The word “collagen� comes from the Greek word “kólla'', meaning ''glue�. It is known as glue because it is the component that produces a glue-like substance when tissues are boiled. It is also considered as the biological glue that holds cells together.
Collagen comprises 25% of the total protein content in the human body and accounts for three-quarters of the dry weight of skin in our bodies. It is the most prevalent component of the skin’s extracellular matrix which is made of an insoluble network, consisting of collagen, polysaccharides, fibrous proteins and adhesive proteins that are secreted by animal cells to provide structural support in tissues.
Most abundant protein
Holding our bodies together, collagen's adhesive qualities are only matched by its ability to provide structure and strength. Found throughout our bodies, it is responsible for maintaining healthy joints, providing structure to bones, forming cartilage, connecting skeletal muscles to bones, helping blood to clot and maintaining skin elasticity. It also provides structural support for cell growth, and is responsible for the mechanical resilience of connective tissues. Hence, it is the single most abundant protein in the animal kingdom.
Simple triple helix
Collagen is composed of three chains, wound together in a rope-like structure to form a tightly packed triple-helix. Each chain is composed of approximately 1000 amino acids of which every third residue is a glycine. Glycine is the only amino acid that takes up the middle of the triple helix because it’s lack of side-chain means that it is the only one that can fit.
Overview of the collagen triple helix. First high-resolution crystal structure of a collagen triple helix from PDB id 1cag. Collagen structure displayed as molecular surface representation, with chains displayed in distinct colours. The water molecules shown as red dots form a cylinder of hydration around the triple helix, with an extensive hydrogen bonding network between water molecules and protein chain. he image was created using .
Collagen has an unusual abundance of three amino acids: glycine, proline and hydroxyproline, a derivative of proline. The most common motifs in the amino acid sequence of collagen are glycine-proline-X and glycine-X-hydroxyproline, where X is any amino acid other than glycine, proline or hydroxyproline.This simple sequence of amino acids is repeated and serves as a characteristic signature to drive spontaneous self-assembly of three chains into a distinctive triple-helical structure of collagen shown below.
Transmission electron microscope image of a thin section cut of mammalian lung tissue showing Collagen Type I. Image by
Video showing self assembly of collagen fibres. Growth of collagen fibrils exhibiting the typical banding pattern can be seen. Video used from .
The interaction between the three chains is stabilized via weak, interchain hydrogen bonds making the molecule fairly resistant to attack by other molecules. What is fascinating is that there are enough of these weak hydrogen bonds to make the fibre strong enough to keep the connective tissues in the body together.
Gelatin
For thousands of years, collagen was used to create glue, obtained from boiling connective tissue rich in collagen like bone broth. This glue is nothing but cooked collagen where collagen chains denature and unfold, resulting in an entangled mass. Heat breaks down the bonds holding the chains together, and on cooling, the protein molecules begin to stick together to regain some of their original helical structure and also trap all the surrounding water making a jelly like texture of gelatin, a common cooking ingredient.
Insoluble fibre
According to the laws of chemistry, "like dissolves like". Water being polar or ‘water loving�, and collagen being completely non-polar or ‘water hating�, means that collagen does not dissolve in water or aqueous solutions. You can imagine that without collagen our bodies would dissolve in rain or even while taking a shower. It is all thanks to the insoluble nature of the non-polar amino acids: glycine, proline and hydroxyproline in collagen, which prevent us from dissolving!
Types of collagen
In some collagens the long triple-helical structures pack together to form long fibrils or ’protein ropes�, found in skin, hair and nails, while others form striated horizontal sheets to provide rigidity to proteins and complement their biological activity.
In nature there are about 30 different types of collagen that have been identified so far. The five most common types are:
Type I: found in connective tissues of the skin, tendon, ligaments, bone
Type II: found in cartilage
Type III: found in reticulate (main component of reticular fibres), commonly found alongside type I
Type IV: makes sheets found between layers of cells in blood vessels, muscles and eyes
Type V: found in cell surfaces, hair, and placenta
Over 90% of the collagen in the human body is type I collagen. It is the strongest type of collagen because it is made of fibres densely packed together. These fibres are capable of being stretched without breaking and, gram-for-gram, they are stronger than steel. Type II collagen isn’t quite as densely packed as type I, but it’s perfect for cushioning joints as cartilage. Muscles, organs, and arteries rely on type III collagen and type IV help with filtering, as a two-dimensional sheet found in our skin.
Fountain of youth
No matter what type of collagen, age is the ultimate enemy for maintaining adequate levels of collagen in the body, especially our skin. It is the most prevalent second layer of our skin (the dermis) and is key to our skin’s appearance. With adequate collagen production the skin looks smooth and firm, giving it good strength, hydration, texture and elasticity.
However, we can lose collagen in the body due to exposure to ultraviolet light, tobacco, excessive intake of sugar, and aging. This loss of collagen plays a big role in the skin appearance making it less elastic with a thinner epidermis, giving rise to wrinkles and sagging, and also making the skin vulnerable and easily damaged.
Growing evidence has shown that supplemental collagen can be favourable to our skin appearance. It is also proclaimed as a way to improve hair and nail health as well as reducing the tell-tale signs of aging, like wrinkles. In addition, it can ease arthritis symptoms, promote wound healing, and fend off muscle wasting.
While studies around the exact benefits are still emerging, there is one thing that can be agreed upon: a little extra collagen will not hurt and there is nothing wrong with boosting your collagen intake, however you see fit.
Deepti Gupta
About the artwork
“I took a sudden interest in the structures of the proteins, specifically the form. Continuing on the structural theme I began to concentrate on bones which compose an animal body. Bones consist of collagen type I which is a fibrillar collagen. To express this protein I combined two photos, one I captured of human ribs and the other is the compound of type I collagen. I changed these images by minimising them to a black and white filter. This was then used for my screenprint on the medium paper.�
Anna Miska Alvarez, , Cambridge, UK
View the artwork in the .
Structures mentioned in this article
Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution. PDB ID 1cag
Structural analyses of von Willebrand factor C domains of collagen 2A and CCN3 reveal an alternative mode of binding to bone morphogenetic protein-2. PDB ID 5nir
Sources
