e., figure 1 the water uptake of tissues and it is well-known that the degradation of the collagen moiety of ECM with enzymes such as collagenase results in a strongly increased water content.22 Therefore, an appropriate ratio between collagen and glycosaminoglycans (GAGs) is crucial in order to provide the specific (bio)mechanical properties of the ECM. It should be noted, that native tripelhelical collagen is not digestable by the majority of proteolytic enzymes with the exception of collagenase, whereas denaturated collagen (gelatin, which can be generated by heating of collagen) is highly susceptible to most proteases.23 Finally, one major difference between cartilage and bone is the calcium content: mammalian bones contain huge amounts of calcium phosphate crystals in a protein matrix.

The major form of the calcium phosphate is hydroxyapatite. Although type I collagen is uncharged, it plays a very important role for calcification and, thus, osteogenesis. These aspects have been recently reviewed.24 Glycosaminoglycans (GAGs) GAGs are natural, very complex, unbranched, polydisperse polysaccharides composed of disaccharide units normally of D-glucuronic acid (GlcUA) or L-iduronic acid (IdoUA) [only keratan sulfate (KS) has a galactose instead of an uronic acid moiety] linked to a D-glucosamine (GlcNAc) or D-galactosamine (GalNAc) residue (Fig. 3). Figure 3. Structures of the disaccharide repeating units of the most important GAGs. In addition to the illustrated modifications, minor variations (e.g., differences of the sulfation patterns) are also common.

HA is the only GAG that is completely non-sulfated containing an unmodified N-acetyl-D-glucosamine (GlcNAc)-GlcUA repeating unit, while the other polysaccharides are generally modified through post-biosynthetic modifications, such as the addition of O-sulfo groups, C5-epimerization to form IdoUA residues, and de-N-acetylation to produce GlcN-sulfo residues.25,26 These modifications often play a key role in a wide variety of biological and pharmacological processes but a more detailed discussion of these aspects is beyond the scope of this review. Although having a rather simple structure, HA is widely used in medicine and cosmetics.27 HA is the by far largest GAG: For instance, the molecular weight of HA within articular cartilage and in healthy synovial fluid is about 107 Dalton.

28 At physiological pH, the majority of carboxyl groups is deprotonated and, therefore, HA (pK ��3.21) negatively charged. To indicate all charge states, hyaluronan is the most commonly used term instead of hyaluronic acid and hyaluronate. We will briefly deal with KS because this GAG is less abundant in the ECM than chondroitin sulfate (CS) or dermatan sulfate (DS). The repeating disaccharide unit of keratan sulfate (KS) [Gal (��1��4) GlcNAc (��1��3)]n Drug_discovery contains a galactose residue instead of uronic acid and the glycosidic bonds are reversed in comparison to HA and CS/DS (Fig. 3).