Carbohydrates II - Disaccharides and Polysaccharides

Learning Objectives

• Understand the formation of disaccharides and their biochemical significance.

• Identify the properties and classifications of reducing and non-reducing sugars.

• Recognize common examples of polysaccharides, their functions, and importance in biological systems.

Disaccharides

Definition: Disaccharides are carbohydrates formed by the linkage of two monosaccharides through a condensation reaction which results in the formation of a glycosidic bond. This bond can vary in type depending on the specific monosaccharides involved.

Glycoside: A glycoside is a type of compound produced as a result of a glycosidic bond, where the anomeric carbon is bonded to another compound (which may be either a sugar or a non-sugar).

Naming: Disaccharides are typically named based on the composition and arrangement of their monosaccharides along with the type of glycosidic connection (e.g., maltose, sucrose, lactose).

Example:

• Maltose:

  • Formed from the condensation of two glucose molecules, resulting in a glycosidic link characterized as ext{α-D-glucopyranosyl-(1→4)-D-glucopyranose}.

  • It plays a role in energy release through the hydrolysis of starch during digestion.

Structure of Disaccharides

Disaccharides consist of two monosaccharides that unite through an elimination of a water molecule during the reaction (condensation), creating an acetal or hemiacetal in a cyclic form. During hydrolysis, these hemiacetals can revert back to free monosaccharides when water is added.

Reducing and Non-Reducing Disaccharides

Reducing Disaccharides:

• Defined by the presence of a free anomeric carbon, allowing them to act as reducing agents. The reducing end can participate in redox reactions, which is significant for biological processes.

• Examples:

  • Lactose: Found in milk, composed of glucose and galactose, can be hydrolyzed by the enzyme lactase.

  • Maltose: Involved in the fermentation process and is also a product of starch digestion.

Non-Reducing Disaccharides:

• These do not have a free anomeric carbon; both anomeric carbons of the constituent sugars are involved in the glycosidic bond, preventing them from acting as reducing sugars.

• Examples:

  • Sucrose: Composed of glucose and fructose, commonly found in table sugar; confirmed as a non-reducing sugar via biochemical tests.

Raffinose (Trisaccharide)

Raffinose consists of three monosaccharides: glucose, fructose, and galactose. It is not classified as a reducing sugar because both anomeric carbons are engaged in glycosidic bonds, preventing any reducing action. This trisaccharide is significant in nutrition as it can cause gas and bloating when metabolized due to the fermentation by gut bacteria. The enzyme α-galactosidase can hydrolyze raffinose, making it more digestible and releasing its constituent monosaccharides.

Polysaccharides / Glycans

Definition: Polysaccharides are long chains of monosaccharide units linked together by glycosidic bonds. These natural carbohydrates exist as either homopolymers made up of the same type of monosaccharide or heteropolymers containing different types.

Types:

• Homopolymers: Composed entirely of a single type of monosaccharide.

• Heteropolymers: Composed of multiple types of monosaccharides.

• They can exhibit linear or branched structures, affecting their physical properties and biological function.

Examples: Starch, glycogen, cellulose, chitin, agar, with each playing crucial roles in energy storage, structural integrity, and interaction with other biomolecules.

Starch

Function: Starch serves primarily as an energy storage molecule in plants and is crucial for the human diet as a primary carbohydrate source.

• Components: Starch is primarily composed of two glucose polymer forms:

  • Amylose: Consists of linear chains of glucose units linked via ext{α(1→4)} glycosidic bonds, forming helical structures.

  • Amylopectin: A branched polymer containing both ext{α(1→4)} and ext{α(1→6)} linkages, allowing for more compact storage and easier enzymatic accessibility during digestion.

Glycogen

Function: Glycogen is the principal energy reserve in animals. It has a structure similar to amylopectin but with a significantly higher frequency of branching, which enhances its solubility and allows for rapid release of glucose when energy is needed.

Cellulose

Function: Cellulose forms the primary structural component of plant cell walls and is crucial for maintaining cell integrity.

• It is a homopolymer of glucose linked via ext{β(1→4)} glycosidic bonds, creating linear chains that pack tightly together to form fibers, which humans cannot digest.

• Functions as dietary fiber, promoting healthy digestion and preventing constipation.

Chitin

Function: Chitin is the major structural component of exoskeletons found in arthropods (such as insects and crustaceans) and in the cell walls of fungi. It is a homopolymer of N-acetylglucosamine molecules linked by ext{β(1→4)} bonds, providing strength and flexibility.

Agar

Agar is a branched heteropolysaccharide consisting of two components: agarose and agaropectin. It serves as a medium in microbiology laboratories for the culturing of bacteria and other microorganisms due to its gelling properties and resistance to microbial degradation.

Glycosaminoglycans

Function: Glycosaminoglycans play a vital role in forming meshworks with fibrous proteins, providing structural integrity and flexibility in connective tissues. They are composed of linear polysaccharides with repeating disaccharide units, which typically include either N-acetylglucosamine or N-acetylgalactosamine, contributing to the physical properties of cartilage and other connective tissues.

Glycolipids

Glycolipids are defined as oligosaccharides covalently bound to lipids within cell membranes. They are particularly important in cellular recognition processes and signaling. In vertebrates, they contribute to blood group specificity, while in bacteria, they form essential components of the cell wall, influencing the structural integrity and protective capabilities of these cells.

Conclusion

A comprehensive understanding of carbohydrates, specifically disaccharides and polysaccharides, is fundamental to grasping their multifaceted roles in biological systems, including energy storage, structural functions, and interactions at the molecular level. Recognition of the differences between reducing and non-reducing sugars is crucial in biochemistry, food science, and nutrition-related fields.