Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Fibrous proteins
Silk fibroin

The ß-structural Fibrous Proteins include silk Fibroin and spider silk protein. In silk fibroin, polypeptide chains rich in Glycine and Alanine contain extended repeating sequences of Ala—Gly, for example,

Class="center">—(Ala—Gly)₄—Туг—Gly—(Ala-Gly)5—Туг—Gly—Ala—Ala—Ser—Gly—(Ala—Gly)₂,

and form an antiparallel ß-Structure. The side chains of alanine residues, alternating with glycine in the fibroin sequence, lie on one side of the ß-pleated sheet, so that hydrophobic contacts between them further stabilize the structure. Polypeptide chains of fibroin that are close to fully extended impart strength to the fiber; however, its elasticity is low and is predominantly determined by less regular structural regions. In a natural silk thread, fibroin, a typically fibrous protein, is embedded in a sort of matrix formed by another structurally less ordered protein, sericin.

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Concluding our review of the features of certain fibrous proteins, we should note that their Primary Structure typically contains characteristic "motifs," i.e., repeating identical or similar Amino acid sequences that ensure The formation of a very extended, regular Secondary structure.

The boundary between globular and fibrous proteins is not as sharp as it might seem. Frequently, compact globular and elongated fibrous elements are combined within the same structure. This is characteristic of fibrous protein precursors, such as procollagen, but is also found in mature Globular proteins, such as Influenza virus hemagglutinin, various proteins regulating DNA activity via a "leucine zipper," and the like.

At the same time, considerable interest is generated by proteins formed by very long polypeptide chains which, however, unlike the fibrous proteins discussed above, form a Spatial Structure in the form of a long chain of generally small, functionally specialized globules. In such multidomain proteins, the aspect ratio of the molecule is characteristic of a fibril, which affects their physicochemical properties, yet features typical of globular formations are also present. The shape of their giant macromolecules is often far from a linear fibril and is sometimes quite complex.

Proteins of this kind play a vital role in the Formation of the Extracellular matrix of Connective Tissue—a specific environment in which Cells are embedded. They participate in tissue embryonic development, interstitial Cell motility, Cell Adhesion, and, ultimately, in the dissemination of tumor cells, or metastasis.