A single collagen molecule consists of three polypeptides known as  chains. The chains intertwine, forming a right-handed triple helix (see Fig. 6.7d). Every third amino acid in the chain is a glycine molecule, except at the ends of the chains. A hydroxyproline or hydroxylysine frequently precedes each glycine in the chain, and a proline frequently
FIGURE 6.6 • Collagen fbrils in dense irregular connective tissue. This atomic force microscopic image of type I collagen fibrils in the connective tissue shows the banding pattern on the surface of collagen fibrils. Note the random orientation of collagen fibrils that overlie and crisscross each other in the connective tissue matrix. 65,000. (Courtesy of Dr. Gabriela Bagordo, JPK Instruments AG, Berlin, Germany.) follows each glycine in the chain. Along with proline and hydroxyproline, the glycine is essential for the triple-helix conformation (see Fig. 6.7e). Associated with the helix are sugar groups that are joined to hydroxylysyl residues. Because of these sugar groups, collagen is properly described as a glycoprotein.
The chains that constitute the helix are not all alike. They vary in size from 600 to 3,000 amino acids. To date, a. Fibril D 68 b. Packing of (0.6 D) 0.4 D c. Collagen molecule 300 nm (4.4 D) 1.5 nm diameter d. Triple helix 10.4 nm (0.15 D) e. Typical sequence 1 of 1 and 2 1.74 nm .87 nm
FIGURE 6.7 • Diagram showing the molecular character of a type 1 collagen fbril in increasing order of structure. a. A collagen fibril displays periodic banding with a distance (D) of 68 nm between repeating bands. b. Each fibril is composed of staggered collagen molecules. c. Each molecule is about 300 nm long and 1.5 nm in diameter. d. The collagen molecule is a triple helix. e. The triple helix consists of three chains. Every third amino acid of the chain is a glycine. The X position following glycine is frequently a proline, and the Y position preceding the glycine is frequently a hydroxyproline.
at least 42 types of chains encoded by different genes have been identified and mapped to loci on several different chromosomes. As many as 28 different types of collagens have been categorized on the basis of the combinations of chains they contain. These various collagens are classified by Roman numerals I to XXVIII according to the chronology of their discovery. A collagen molecule may be homotrimeric (consisting of three identical chains) or heterotrimeric (consisting of two or even three genetically distinct chains).
For example, type I collagen found in loose and dense connective tissue is heterotrimeric. Two of the chains, identified as 1, are identical, and one, identified as 2, is different. Thus, in collagen nomenclature it is designated [ 1(I)]2 2(I) (Table 6.2). Type II collagen is homotrimeric and present in hyaline and elastic cartilage, where it occurs as very fine fibrils. The collagen molecules of type II collagen are composed of three identical chains. Because these chains differ from those of other collagens, type II collagen is designated [ 1(II)]3.
Several classes of collagens are identified on the basis of their polymerization pattern.
Most of the collagen molecules polymerize into supramolecular aggregates such as fibrils or networks, and they are divided into several subgroups on the basis of their structural or amino acid sequence similarities.