Type IV collagen C4 domain
In molecular biology, the type IV collagen C4 domain (or collagen IV NC1 domain) is a duplicated domain
Protein domain
A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

 present at the C-terminus of type IV collagens. Each type IV collagen contains a long triple-helical collagenous domain flanked by a short 7S domain of 25 amino acids and a globular non-collagenous C4 domain of ~230 amino acids at the N and C terminus, respectively. In protomer assembly, the C4 domains of three chains interact, forming an C4 trimer, to select and register chains for triple helix
Collagen helix
In collagen, the collagen helix, or type-2 helix, is a major shape in secondary structure. It consists of a triple helix made of the repetitious amino acid sequence glycine - X - Y, where X and Y are frequently proline or hydroxyproline....

 formation. In network assembly, the C4 trimer
Trimer (biochemistry)
In biochemistry, a trimer is a macromolecular complex formed by three, usually non-covalently bound, macromolecules like proteins or nucleic acids. A homo-trimer would be formed by three identical molecules. A hetero-trimer would be formed by three different macromolecules. Collagen is an example...

s of two protomers interact, forming a C4 hexamer structure, to select and connect protomers.

The collagen IV C4 domain contains 12 cysteines
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

, and all of them are involved in disulphide bonds. It fold
Tertiary structure
In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...

s into a tertiary structure with predominantly beta-strands. The collagen IV C4 domain is composed of two similarly folded
Protein folding
Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....

 subdomains stabilised by 3 intrachain dissulphide bonds involving the following pairs: C1-C6, C2-C5, and C3-C4. Each subdomain represents a compact disulphide-stabilised triangular structure, from which a finger-like hairpin loop projects into an incompletely formed six-stranded beta-sheet of an adjacent subdomain of the same or of an adjacent chain clamping the subdomains tightly together.
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