The Biochemistry of Homopolysaccharides: Structure and Energy Storage

Homopolysaccharides: Structure and Function

Homopolysaccharides are the most abundant type of polysaccharide, consisting of only one type of monosaccharide. Their primary function is to provide support, protection, or energy storage.

Structural Homopolysaccharides

These polysaccharides are integral parts of the cell structure, providing support and protection. They are found in the cell walls of fungi and the shell structure of arthropods, often serving as linking molecules.

Cellulose

• Cellulose is a linear polymer composed of D-glucose molecules linked by $\beta$-D(1→4) bonds.
• Among the glucose molecules of the same chain, it forms intrachain hydrogen bonds.
• The linear chains are arranged in parallel and held together by interchain hydrogen bonds.
• The union of 60 to 70 cellulose chains forms what is called a micelle.
• The association of 20 or 30 micelles forms a microfibril that binds together to form fibers. These fibers form sheets that constitute the plant cell wall.
• It is insoluble in water and is hydrolyzed to glucose only by specific enzymes (cellulases) produced by microorganisms, such as bacteria from the gut flora of animals.

Chitin

• Chitin is a polymer of N-acetylglucosamine.
• It is a key component of the exoskeleton of arthropods and the cell walls of fungi.
• Its structure is similar to cellulose, forming alternating layers. This quality provides organisms with great strength and hardness.

Reserve Homopolysaccharides (Storage)

The molecules that carry the alpha ($\alpha$) anomer configuration are intended to be stored as energy because the $\alpha$ linkage is relatively weak, making it easy to break and hydrolyze.

The hexoses provided in this group of polysaccharides are primarily glucosans (polymers of glucose), as they are fundamental energy molecules.

Starch

Starch is the reserve polysaccharide of plant cells. It is a mixture of two components:

• Amylose: Consists of long, unbranched chains of $\alpha$-D-glucose molecules linked by $\alpha$(1→4) bonds, which adopt a helical coil structure.
• Amylopectin: Highly branched, with a skeleton of $\alpha$-D monomers linked by $\alpha$(1→4) bonds and branching points linked by $\alpha$(1→6) bonds.

Starch (also known as amidon) is found in the plastids of vegetal cells and is abundant in the reserve organs of plants, such as tubers or roots. It is hydrolyzed by specific enzymes called amylases, which are synthesized in most organisms and yield glucose, maltose, and fragments containing $\alpha$(1→6) branch points. The resulting fragments containing ramifications require debranching enzymes capable of hydrolyzing the $\alpha$(1→6) bonds.

Glycogen

Glycogen is the reserve homopolysaccharide of animal cells. It is stored as granules primarily in the liver and skeletal muscle, and it yields a large amount of glucose upon hydrolysis.

Dextrans

Dextrans are polymers of $\alpha$-D glucose. They are characteristic of bacteria. Depending on the species, the ramifications can involve $\alpha$(1→2), $\alpha$(1→3), $\alpha$(1→4), or $\alpha$(1→6) linkages.