(B) a drawing of the giant aggrecan aggregate shown in (a). It consists of about 100 aggrecan monomers (each like the one shown in Figure 19–36) noncovalently bound through the N-terminal domain of the core protein to a single hyaluronan chain. a link protein binds both to the core protein of the proteoglycan and to the hyaluronan chain, thereby stabilizing the aggregate. the link proteins are members of a family of hyaluronan-binding proteins, some of which are cell-surface proteins. the molecular mass of such a complex can be 108 daltons or more, and it occupies a volume equivalent to that of a bacterium, which is about 2 × 10–12 cm3. (a, courtesy of Lawrence rosenberg.)
Figure 19–38 Proteoglycans in the extracellular matrix of rat cartilage.
the tissue was rapidly frozen at –196°C, and fixed and stained while still frozen (a process called freeze substitution) to prevent the GaG chains from collapsing. In this electron micrograph, the proteoglycan molecules are seen to form a fine filamentous network in which a single striated collagen fibril is embedded. the more darkly stained parts of the proteoglycan molecules are the core proteins; the faintly stained threads are  the GaG chains. (reproduced from
E.B. hunziker and r.K. Schenk, J. Cell Biol. 98:277–282, 1984. With permission from the rockefeller University press.)
Figure 19–39 The structure of a typical collagen molecule. (a) a model of part of a single collagen α chain, in which each amino acid is represented by a sphere. the chain is about 1000 amino acids long. It is arranged as a left-handed helix, with three amino acids per turn and with glycine as every third amino acid. therefore, an α chain is composed of a series of triplet Gly-X-Y sequences, in which X and Y can be any amino acid (although X is commonly proline and Y is commonly hydroxyproline, a form of proline that is chemically modified during collagen synthesis in the cell). (B) a model of part of a collagen molecule, in which three α chains, each shown in a different color, are wrapped around one another to form a triple-stranded helical rod. Glycine is the only amino acid small enough to occupy the crowded interior of the triple helix. Only a short length of the molecule is shown; the entire molecule is 300 nm long. (From a model by B.L. trus.)
Collagens are extremely rich in proline and glycine, both of which are important in the formation of the triple-stranded helix.
The human genome contains 42 distinct genes coding for different collagen α chains. Different combinations of these genes are expressed in different tissues. Although in principle thousands of types of triple-stranded collagen molecules could be assembled from various combinations of the 42 αchains, only a limited number of triple-helical combinations are possible, and roughly 40 types of collagen molecules have been found. Type I is by far the most common, being the principal collagen of skin and bone. It belongs to the class of fibrillar collagens, or fibril-forming collagens: after being secreted into the extracellular space, they assemble into higher-order polymers called collagen fibrils, which are thin structures (10–300 nm in diameter) many hundreds of micrometers long in mature tissues, where they are clearly visible in electron micrographs (Figure 19–40; see also Figure 19–38). Collagen fibrils often aggregate into larger, cablelike bundles, several micrometers in diameter, that are visible in the light microscope as collagen fibers.