## 2 Understanding the Key Extracellular Matrix Proteins
### Collagen  
Collagen comprises about one third of the total protein content in the human body [1]. Collagen is a large family of molecules with more than 28 different types of collagen identified to date [32]. All collagens have the same fundamental characteristic; a protein comprised of three polypeptide chains that has at least one repeating amino acid sequence [32]. The most abundant fibrillar collagen, type I, is composed of well organised fibrils that provide tensile strength and resistance to deformation and stress to the ECM and is found in various tissues including the skin, the vasculature, the heart tissue and bones [33]. Type I collagen is formed by two types of \(\alpha\) helices; specifically, \([\alpha 1(I)]_{2}\alpha 2(I)\), thus forming a triple helix consisting of two \(\alpha 1(I)\) and one \(\alpha 2(I)\) helix [34]. Other types of collagen include; type II found mainly in cartilage; type III found in the skin, muscle and blood vessels and type IV found in all basement membranes [35].  
All collagens are characterised by the repeat sequence of: GLY-X-Y, where GLY is glycine, X is often proline and Y is often hydroxyproline [36; 37]. The precursor molecule of collagen is _procollagen_, a glycoprotein that consists of three extended polypeptide chains and is believed to have a stiff structure with flexible short end regions [37]. The synthesis of collagen initiates in the nucleus where the DNA is transcribed into mRNA that is then translated in the rough endoplasmic reticulum (RER), synthesising collagen \(\alpha\)_-chains_ (Figure 3). These molecules are transported to the Golgi apparatus where post-translational modifications generate the self-assembled triple helical _procollagen_ structure that is then secreted to the extracellular space [37]. During this exocytosis, procollagen is converted into _tropocollagen_, by the proteolytic cleavage of the C- and N-terminus propeptides of the procollagen molecule [38]. Collagen fibres are assembled by a process called fibrillogenesis [32] through a variety of cross-linking mechanisms. Three key cross-liking mechanisms occur in vivo: lysyl oxidase crosslinking, sugar mediated and transglutaminase crosslinking [38]. Lysyl Oxidase (LOX), a copper-dependent enzyme, specifically crosslinks collagen and elastin [39] creating intramolecular bonds by catalysing the deamination of lysine to allysine and hydroxylysine to hydroxyalllysine (Figure 3). The allysine aldehyde product subsequently undergoes spontaneous aldol condensation resulting in crosslinks between the fibrils. Alternatively, Schiff base crosslinks are formed as the aldehyde groups react with the amino groups of lysine and hydroxylysine residues on adjacent molecules [39]. The hydroxylysine pathway results in two cross linked molecules: ketoimine and aldimine, which connect individual collagen triple helices [38]. Sugar-mediated crosslinking is characterised by a Maillard reaction, also known as a non-enzymatic browning reaction, which increase the turnover of collagen linearly. In diabetic patients studies have shown that there is an acceleration of the browning reaction [40]. Finally, transglutaminase (TGase), that are thiol- and calcium dependent multifunctional enzymes [31; 41], also induce collagen crosslinking. However, not a lot is currently known about their activity. To date, we know that Tgase catalyses the formation of covalent bonds between the \(\gamma\)-carboxamide group of the peptide-bound glutamine residue and the \(\varepsilon\)-amino group of lysine of the collagen molecules [31].