Collagen is a protein found almost everywhere in the body, and extremely important for a number of functions of the body. It is worth putting a whole article about collagen because it is so important, and there are a few things worth knowing about it.
In many ways, the structure of collagen can be compared to an alpha-helix - a commoon structure found in many proteins. However, there's a few key differences. The first major difference is the shape of the helix. An alpha-helix is shaped into a right handed helix. Collagen strands are each shaped into a left-handed helix (i.e. the opposite way round), and then three of these coiled strands get together and form a right-handed triple helix.
The order of the amino acids is also different. In an alpha-helix, you could find a whole range of different amino acids. In collagen, you tend to get a particular pattern of three: glycine, proline, and hydroxyproline. In fact, there is much more variation than that, but glycine very commonly takes the first place in each triplet; and proline and hydroxyproline are frequently in second and third place respectively.
Why does anyone care about this? Well, the reason is that this structure makes collagen very stable, and very strong. The tiny glycine amino acid fits snuggly pointing into the helix, while proline and hydroxyproline point outwards. This stable twisting structure can then be used all around the body.
One very important details is knowing that it makes up an important part of connective tissue (the tissue that fills up the space between other organs). Because connective tissue is so widespread, a disorder of someone's collagen will have an effect all around the body.
It's also an important part of muscle - not the bit that contracts, but the bit which surrounds it and keeps it into bundles of muscle fibres.
There's loads of types of collagen! People keep discovering more and more, which are sufficiently different to take us to a new type. However, the majority of the collagen that you'll deal with on a day-to-day basis, the kinds which are most important and common, are types 1 to 4 - or, as they're more commonly written, types I to IV.
Type I is found everywhere in the body, and is the most common type of collagen - which isn't much of a surprise. You'll particularly find it in blood vessel walls, tendons, skin, joints, bones and muscles .... it's not a short list!! Collagen I is obviously an extremely important protein, and problems with it include the condition Osteogenesis Imperfecta, a condition which can kill. Type I is also the kind of collagen that you'll find in the later part of healing, in scars.
Type II is the type you'll find in cartilage - particularly hyaline cartilage, the type of cartilage found in synovial joints.
Type III is the kind that is used in early healing, before the tougher Type I comes along. It's produced more quickly, which makes it useful in the early healing process, but it also explains why it isn't so tough and needs replacing with Type I. You also find it in the walls of things like arteries, the intestines and the uterus.
Type IV is the kind you find underneath a row of cells when they're lined up on the basal lamina. It's particularly important in the lens of the eye, and for providing something through which stuff is filtered - for instance, in capillaries when oxygen diffuses out of blood, or for filtration in the kidney.