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A collagen may be defined as a protein containing sizable domain(s) of triple-helical conformation. A further essential feature of the definition is that the protein participates in the construction of extracellular aggregates functioning primarily as supporting elements. At least 11 genetically distinct collagen systems have been identified in vertebrate tissues which have been characterized to varying degrees. These systems are more commonly referred to as collagen types. The known collagens are collectively composed of approximately 20 unique polypeptide chains. In some cases, the chains associated with a given collagen type may be utilized to form more than one molecular species. There exists, therefore, far more tripled helical molecular species than the designated collagen types. For this reason, the term "collagen system" may be more appropriate than "collagen type", particularly when several molecular species are identified. The relatively large number of collagen chains plus numerous post-translational modifications of the chains and molecules derived from them generate enormous diversity in the chemical and structural characteristics of the collagen family of proteins.
Primary Structure of Chains
TABLE 1 - Chains Involved in Formation of the Vertebrate Collagens.
Table I lists collagens and the chains associated with each type. The collagens are listed in three groups based largely on the properties of the associated chains. In this regard, the chains composing type I, II, III, V and K collagens have an Mr, equal to or greater than 95,000 and are further characterized by the presence of an uninterrupted Gly-X-Y repetitive triplet structure extending over slightly more than 1000 amino acid residues. The collagen designated as type K is also referred to as Type XI, but there is considerable uncertainty with respect to the molecular organization of the chains associated with this collagen. Moreover, one of these chains, 3a , appears to be identical to a 1 (II). In any event, the chains associated with type I through K collagens and the molecules derived from them are quite similar in structure and these collagens may be classified as the fibril- and fiber-forming collagens. The chains composing type IV, VI, VII and VIII collagens are likewise equal to or greater than 95,000 in Mr, but are characterized by somewhat smaller stretches of uninterrupted Gly-X-Y repetitive triplet structure. With respect to size and topology of structural features, the chains of the latter collagens are not as closely related as the chains of the fiber-forming collagens. Similar considerations are applicable to the chains of type IX and X collagens. The chains of these collagens are, however, clearly distinguishable from the chains of the other collagens in that they exhibit an Mr, less than 95,000.
TABLE 2 - Molecular Species of the Vertebrate Collagens
Table 2 shows the molecular species of the collagen types. Of the nine unique chains associated with the fiber-forming collagens, extensive sequence data encompassing the complete primary structure of pro-a 1 (I), pro-a (II) and pro-a (III) and the C-terminal third of pro-a 2 (V) are available. The major structural features along the pro-a chain of a fiber forming collagen include alternating non-triplet and repetitive Gly-X-Y triplet segments. The chain may contain as many as 1441 amino acid residues as in pro-a 1(I). Although the non-triplet sequences of these chains are generally characterized by relatively high levels of acidic and large hydrophobic residues, the N-terminal non-triplet sequence of pro-a 1(II) contains a large complement of basic amino acid residues. The pro-a chains of type I, II, and III collagens exhibit a high degree of overall homology. Not counting invariant glycyl residues in triplet sequences, the levels of homology range from 65% identical residues for pro-a 1(I) and pro-a 1(II) to 45% identical residues for pro-a 2(I) and pro-a 1(III). Pro-a 1(III) is least like the other three chains. Initial sequence data for pro-a 2(v) have shown that the latter chain exhibits a significant, albeit reduced, level of homology to the chains of type I, II, and II collagens. The native procollagen molecule contains a globular domain at each end of a lengthy triple-helical region.
Type I and type II collagens undergo extracellular processing of the procollagen molecule to remove the propeptide sequences at each end of the molecule, whereas type III collagen retains a substantial portion of its N-terminal propeptide sequence. Type V collagen may be similar to type III in its processing. No conclusive data exists for the extracellular processing of type VII and VIII molecular species and results for type VI and IX collagens remain equivocal. Type X molecules form chains with an Mr of about 49,000.
TABLE 3 - Macroscopic Distribution of the Collagens
Table 3 shows the macroscopic distribution of collagens. Type X is usually prepared from the larger collagen-rich connective tissues indicated in the table. Although it is a relatively minor constituent in any tissue or organ, type V collagen appears to have a distribution similar to that of type I. Type III collagen, likewise, occurs in tissues in a pattern similar to type I. Nevertheless, the proportion of type III collagen in mineralized tissues such as bone and dentin, as well as in tendon, is normally quite low and often undetectable by chemical assays. Type III collagen, however, along with type I collagen, is a major constituent of tissues such as the dermis and vascular walls. This collagen appears to be prevalent in all of the more distensible connective tissues. The remaining two fiber-forming collagens are located chiefly in hyaline cartilage as well as cartilage-like tissues, such as the nucleus pulposus of the vertebral body and vitreous body of the eye. Type II is by far the most prevalent of these collagens and is composed of chains which are highly homologous to the chains of type I and III collagens. Moreover, two of the chains of type K collagen are highly homologous to those of the type V collagen. It would appear that hyaline cartilage and cartilage-like tissues are quite unique with respect to their collagen complement.
Type IV collagen is a major macromolecular constituent of basement membranes and can be readily isolated from basement-membrane-rich tissues or highly vascularized tissues such as the placental villi. This collagen appears to be largely restricted to structures identifiable as basement membranes. In contrast, type VI collagen appears to be prevalent in several tissues even though it has been isolated largely from placental villi preparations. The extent to which type VII and VIII collagens are distributed is not known. These collagens can be isolated from the sources given in Table 3. Type IX and X collagens constitute additional minor collagenous constituents of hyaline cartilages. Their presence in these tissues further emphasizes the large and unique complement of collagens in these types of tissues.
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