Biopolymers – the unofficial building blocks of human body 

Nature produces and gives us an array of useful substances. One of those, highly beneficial substance is biopolymers. These are natural and biological polymers that are produced by enzyme-catalyzed metabolic reactions in living organisms. Some of the prominent biopolymers that have immense application in the medical field are collagen, elastin, chitin, and chitosan. This blog is about these 4 biopolymers and structural proteins.

Collagen

Collagen is the most abundant protein found in vertebrates. These are insoluble fibrils present in the ECM (Extra Cellular Matrix) of cartilages and tendons to support stress during multi-dimensional movement. They are like insoluble scaffolds maintaining the structure of tendon, ligament, bone, and teeth and are net structures in the epithelium.

Collagens have three polypeptide chains (alpha, beta, gamma) wound together to form a helix called tropocollagen. The following is the amino acid sequence for the triple helix with intra and inter crosslinks.

Gly (glycine)- Pro (proline)- Hyp (hydroxyproline) – Gly- X (any amino acid)

These crosslinks are derived from lysine and histidine chains and tend to form at the N, and C ends of collagen molecules. This degree of crosslinking increases along with age. There are three types of collagens based on arrangement (I, II, III). There are about 10 variants and 17 polypeptide chains available in different structures of the human body. The following table shows the arrangement of collagen in different body structures.

Collagen is known for its good tensile strength. This is due to its rigid helical structure. When tension is applied to the collagen, it converts it to compressible force and it gets applied to the helix which is incompressible due to its twisted nature. In this way, it prevents out pulling during tension and maintains stability.

Factors affecting

However, this stability can be disrupted by certain factors like heat, dehydration (reduce hydrophobic interaction), aging, acid mucopolysaccharides (form protein complex), denaturation by collagenase (removal of telopeptides and reduction in immunity) etc. Collagen treated below the denaturation temperature with any proteolytic enzyme (except collagenase), the coiled telopeptides are removed but the helix is left intact. In bovines, it was observed that collagen was digested acidically, in the presence of protease enzymes. Hence there was a motive to improvise the collagen in order to overcome these problems. Reconstituted collagen was developed by using metal ions to increase Ph and ionic strength. Also, the activity of collagenase was drastically reduced.  In another case, the crosslinking was done in severe dehydration and with glutaraldehyde vapors, the ultimate tensile strength improved to 20-60 MPa.    

One of the products obtained from the digestion of collagen is gelatin. These are used in the medical field as a tissue adhesive, sponges, and in spine surgery. The following image shows the application of collagen in the medical field.

Elastin

Elastin is another biopolymer mainly found in yellow connective tissue like skin, aorta, ligaments, and lungs. As the name suggests, these are known for their elasticity where the fibers stretch many times longer than natural. It has its own amino acid sequence, devoid of secondary structure with one-third glycine, over one-third alanine, valine; more proline; less hydroxyproline, and no hydroxylysine. The crosslinks are formed by desmosine and isodesmosine which are also responsible for the yellow color.

Unlike collagen, elastin is stable at higher temperatures and chemicals due to less polar amino acids. Elastase is the enzyme that completely degrades elastin. Elastin is used in anti-aging creams, suture,s and wound healing materials.

Chitin

Chitin is the second most abundant natural polysaccharide. It is similar to cellulose but with the acetamido group replacing hydroxyl. It is available in crabs (25-30%), shrimp (30-40%), and fungi (15-40%) and also in skeletons of arthropods, diatoms, shells of mollusks, and cell wall of fungi, yeast, etc. The shells are crushed, demineralized with HCl, deproteinated with NaOH to get chitin. The fungi are dried and pulverized with NaOH and chitin is extracted using LiCl. This is then dissolved in the solvent and made to extrude in solution through holes using rollers to form fibers. The following are the properties of chitin: –

Properties

• Biocompatible
• Biodegradable
• Non-toxic
• Protein affinity
• Renewable
• Abundance

Chitosan

Chitosan is the N-acetylated derivative of chitin and is obtained by treatment with 50% NaOH. It is a cationic polyamine with low pKa and high charge density at Ph less than 6.5, amenable to chemical modification, forms a gel with polyanions, adheres to negative charges, and chelates with metals. N-Carboxylmethyl Chitosan (NCMC) is water-soluble and is an efficient metal chelator. N- Carboxybutyl chitosan is water-soluble due to the butyl group and is used as a wound dressing and reconstructive tissue. Chitosan is biocompatible, common and is non-toxic. The following are properties of chitosan.

Properties

• Hemostatic
• Bacteriostatic
• Fungistatic
• Spermicidal
• Anti-cancer
• Anti-cholesteremic

The following are the biomedical applications of chitin and chitosan: –

• Wound Dressing- Protect skin from contamination and infection.
• Prevent bacteria, accelerate wound healing and avoid water loss.
• Used as a wound dressing and surgical threads.
• Anticoagulant -Prevent blood clots essential for open-heart surgeries using sulfated chitin.
• Tissue Engineering- chitosan permits cell growth used to seed cells onto the matrix.
• Orthopedic- chitin can replace collagen in the bone composite as temporary ligaments.
• Hydrogel- absorb large amounts of water.
• Chitosan hydrogel is used in drug delivery.
• Tablets and Microcapsules.

HAPPY READING!!!

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