**3. Collagen**

Collagen is the most abundant structural protein found in the vertebrate body. Collagen is a rigid, inextensible, fibrous protein that is the principal component of connective tissue in animals, including tendons, cartilage, bones, teeth, skin and blood vessels. As a structural protein it is mainly used to give strength to structures in the body, however, it has different functions depending on the location of the body [17]. One-third of the total protein content in the mammalian body is collagen and accounts for three-quarters of the dry weight of the skin.

#### **3.1 Collagen structure**

The triple-helix of collagen consists of three distinct alpha chains coiled around each other and this is termed as tropocollagen. The tropocollagen units are arranged

approximately 1000 amino acid residues. The accurate folding of these chains requires a glycine residue to be present in every third position of the polypeptide chain [1]. One-third of the amino acids in collagen in glycine and it always occupies the first position of the triplet. This is due to glycine being a small and an uncharged amino acid near the axis of the collagen triple helix. Glycine is a very crucial part of collagen molecule inherent characteristic as substitution of a single glycine for another amino acid disrupts the triple helix and results in skeletal deformities such

Imine acids make up approximately 25% of the residues in the collagen triplehelix. Imine acids – proline and hydroxyproline are typically found around the outside of the trip helix and the pentagon structure of these two amino acids includes the amine nitrogen and the α-carbon of the backbone chain. These limit the possible rotation in the amino acid (**Figure 1**) and hence forcing each collagen chain to form a left-handed helix. The high content of these imine acids makes the α-helix and β-sheet arrangements (generally found in proteins) unstable. Collagen triplehelix is held together by hydrogen bonding between chains. The NH group in

as ontogenesis imperfect.

**Figure 2.**

Preservation

*Collagen: From Waste to Gold*

*DOI: http://dx.doi.org/10.5772/intechopen.94266*

Soaking

De-hairing

Fleshing

Splitting

De-liming

**Table 1.**

**207**

*Process flow of transformation of hides into leather [12].*

**Tanning step Chemicals Wastes generated**

Salt Contaminated salt, raw hide trimmings

Water, surfactants, and enzymes Salted and contaminated wastewater

Water, sodium sulphide, and enzymes Hair, alkaline water

Skin/hide Limed hide

*Chemicals used at each stage of hide to leather conversion and wastes generated [13].*

Water, ammonium sulphate and weak acids Acidic wastewater

Water, mechanical processes Flashings, alkaline water

as fibres or sheets. A tropocollagen unit is about 285 kDa, 3000 Å in length and 15 Å in diameter. The triple helix is composed of repeating units of (Gly-X-Y)N amino acids, where X and Y are any amino acids, however, often X is proline and Y is hydroxyproline. The individual polypeptide chains of collagen each contain
