Collagen
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Introduction
Collagen, found in various extracellular matrices, is the most abundant protein in the body - comprising ~30% of it. Collagens provide critical structural support to various tissues and also provide various cellular signals.
Collagen Structure
There are over 20 types of collagen, but a typical collagen molecule is a long fibrous structure with three polypeptide chains wound around each other to form a triple-helix. These chains, which are more extended than alpha helices, are held together by H-bonds.
Collagen primary structure, which is usually around 1000 residues, follows a repeating sequence of Gly-X-Y, where X is often hydroxyproline and Y is often hydroxylysine. Glycine residues allow tight packing of the triple helix, while hydroxyproline stabilizes interchain H bonds.
Collagen Synthesis
Collagens are made by stromal cells - fibroblasts, osteoblasts, or chondrocytes. Procollagen is formed within the cell and formed into tropocollagen once excreted.
Hydroxylation of proline or lysine residues occurs in the ER and requires oxygen and the reducing agent ascorbic acid (vitamin C) to occur. Vitamin C dieficiency results in scurvy, which often manifests with bruising due to capilary fragility and blood extravasation.
Types of Collagen
Collagens can be organized into three groups:
1. Fibril-forming collagens, such as Types I, II, and III, are rope-like and provide high tensile strength.
Type I collagen is found in tendon, bone, and mature skin (90% of total body collagen)
Type II collagen is restricted to hyaline cartilage and the cornea
Type III collagen is common in distensible tissues such as blood vessels, as well as healing wounds
- normal ratio of type I:type III is 5:1; hypertropic and immature scars have ratio of 2:1 or less
2. Network-forming collagens form a three-dimensional mesh work
Type IV is a major constituent of the basement membrane, providing support to cells and functioning as a semi-permeable membrane
Type V: similar to type IV.
3. Fibril-associated collagens bind to collagen fibrils and link them to one another and other components of the ECM.
Resources and References
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