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Collagen: Type I, II or III

Collagen is the most common protein in animals and Collagen types I, II, and III are the most common collagen in the humans. These collagens are known as fibrous collagen, because they form long strands. Collagen type I is abundant in the skin, tendon, ligaments, bone, teeth, and in between organs. Collagen type II is abundant in the cartilage and eyes. Collagen type III is abundant in the muscle, blood vessels, and skin. All together, collagen types I, II, and III are 80% to 90% of the collagen in people. Because they are structurally similar, the processes for producing collagen types I, II, and III are in tissues also are similar.

Collagen Type I, II, and III Form in Cells
The instructions for making any protein are contained in the DNA. The DNA resides in the nucleus of the cell far from the organelles that produce proteins. The cell transcribes the instructions from DNA onto a related molecule called RNA. Transcribed RNA with the instructions for creating collagen leaves the nucleus and goes to the ribosomes where proteins are assembled.

Ribosomes are responsible for producing proteins by translating the instructions on RNA into a chain of amino acids called a polypeptide. Some ribosomes are spread throughout the cell. Other ribosomes are attached to a network of tubes called the endoplasmic reticulum. Collagen production occurs at the ribosomes attacked to the endoplasmic reticulum.

The polypeptides that will become collagen are made mostly of glycine, proline, and hydroxyproline. The chemical structures of these three amino acids give the collagen polypeptides their properties. Bonds form between the amino acids on adjacent polypeptides to pull them together in a triple helix. This triple helix has impressive strength and resiliency. On a per mass basis, collagen type I has more strength than steel.

Collagen Type I, II, and III Matrix Outside of Cells
The triple helix of polypeptides undergoes some more processing and is then secreted from the cell. The final steps in collagen production must happen outside the cell so that completed collagen does not form a matrix inside the cell, clogging it. During or following secretion, the ends of the triple helix are cut off. This allows the small strands to attach to each other forming a strong, elastic matrix.

Roles in the Body
Collagen types I, II, and II are the foundation of many tissues. Tendons are made of collage type I fibrils that are lined up. Bone is a matrix with collagen type I. Cartilage is made with a combination of collage types II and IX. Collagen type IX has a kink in it that contributes to cartilage's qualities. The combination of collagen types II and IX make collagen strong and compressible so that it can absorb shocks. Skin contains collagen types I and III along with other polymers.

Most of collagen production occurs inside the cell, except the last step. DNA, RNA, ribosomes, and the endoplasmic reticulum all contribute to collagen production. Collagen types I, II, and III are important components of skin, bone, teeth, ligaments, tendons, cartilage, eyes, muscles, and blood vessels. The chemical structures of collagen types I, II, and III are responsible for the properties of the tissues formed out of them.

Collagen Types I, II, and III are in many tissues
Collagen Type I Skin, Bone, Teeth, Tendons, Ligaments, Interstitial Tissue
Collagen Type II Cartilage, the Fluid in the Eyeballs
Collagen Type III Skin, Muscle, Blood Vessels

References:

1. Collagen: The Fibrous Proteins of the Matrix. National Center for Biotechnology Information
National Institute of Health
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A6542

2. Collagen. RCSB Protein Data Bank
Rutgers University and University of California San Diego
http://rcsb-legacy-mirror.rutgers.edu/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb4_1.html