Collagen is the predominant protein in animal bodies, constituting approximately 30% of the total

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Collagen is the predominant protein in animal bodies, constituting approximately 30% of the total protein (Birk & Bruckner, 2005; Unlua et al. 2014). It is an extracellular matrix protein that can either directly or indirectly interact with cells to alter cellular activity. At least 27 different types of collagen have been indentified, correspondingly named types I to XXVII (Birk & Bruckner, 2005).
Structure
Collagen is characterized by the right-handed triple helical rod structure which is formed by three left handed helix polypeptide chain. Although the triple helical structure is the main characteristic of collagen, some globular proteins are indentified as collagen due to the repeating trilets and small triple helical rod (Prockop &
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It includes types I-III, V, and XI, which are similar in size, approximately about 1000 amino acids. Network-forming collagen is type IV collagens found in basement membranes and type VIII and X collagens. This class of collagen forms net-like structures in which monomers associate at the C-termini to form dimers and at the N-termini to form tetramers. In addition to these end-to-end interactions, the triple-helical domains intertwine to form supercoiled structures. FACIT collagen stands for fibril associated collagen with interrupted triplehelice, which includes types IX, XII, XIV, XVI, and XIX collagens (Prockop & Kivirikko 1995; Van Der Rest & Garrone 1991). They do not form fibrils themselves but are found attached to the surfaces of preexisting fibrils of the fibril-forming collagens. Type VI is the only known collagen that can form beaded filament. The type VI collagen forms a dimer with another type VI collagen with end to end interaction. The globular domain of the type VI collagen dimer forms a tetramer with other type VI collagen dimer. The tetramer further aggregates with another tetramer to form the beaded filament (Prockop & Kivirikko 1995). Collagen of anchoring fibrils is composed by type VII collagen. Type VII collagen has two non collagen domain, NC1 and NC2. Type VII collagen forms a dimer with another type VII collagen by the interaction with two NC2 domains. The dimer further aggregates with other dimmers to form fibrils.

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