The composition of extracellular matrix - Collagen
Collagen is the most abundant protein in animals, accounting for more than 30% of the total body protein. It is distributed in various organs and tissues in the body and is a framework structure in the extracellular matrix that can be synthesized and secreted into cells by fibroblasts (Figure 10-3), chondrocytes, osteoblasts and certain epithelial cells.
At least 19 collagen (Table 10-1) have been found, encoded by different structural genes, with different chemical structures and immunological properties. Ⅰ, Ⅱ, Ⅲ, Ⅴ and Ⅺ collagen is a striped fibrous collagen.
Collagen is composed of three identical or different peptide chain formation of three strands, containing three structures: spiral, non-spiral and spherical domain. The structure of type Ⅰ collagen is the most typical.
The type of collagen
Collagen structure
Type I collagen fibrils arranged in parallel into a coarse beam, a light microscope can be seen under the collagen fibers, tensile strength than steel. The triple helix consists of two α1 (I) chains and one α2 (I) chain. Each alpha chain contains about 1050 amino acid residues and consists of repeating Gly-X-Y sequences. X is often Pro (proline), Y is often hydroxyproline or hydroxyl lysine residues. The repeating Gly-X-Y sequence renders the alpha chain curled into a left-handed helix with 3 amino acid residues per lap. Three shares of such a spiral and then coiled into the right hand super-spiral, that is, the original collagen.
The procollagen molecules are aggregated into lateral fibers with a diameter of 50 to 200 nm and a length of 150 nm to several micrometers in a stepwise manner, and are crossed at intervals of 67 nm under electron microscopy. The crosslinking bonds in the collagen fibrils are formed by condensation between two adjacent aldehyde groups produced by the oxidation of the adjacent adjacent lysine or hydroxyl lysine residues.
The original collagen is covalently cross-linked to become insoluble collagen with tensile strength. Embryos and neonatal collagen are prone to cross-linking between molecules and are easily extracted. With age, increasing cross-linking, skin, blood vessels and various tissues become stiff, become an important feature of aging.
Human α1 (I) chain of genes containing 51 exons, and thus the gene after the transcription is very complex. The translated peptide chains are referred to as the anterior α-chain and each end has a propeptide which does not contain the Gly-X-Y sequence. The C-terminal propeptide of the three pre-α chains forms a cross-link between the disulfide bonds, so that the three pre-α chains are aligned with each other. And then from the C-side to the N-terminal formation of three-strand spiral structure. The prepeptide fraction was non-helical. A triple helix collagen molecule with a propeptide is called procollagen. After the denaturation of collagen can not be natural refolding to form a three-strand spiral structure, because the mature collagen molecules peptide chain does not contain propeptide, and therefore can not be "aligned" arrangement.
The anterior α-chain is synthesized on the rough endoplasmic reticulum and is hydroxylated on the proline and lysine residues before the formation of the triple helix. The hydroxylation of the proline residue is carried out in the presence of membrane- Acyl-4-hydroxylase and prolyl-3-hydroxylase. Vitamin C is a cofactor necessary for both enzymes. Vitamin C deficiency leads to the hydroxylation of collagen can not be fully carried out, can not form a normal collagen fibrils, the results of non-hydroxylated α chain in the cell was degraded. Thus, the lack of vitamin C in the diet can lead to blood vessels, tendons, skin brittle, easy bleeding, known as scurvy.
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