.

• Conformational Isomers: Same connectivity, differ by rotation about single bonds.

• Stereo-isomers: Same connectivity, differ in spatial arrangement.

• Enantiomers, Diastereomers, Epimers, Anomers: See above for definitions.

Monosaccharides: Structure and Classification

Aldoses and Ketoses

Monosaccharides are classified based on the presence of an aldehyde (aldose) or ketone (ketose) group.

• Aldoses: Monosaccharides with an aldehyde group (e.g., glucose, galactose).

• Ketoses: Monosaccharides with a ketone group (e.g., fructose).

Common Monosaccharides and Their Structures

• Glyceraldehyde (aldotriose)

• Dihydroxyacetone (ketotriose)

• Erythrose (aldotetrose)

• Ribose (aldopentose)

• Ribulose (ketopentose)

• Xylose (aldopentose)

• Glucose (aldohexose)

• Fructose (ketohexose)

• Mannose (aldohexose)

• Galactose (aldohexose)

Monosaccharides can be represented in Fischer (linear) or Haworth (cyclic) projections.

Formation of Hemiacetals and Hemiketals

Monosaccharides cyclize to form hemiacetals (from aldoses) or hemiketals (from ketoses), resulting in ring structures.

• Hemiacetal Formation: Aldehyde group reacts with an alcohol group within the same molecule.

• Hemiketal Formation: Ketone group reacts with an alcohol group within the same molecule.

Glycosidic Linkages and Oligosaccharides

Formation of Glycosidic Bonds

Monosaccharides are linked via glycosidic bonds to form disaccharides, oligosaccharides, and polysaccharides.

• O-glycosidic bond: Formed between the anomeric carbon of one sugar and a hydroxyl group of another.

• N-glycosidic bond: Formed between the anomeric carbon and an amine group (e.g., in nucleotides).

Common Disaccharides

• Sucrose: Glucose + Fructose (α1→β2 linkage)

• Lactose: Galactose + Glucose (β1→4 linkage)

• Maltose: Glucose + Glucose (α1→4 linkage)

• Cellobiose: Glucose + Glucose (β1→4 linkage)

Reducing and Non-Reducing Sugars

The ability of a sugar to act as a reducing agent depends on the presence of a free anomeric carbon.

• Reducing Sugars: Have a free anomeric carbon (e.g., glucose, maltose, lactose).

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

Polysaccharides: Structure and Function

Types of Polysaccharides

• Starch: Storage polysaccharide in plants; composed of amylose (unbranched, α1→4) and amylopectin (branched, α1→6).

• Glycogen: Storage polysaccharide in animals; highly branched (α1→4 and α1→6 linkages).

• Cellulose: Structural polysaccharide in plants; β1→4 linked glucose units.

• Peptidoglycan: Major carbohydrate (polysaccharide) constituent of bacterial cell walls.

Table: Comparison of Major Polysaccharides

Enzymes Involved in Carbohydrate Metabolism

• Amylase: Enzyme that digests starch (α-amylases, exoglycosidases, debranching enzymes).

• Cellulase: Enzyme that digests cellulose (not produced by humans).

Mutarotation and Ring Forms

Monosaccharides can interconvert between α and β anomers in solution, a process called mutarotation.

• Mutarotation: Conversion between α and β forms via the open-chain structure.

Axial and Equatorial Positions in Pyranose Rings

• Axial: Groups above or below the plane of the ring; large groups here are destabilizing.

• Equatorial: Groups in the plane of the ring; large groups here are stabilizing.

Glycosylation of Proteins

Proteins can be glycosylated with oligosaccharides via N-linkages (to Asn residues) or O-linkages (to Ser or Thr residues), affecting protein function and stability.

Vitamin C (Ascorbic Acid)

Vitamin C is an essential dietary antioxidant and a cofactor for collagen synthesis.

• Antioxidant: Protects cells from oxidative damage.

• Cofactor: Required for the hydroxylation of proline and lysine in collagen.

Key Terms and Definitions

• Tautomers: Isomers differing by the position of hydrogen and double bond (e.g., keto-enol forms).

• Mutarotation: Interconversion of anomers in solution.

• Isomers: Molecules with the same chemical formula but different structures.

Summary Table: Types of Isomers in Carbohydrates

Important Equations

• General formula for carbohydrates:

• Mutarotation equilibrium:

Applications and Examples

• Dietary Importance: Starch and glycogen are major energy sources in human diet.

• Medical Relevance: Defects in carbohydrate metabolism can lead to diseases such as diabetes and lactose intolerance.

• Biotechnological Use: Enzymes like amylase are used in food processing and biotechnology.