INTRODUCTION TO COLLAGEN STRUCTURE
As you might already know, human body is comprised of proteins and collagen is the main protein fiber of connective tissues. Collagen can be found everywhere, e.g.bones, cartilages, tendons, teeth, skin, cornea, lungs, liver, blood vessels and other organs and tissues. Collagen shapes and strengthens tissues and is known to exist in over 20 different types. The type of collagen depends on its structure and can take a variety of shapes and forms, e.g. gel-like in the vitreous body or fiber-like intendons.
Collagen is distinguished from other proteins by its unique composition of amino acids that can come in four types: glycine, proline, hydroxyprolineand hydroxylysine. The structural unit of collagen is tropocollagen comprised of 3 polypeptide chains, each consisting of 1,000 amino acids. Tropocollagen connects in transversal bands making up long fibers called fibril.
How is the standard collagen molecule formed?
- A single molecule is comprised of 3 polypeptide alpha Each chain is a left handed alpha helix with a sequence of three amino acids. Three left handed helices twist into a right handed superhelix.
- The C-terminus polypeptide initiates the helix formation.
- Procollagen is secreted into the extracellular space.
- Enzymes break down the polypeptides into collagen.
A rigid stick-shaped molecule is formed. An important feature of the primary collagen structure is that every third amino acid of the alpha chain is glycine. The repeating sequence can be written down as a formula: (Gly – X – Y)n, where X and Y mean any amino acids. As mentioned before, the triple helices of collagen molecules form fibrils. The fibrils of the same layer connect at their endings. The helices of nearby layers are not situated equally: they lie sideways by a quarter of their length. Such an arrangement of fibrils is an important factor of the mineralization process.
No hydrogen bonds are formed in the helix itself. The triple helix (superhelix) is stabilized by hydrogen bonds between separate helices and the bond of hydroxyproline. The fibers of collagen are connected by covalent bonds between the lysine and hydroxylysine molecules in single or several tropocollagen molecules. The glycine which is third in the sequence is the reason behind the close arrangement of the chains. The lateral group (R = H) of glycine is the smallest group of all amino acids. A single section of the helix has 3 amino acids which means that the glycine groups are situated on the same side of the helix in each chain. The edges of glycine groups are the connection points of the three polypeptide chains.
An enlarged model of the superhelix is shown in Image (a) while Image (b) demonstrates a more detailed arrangement of glycine groups. The lighter connections mean the hydrogen bonds that are important to maintain the molecular structure.
When collagen is synthetized, amino acids, proline and lysine are connected into a single polypeptide chain. The prolyl-hydroxylase is the catalyst of the hydroxylation reaction of proline in the protein molecules. This enzyme contains the ion of Fe2+ in its center and requires ascorbic acid (vitamin C) to maintain the said ion in a redox status. The collagen structure becomes unstable in case of a lack of vitamin C.
To sum up, one should note that collagen is a vital nutrient to both humans and animals as it is characterized by a variety of different body functions, including but not limited to thermal stability, mechanical resilience and ability to interact with other biomolecules in a certain way. Deeper comprehension of the way the triple helix structure and stability are achieved is required to understand the benefits gained from the main molecular structure of collagen and its triple helix.