The protein collagen is the main substance of connective tissue and is present in all multicellular organisms. In mammals collagen is the most abundant protein making up a quarter of the total weight of proteins. Collagen gives many different organs and tissues substantial, stout and elastic properties. It has been found in many different tissues and organs like bones, tendons, (hyaline) cartilage, blood vessels, teeth, cornea, inter vertebral disks, vitreous bodies, placenta, (fetal) skin, etc. Collagen is a protein that enwraps the organs and parts of it to hold specialized cells together in discrete units. It prevents organs/tissues to tear or loose their functional shape when they are exposed to sudden and wild movements. Besides the structural role in mature tissues, collagen plays a regulating role in developing tissues as well. Collagen functions as a kind of trigger that influences the proliferation and differentiation of unspecialized cells. It has a key function in the regulation of cell-type- specific gene expression and developmental control and diseases like cancer (Eyre, 1980).
Besides glue, many new applications have been developed in industry for these proteins. The most popular uses are found in: cosmetics, nutrition, and medical applications for example as surgery stitch cotton (Chvapil, 1979).
However, collagens attract attention not only for commercial motives. Also from a clinical point of view there is much interest in collagens, because many different diseases are related to disorder in collagen. A better understanding of the spatial structure will give us more insight in collagen related disorder diseases. These can be congenital, for example like the Ehler-Danlos syndromes, or the consequence of a deficiency like scurvy. Some of the most important defects are tabulated in appendix 1.
The collagen proteins belong to the larger family of fibrous proteins, like the fibrines from blood. They form mostly insoluble super strong fibres that have a high tensile strength. A fibre of only 1 mm thick can easily resist a force of 10 kg.
Collagen occurs not only in the shape of fibres. Collagen has been observed as ropes, straps, woven sheets, filtration membranes, supporting skeletal frameworks, bearing materials lubricated with proteoglycans and in all other specialized tissues that must be strong and yet have unusual properties, for example as the light-transmitting cornea and fatigue-resistant heart valves (Martin, et al., 1985). All these examples concern genetically distinct types of collagen that have evolved into a particular structural function outside the cell (An overview of the synthetic pathway in the the will be given in chapter 4). At least 14 types of collagen have been described and characterized so far (see table 4, page 9). They are distinguished on the basis of their chemical differences. These 14 types differ in the way they associate with one another and the way they interact with other molecules.
The most important types are the fibrillar collagen types I, II, III, IV and V. Type I is with 90% the most abundant collagen type. It forms the largest and strongest fibrillar component that provides tensile strength to bones, skin, tendons and ligaments. Type II collagen is unique to articular cartillage and fibrocartilage, the vitreous body of the eye and certain other organs. Type III collagen is similar in structure to type I but less abundant and is often encountered in areas of rapid new collagen synthesis. Type IV is a major component of all basement membranes and type V is found in some veins and arteries.
Collagens are known to form highly ordered agregates. The periodicity in these macromolecular structures makes them suitable for investigation by means of X-ray diffraction.
This paper will describe the spatial organisation of collagen structures. In particular of the collagen type I. Collagen type I is the most regular one and most structure research has been done for this collagen type. Especially that extracted from rat tail tendon.
First, a detailed description will be given of the collagen monomers and the different types. Next it will be shown how they are synthesised. After that the packing of collagen into collagen fibres will be discussed. The mode of packing will be divided in an axial and an equatorial direction.