Fibrous protein
One of two major classes of protein molecules, the other being termed globular protein. Fibrous proteins function either in a filamentous aggregate or as long, thin molecules. One major class is thea fibrous group. It includes intermediate filaments (occurring in skin, wool, and neurones), muscle proteins (myosin, paramyosin, and tropomyosin), an fibrinogen (a plasma protein that forms clots).

The protein chain forms a right handed a helical structure that is stabilized by hydrogen bonds that lie parallel to the axis of the helix (see illus.)

The chains also have a seven-residue periodicity in which apolar (that is, without a charge) residues occur alternatively three and four apart. This forms an apolar stripe on the surface of the helix. Stability in water is achieved only by two or more a helices combining to form a left handed ropelike structure.

The second major class is based on the b structure. Feather and scale keratin have a twisted b sheet structure, that is, an extended array of chains which are held together by hydrogen bonds positioned perpendicular to the chain axis. The sheet aggregates laterally and helically to generate a filamentous structure. Many types of silk have a b structure.

The third major class is exemplified by collagen, a major component of skin, tendon, bone, cornea and cartilage (seeillus.). The collagen chains have a repeating sequence in which glycine occurs at every third position, with proline and hydroxyproline commonly occurring elsewhere in the chain.

The molecules contain three chains, each of which coils up into a threefold helix. The chains then coil around one another in a right handed manner to generate the collagen molecule. In turn, molecules generally aggregate to form fibrils with a characteristic axial period. Many types of collagen molecules are known, and these differ in the extent of triplet substructure and in function.

Filial generation
See F1; F2.

Filler
An inert foreign substance added to a matrix to improve or modify its properties.

Fission
(L. fissilis, easily split) Asexual reproduction involving the division of a single celled individual into two new single celled individuals of equal size.