Fibrous Proteins

The two major classes of proteins are the fibrous proteins and the globular proteins. Fibrous proteins are distinguished from globular proteins by their filamentous, elongated form. Most of them play structural roles in animal cells and tissues, holding things together. Fibrous proteins have amino acid sequences that favor a particular kind of secondary structure which, in turn, confer particular mechanical properties on the proteins.

Table 6.2 lists the amino acid compositions of four different fibrous proteins, namely -keratin, fibroin, collagen, and elastin. Notice the relatively high abundance of amino acids with non-bulky side chains, such as glycine, alanine, serine, glutamate, and glutamine. A notable exception to this is the quite high amount of proline in collagen and, to a lesser extent, in elastin. Each of the proteins in Table 6.2, however, has a unique amino acid composition, because each is comprised of a unique sequence of amino acids.

Keratins - -Keratins are the major proteins of hair and fingernails and a compose a major fraction of animal skin. -keratins are classified in the broad group of intermediate filament proteins, which play important structural roles in nuclei, cytoplasm, and cell surfaces. Their secondary structure is composed predominantly of -helices. Figure 6.11 shows the coiled-coil structure of the -keratin in hair. The chemical composition of the cysteine residues in -keratin affects its macromolecular structure and function. For example, hair has relatively few cysteine cross-links, whereas fingernails have many such cross-links. -keratins, on the other hand, contain much more pleated sheet secondary strucure than -keratins and are found in feathers and scales.

Fibroin - Fibroin is made by silkworms and spiders and it contains long regions of antiparallel sheets. Other parts of fibroin are not in the form of sheets. These contain bulky amino acids that interrupt the sheet structures and may account for the "stretchiness" of silk fibers. The sheet regions contain, almost exclusively, multiple repetitions of the sequence:

[Gly-Ala-Gly-Ala-Gly-Ser-Gly-Ala-Ala-Gly-(Ser-Gly-Ala-Gly-Ala-Gly)8]

Notice that almost every other residue is a glycine and that between them lie either alanine or serine residues. This repeating structure results in simple, tightly organized structures, such as the structure of silk fibroin shown in Figure 6.12.

Collagen - Collagen is the single most abundant protein in most vertebrates - up to a third of the total protein mass. Collagen fibers provide the matrix of bone on which mineral constituents precipitate. The fibers constitute a major portion of tendons and a network of collagen fibers is present in skin. The triple-helix tropocollagen molecule is the basic unit of the collagen fiber. Composed of about 1000 amino acids each, the individual chains of tropocollagen contain a left-handed helical structure, but are wound together with the other two chains of the fiber in a right-handed manner. This unique structure is shown in Figure 6.13. Every third residue lies near the center of the triple helix and can only be glycine, because all other amino acid side chains would be too bulky. The repeating amino acid sequence is Gly-X-Y, where X is often proline and Y is proline or hydroxyproline (a modified form of proline). Collagen is unusual not only in having modified amino acid residues, such as hydroxyproline and hydroxylysine, but also in having so many of them. Hydroxyproline helps to stabilize the triple helix via hydrogen bonds. Hydroxylysine functions to form attachment sites for polysaccharides. Hydroxylation of proline requires ascorbic acid (vitamin C). Deficiency of vitamin C reduces hydroxyproline production, leading to weakened collagen fibers and the condition known as scurvy. Part of collagen's toughness arises from cross-links between lysine residues of adjacent chains. This cross linking reaction occurs throughout life and makes bones, skin, and tendons less elastic - properties we associate with aging.

Elastin - Elastin is a highly elastic fiber present in ligaments and arterial blood vessels. The polypeptide is rich in glycine, alanine, and valine. Its secondary structure is the most random of the fibrous proteins described here. Like collagen, elastin contains lysine groups involved in cross-links between the chains. In elastin, however, four lysine chains can be combined to form a desmosine cross-link (see here). Thus, fewer cross-links are needed to provide strength for the chains and a more elastic network is created.

See also: Collagen Synthesis, Globular Proteins