Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Fibrous proteins
Polypeptides in collagen form triple-helical structures

Collagen fibrils are composed of repeating polypeptide subunits called tropocollagen. These subunits are arranged longitudinally within the fibril in parallel HEAD-to-tail bundles (Fig. 7-14). The heads of the adjacently positioned tropocollagen molecules are staggered relative to one another in a step-like manner by a uniform distance along the longitudinal axis. This accounts for the characteristic transverse banding pattern of collagen fibers, which corresponds to a 64 nm periodicity.

Collagen fibers yield an X-Ray Diffraction pattern distinct from those of a- and ß-Keratins. X-ray crystallographic data led to the Conclusion that tropocollagen subunits consist of three polypeptide chains tightly wound into a triple-stranded rope.

Tropocollagen is a rod-shaped molecule measuring 300 nm in length and a mere 1.5 nm in thickness, with a Molecular Weight of approximately 300,000. The three helically intertwined polypeptide chains are of equal length, each containing about 1,000 amino acid residues. In some collagens, all three chains share an identical Amino Acid Sequence, whereas in others, only two chains are identical while the third differs. Significant progress has been made in determining the Amino acid sequences of The major types of collagen chains, which rank among the longest known protein polypeptide chains.

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Fig. 7-14. Arrangement of tropocollagen molecules in collagen fibrils.

Each tropocollagen molecule exhibits four transverse bands repeating at 64 nm intervals. The heads of the tropocollagen molecules are positioned such that they are staggered relative to each other by 64 nm. Below the schematic representation of the fibril, a segment of the tropocollagen molecule is shown as a triple-helical backbone. At the very bottom, an even higher-magnification view of the triple helix illustrates that each of the tropocollagen polypeptide chains is itself a helix; the pitch and repeat of this helix are dictated by the geometry of the rigid R-groups of numerous Proline and hydroxyproline residues.

X-ray diffraction studies have demonstrated that each tropocollagen polypeptide chain is also helical, although its pitch and dimensions differ markedly from those of the a-helix. The tropocollagen polypeptide chain forms a left-handed helix with only three amino acid residues per turn. Because collagen is rich in proline and hydroxyproline residues—which impart a rigid, bent conformation to the chain—the three helical polypeptide chains are tightly wound around one another. They are further stabilized by interchain Hydrogen Bonds. In addition, collagen features unusual types of covalent cross-links formed between two Lysine residues located in adjacent chains (Fig. 7-15). Adjacent tropocollagen triple helices are also cross-linked to one another. Tropocollagen is virtually inextensible due to the tight coiling of its triple helices and the presence of cross-links. Furthermore, tropocollagen contains carbohydrate side chains attached to the hydroxyl groups of hydroxylysine.

As we age, an increasing number of cross-links form within and between tropocollagen subunits, making collagen fibrils in Connective Tissue progressively stiffer and more brittle. Because collagen is a major component of numerous structures, its age-related embrittlement and increased rigidity alter the mechanical properties of Cartilage and tendons, make bones more fragile, and reduce the transparency of the cornea.

The Collagen helix is unique, as it does not occur in any Proteins other than collagen, unlike the a-helix and ß-conformation, which are found—at least in small segments—in many Globular proteins.