Collagen – An Overview
The word collagen is derived from the Greek word meaning glue.
The protein collagen is the main substance of connective tissue and is present in all multi-cellular organisms. It is found in a many different tissues and organs like bones, tendons, cartilage, skin, blood vessels, teeth, cornea, inter-vertebral disks, vitreous bodies, placenta, etc.
The main function of collagen is mechanical reinforcement of connective tissues of vertebrates. It enwraps the organs and holds specialized cells together in discrete units. Thus, it prevents organs and tissues from tearing or loosing their functional shape when exposed to sudden or rough movements. In addition to this structural role in mature tissues, collagen plays a regulating role in developing tissues, influencing the proliferation and differentiation of unspecialized cells.
Collagen is a highly conserved protein preserving the amino acid sequence and typical triple helix structure across species lines. At present, over fifteen different types of collagens have been identified. All collagens contain the unique triple helix structure; however, the length and nature of the helix as well as the size of non-helical portions of the molecule vary from type to type.
The predominant collagen of the skin, tendon and bone is type I collagen; type II collagen is essentially unique to cartilage; type III collagen occurs in adult skin (5-10%), blood vessels and internal organs; type V is found in bone, skin, tendons, ligaments, and cornea; types IV and VIII are network-forming collagens.
Aging and Maturation of Collagen
The aging and maturation of collagen in the body is controlled by two main processes: cross linking and degradation.
Collagen Cross-Linking Process
There are two distinct mechanisms of collagen cross-linking in the body. One is an enzymatic-mediated process and the second is a non-enzymatic reaction called glycation which is mediated by a reducing sugar, glucose.
Non-enzymatic glycation is the process by which sugars in the body, mainly glucose, act as reducing agent to cross-link native collagen, leading to stiffness of the tissues. Over time, the initial products of the glycation reaction slowly undergo further re-arrangements, resulting in the irreversible formation of a family of cross-linked structures. Watch the video to learn more.
This glycation induced cross-linking is considered to be the major mechanism in extending biological half life of the native collagen. Indeed in studies examining the effect of glycation on collagen scaffolds and tissue, it has been shown to decrease the rate of degradation of the scaffold matrix. Additionally it has been demonstrated that changes in glycation levels are associated with alterations in mechanical strength, solubility, ligand binding and conformation.
Collagen Degradation Process
The native collagen is degraded in two steps. First the triple helical molecule is specifically split into two parts by specific enzymes called collagenases. Second, the a-chains of the split fragments unfold and become susceptible to digestion by gelatinases and non-specific proteinases. This activity initiates a breakdown of the collagen molecules, leading ultimately to harmless absorption by the body.