Metabolism of Collagen

The body can repair broken bones by breaking down old collagen and generating new.

There are at least 19 different human collagens, many of which form tough, fibrous tissues that serve essential structural functions: collagens hold ligaments and tendons together, enhance the cartilage of the knee and even serve as the template for bone mineralization. Without collagen, our bodies would fall apart. The biological and chemical processes that create collagen and break it down are collectively called collagen metabolism.

The Collagen Family

Collagen is the generic name for a family of proteins whose members can be found throughout the body. Type I collagen is the primary component of ligaments and tendons, while Type II is a component of cartilage. Type IV collagen forms the basal laminae, the thin layer of tissue that underlies and supports the skin. Some collagen types seem to play helper roles for other types, and a few collagen types don't form typical fibers. The structures and functions have not been established for all types of collagen, but they share certain structural similarities, such as a helical structure, and they are subject to similar metabolic processes.

Structure

Collagen is made inside cells called fibroblasts, which are found in tendons, ligaments and other connective tissues. When first produced, the proteins are tagged with extra pieces that help them associate into groups of three and make the triple helix that is their functional shape. After assembly into the triple helix, the new structure, called procollagen, is ejected from the fibroblast through the Golgi apparatus. Once outside the cell, special proteins called enzymes cut off the extra pieces, leaving a collagen triple helix molecule that is essentially rod-shaped. This unit is called tropocollagen, and it is the smallest complete collagen molecule.

Fibers

Once the concentration of tropocollagen is sufficiently high, the molecules will begin to aggregate in a very specific way. The tropocollagen rods have "sticky ends" that link together to make long strands, and the rods also stack laterally like bundles of wood. The final result is called a collagen fiber. These fibers make up the collagenous tissues of our bodies.

Regeneration

Like all tissues of the body, collageneous tissues are subject to wear and tear. Unlike some proteins, such as elastin, collagen can be synthesized throughout the body's lifetime. This is why bones (which are built on a collagen template) can heal naturally even after adulthood. A number of disease states (such as rheumatoid arthritis, cancer, lupus and bacterial infection) can interfere with the normal balance of collagen generation and degradation.

Breakdown

Collagen degradation is less well-understood than collagen formation. It is, however, known that collagen is relatively resistant to being broken down. Its tightly packed triple helical structure and fibrous nature offers few weak points for protein-snipping enzymes called collagenases to exploit. It is much easier for the enzymes to cut unraveled, or denatured, collagen, such as that found in injured tissue. Whereas healthy collagen initially self-assembles spontaneously as described above, breakdown of collagen must in many tissues be assisted by nearby cells that partially digest the protein.

References

• "Molecular Cell Biology"; Harvey F. Lodish, Arnold Berk; 2008
• "Extracellular Matrix, Volume 1"; Wayne D. Comper; 1996
• "Color Atlas of Cytology, Histology, and Microscopic Anatomy; Wolfgang Kühnel; 2003
• "Cell Biology of Extracellular Matrix"; Elizabeth D. Hay; 1991
• "Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects"; Peter Royce, Beat Steinmann; 2002

Resources

• "Collagen: Primer"; Jürgen Brinckmann, Holger Notbohm, P. K. Müller; 2005.