**3.5 Collagen types**

**3.3 Collagen fibres**

*Biotechnological Applications of Biomass*

**3.4 Collagen maturation**

**Figure 5.**

**210**

*permission).*

The assembly of collagen fibrils into parallel bundles forms collagen fibres that have high strength and flexibility. When tropocollagen is assembled into collagen, it forms fibrous or sheet-like staggered structures. These fibrous structures have striations every 680 Å consisting of a dense-packed region where fibres overlap, and a loose-packed region is formed (**Figure 5**). In one single row, tropocollagen units are separated by 400 Å gaps, and these gaps are found in the loose-packed region. If the tropocollagen rows are aligned next to each other, each adjacent row is

Hydrophobic and charged amino acid residues along the length of tropocollagen cause the staggered arrangement of tropocollagen. Tropocollagen units are aligned where the sum of the hydrophobic and charged region interaction between two

Inter-and intra-molecular covalent cross-links are formed between and within tropocollagen (collagen triple-helix) units giving strength to collagen fibres. Intramolecular cross-links form between adjacent lysine groups and within individual triple-helix units and intermolecular cross-links occur between two triple-helix

hydroxylysine sidechains to an aldehyde that then undergoes a condensation reaction forming an adol cross-link with other converted lysine sidechains. In each tropocollagen unit, four groups can contribute in the intermolecular cross-linking;

hydroxylysines in the helical region. A hydroxyl-pyridinium cross-link is formed between one lysine and two hydroxylysine between residues near the amino-acid

*Collagen fibre showing the striations where tropocollagen is densely packed (light sections) [23] (used with*

<sup>+</sup> group on the lysine and

offset by 680 Å, forming a structure that repeats every five rows.

units comprising of two hydroxylysine groups and a lysine group.

lysines near the amino and carboxyl ends in the non-helical regions and

The enzyme Lysyl oxidase converts the NH3

units is strongest, hence the 680 Å staggering between units.

Collagen has a wide range of structural roles in mammalian and aquatic tissue. It is the major constituent of skin, bone, tendon, cartilage, blood vessels and teeth. Collagen is found in almost every organ of the body, starting from skin to the cornea of the eye. To serve functions in such diverse tissues, there are different types of collagen that differ in how they interact with each other and with other tissue.

There are more than 28 types of collagen identified. Collagen types I, II, III are the most abundant and most investigated for various applications. However, over 90% of the collagen found in the body is type I. The variations are due to the differences in the assembly of basic polypeptide chains, different lengths of the helix, and differences in the terminations of the helical domains [24].

Each collagen molecule is composed of three different polypeptide chains (α1, α2, and α3). Each chain is identified by its amino acid composition (**Table 2**). Collagen type I, for example, is identified for its constitution of α1 (I) and/or α2 (I) chains. The most commonly occurring variant of type I collagen consists of two α1 (I) and one α2 (I) chain. The alpha symbol is used to indicate a single chain component seen after collagen denaturation and the letter β, γ, and δ have been used to indicate covalently linked dimers, trimers or tetramers of the alpha chain.

The most common types of collagen are:



**Table 2.** *Collagen and its features [25].*

### **3.6 Collagen sources**

As collagen is one of the most abundant proteins on earth, it can be extracted from various sources. Collagen can be extracted from almost every living animal, including alligators and kangaroos. However, common sources of collagen for the food industry and tissue engineering applications include bovine skin and tendons, porcine skin and rat-tail. Collagen can also be extracted from marine life; it can be extracted from sponges to fish and jellyfish. All collagen sources are worth investigating as each source differs in the collagen type in terms of characteristics.

*3.6.1.1 Properties of bovine hides*

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

*Collagen: From Waste to Gold*

(**Figure 7**) [29]:

with age.

technology [33].

**Figure 7.**

**213**

*Structure of bovine hide [31] (used with permission).*

*3.6.3 Porcine collagen*

*3.6.2 Collagen from fish*

Each section of the animal hide for its properties is discussed further [29]

• *Epidermis:* There are two epidermis layers; one being the thin protective layer of cells during the life of the animal and the other being the flesh remain which is removed during tanning (leather production) by a process called liming.

• *Grain:* This layer is composed of elastin and collagen protein fibres. This layer is mainly used in the cosmetic industry for moisturisers and facial creams.

• *Corium:* The corium layer is made of collagen fibres, arranged in bundles and interwoven to give the structure strength, favourable elasticity and durability. Calf hides corium layer is thinner and smoother than the hides of mature animals; this is because the thickness of the corium increases

Collagen from aquatic animals have been used as a safe substitute for bovine collagen, this is due to collagen from bovine sources have shown to be contaminated with some diseases. Fish solid wastes constitute 50–70% of the original raw mate-

Shark type I collagen forms fibrils under different conditions compared to bovine and porcine collagen [32]. For example, shark type I collagen gels and membranes have stronger rigidity and higher affinity to water vapour than those of porcine collagen, thus indicating the potential for utilising shark collagen as a new type I collagen material for various uses such as cell culture and medical

Pigskin is a by-product of the pork production industry. Collagen extracted from pigskin or bone is not favourable to be a component of foods or pharmaceuticals

rial; however, this depends on the method of meat extraction [6].

#### *3.6.1 Bovine collagen*

Collagen is extracted from many different sources; however, bovine collagen is seen to be the most used collagen type in a variety of different applications, such as the food industry, cosmetics, and medical applications. As the name implies, bovine collagen is a by-product of cows, mainly from the hides. It is a naturally occurring substance found in the skin, muscle, bones and tendons of cows. In the 1970s, the research on bovine collagen gained momentum, as researchers developed a system of extracting collagen and processing it in a liquid form [26].

The natural, unbleached skin and hair of cattle is the bovine hide (skin). Bovine hides are a by-product of the food industry from cattle. Bovine hides without complex processing can be manufactured into leather, which in turn can be used in the shoes and clothing industry. However further complex processing of the hides can be carried out to obtain the corium section of the hide for a variety of different medical and scientific applications [27]. One of the main applications of the corium is in the production of collagen.

Animal hide constitutes 60–65% water, 25–30% protein and 5–10% fats. The protein is mainly collagen [28]. Raw hides have four main parts; epidermis (6– 10%), grain (less than 10%), corium (55–65%) and flesh and the thickness vary all over the animal (**Figure 6**) [29].

The epidermis and flesh layers are removed during tanning leaving the grain and corium layers. The grain is made up of collagen and elastin protein fibres. The corium is packed with collagen protein fibres. The thickness of corium also increases with age [30].

**Figure 6.**

*The approximate composition of bovine hide [28] (used with permission).*
