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Building blocks for cartilage, tendons, ligaments & skin

Cartilage, tendon and ligament tissues have strong common traits. They are part of what is called the “extracellular matrix” or connective tissue. The main building blocks for these materially different tissues are the same: collagen fibres, elastin fibres and proteoglycans.

Collagen

Collagen is a protein only found in animals and humans. Plants do not hold any. Collagen is the most common protein in the human body and is constituted of amino acids assembled in a precise sequence. Collagen fibres are very resistant to pull and hardly stretch. They provide resistance and strength to tissues. For this reason, they are often termed as « structure proteins ». There are different sub-groups of collagens: types 1 and 3 are mainly found in skin, tendons, ligaments, bones and teeth, whereas collagen type 2 is mainly found in articular cartilage (hyaline and elastic cartilage).

Procollagen is made up of three long chains of proteins which hold on to one another through disulfide and hydrogen bonds.  Together these three long protein chains form a triple helix collagen, or procollagen (see image).

Illustration: Collagen triple-helix, or procollagen

Many collagen triple-helix form a fibril and many fibrils form a collagen fibre. We can picture a collagen fibre as a huge steel cable, like the extremely resistant cables used by cable cars.

Amino acids

The smallest building blocks of collagen and elastin fibres are amino acids. All in all, there are twenty different amino acids which are found in all proteins of the human body. Of these twenty amino acids, eight are essential, meaning that the human body does not create them (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, Valine). When essential amino acids are not sufficiently available, the other amino acids cannot be used for the creation of new proteins and will be transformed into sugar and fat. This is why the human body needs a sufficient and regular intake of essential amino acids from food.

In collagen, the following amino acids are particularly present: glycine, proline, hydroxyproline and hydroxylysine. Threonine, an essential amino acid, is a forerunner for glycine synthesis. Methionine is an essential amino acid rich in sulphur and lysine is essential to collagen and elastin synthesis in connective tissues.

Proteoglycans

Proteoglycans are the third important element in connective tissues, particularly in cartilage.

Proteoglycans are negatively charged and can bind water molecules. They confer the typical gelatinous consistency to connective tissue. Proteoglycans are situated between collagen and elastin fibres and enable the movement of molecules through the matrix, including feeding the tissues around them.

We can picture the shape of proteoglycans like a fir tree: the trunk is made of hyaluronic acid, branches would be chondroitin, glucosamine, dermatan and keratane sulphates (see illustration).

Elements containing sulphur, in the sulphate form, seem to play a role in maintaining the shape of collagen as well as for the normal function of proteoglycans, since they are negatively charged and enable the retention of water and moisture in cartilage and connective tissue, due to disulfide bonds.

Illustration: Proteoglycan

With age, synthesis of collagen, elastin and proteoglycans (particularly chondroitine sulphates) decrease naturally. Productive cells (chondroblasts and fibroblast) become less productive with age.

 

References:

Prof. Dr. A. Pischinger, Matrix und Matrixregulation, 1995

Oesser et al, Stimulation of type II collagen biosynthesis and secretion in bovine chondrocytes cultured with degraded collagen, Ausgabe: Cell & Tissue Research 311, 2003

Carrino DA, Sorrell JM, Caplan AI., Age-related changes in the proteoglycans of human skin, Archives of Biochemistry and Biophysics, 2000.

Hoffer LJ, Kaplan LN, Sulfate could mediate the therapeutic effect of glucosamine sulfate, Lady Davis Institute for Medical Research and Division of Rheumatology, Canada, 2001.