3 SILK: FIBROIN AND SERICIN

Silk fibroins (Howitt 1946; Lucas et al. 1958; Rudall 1960) are the extracellular proteinaceous filaments produced by certain species of the phylum Arthropoda, classes Insecta and Arachnida. Many species belonging to the class Insecta spin cocoons in which they pupate, such as the classic example, Bombyx mori (order Lepidoptera, subclass Bombycidae). This domesticated silkworm, actually a caterpillar, spins its cocoon after feeding on mulberry leaves for nearly a month. The cocoon consists of two extruded triangular-shaped filaments of a proteinaceous polymer called fibroin, held together by a gumlike protein called sericin. Removal of the sericin from silk fibroin is accomplished by a process called “degumming,” usually by one of three methods: (1) extraction with water at high temperatures, (2) extraction with dilute aqueous alkali or soap solutions, or (3) removal by proteolytic enzymes.

A fundamental distinction between fibroin and sericin is the composition of amino acids that make up these two proteins (see table 1). Characteristic features of the fibroins in general are the high proportion of the smaller side group amino acids glycine, alanine, and serine and their insolubility in aqueous solutions. The large percentage of the small side-chain amino acids allows for a close-packing arrangement of the molecules characteristic of the fibroins and is a general feature of extracellular matrix proteins. Sericin, on the other hand, has over 30 mol % serine (Komatsu 1980) and is readily soluble in hot, dilute alkali solutions (Lucas et al. 1960).

TABLE 1 AMINO ACID COMPOSITIONS OF BOMBYX MORI SILK PROTEINS (mol %)