Page 202 - Advances in Textile Biotechnology

Enzymatic treatment of wool and silk fi bres 183

degumming with soap in alkaline or acidic solution, and enzymatic degum-
ming. The most extensively used method is degumming with soap or deter-
gents. Among them, the expensive Marseilles soap is outstanding for silk
degumming. In alkaline conditions, soap makes sericin swell. It is then

emulsified and removed from the fi broin filaments. Sericin gum can also be
removed by using water only, but a high temperature, around 110–120 °C,
and pressure is required to obtain complete degumming. In recent decades,
research effort has gone into the development of enzymatic degumming of
silk, which can be carried out under mild conditions and at low temperature,
as an alternative to lessen the environmental impact of the chemical process
(Arami et al., 2007; Gulrajani et al., 1996, 2000a; Freddi et al., 2003; Shukla
et al., 1992; Sonthisombat 1993). An understanding of the difference between

the protein structure of silk fibroin and sericin is necessary for the develop-
ment of successful silk degumming.
In recent years, there has been increasing interest in the use of silk fi broin
in the development of various biotechnological and biomedical devices and
applications. Fibroin protein has therefore been further investigated and
attempts have been made to elucidate its structure (Asakura et al., 2002,
2005; Fossey et al., 1991; Lefevre et al., 2007; Mita et al., 1994; Takahashi

et al., 1999). Fibroin has a predominance of five amino acids: 44.4% glycine,
30.2% alanine, 11.9% serine, 4.9% tyrosine, 2.1% valine and 6.5% other
amino acids (Marsh et al., 1955a and 1955b). It consists of heavy-chain
polypeptides (∼350 KDa) and light-chain polypeptides (∼25 KDa). Fibroin
is about two-thirds crystalline and about one third amorphous. The primary

structure of Bombyx mori fibroin may be approximately divided into repet-
itive region (R) and amorphous region (A) along the heavy fi broin chain
(Fig. 8.3). Mita et al. (1994) and Zhou et al. (2000) predicted from the com-
plete sequence of the Bombyx mori fibroin gene that the repetitive region

was subdivided into three regions (Asakura et al., 2002):
(a) highly repetitive -Gly-Ala-Gly-Ala-Gly-Ser- (GAGAGS) sequences
constituting the crystalline region,
(b) relatively less repetitive -Gly-Ala-Gly-Ala-Gly-Tyr- (GAGAGY)
and/or -Gly-Ala-Gly-Ala-Gly-Val-Gly-Tyr- (GAGAGVGY)sequences
comprising semicrystalline regions containing mainly hydrophobic
moieties,
(c) sequences similar to (a) plus an extension by AAS.
The crystalline blocks are composed of short-side chained, highly repetitive
amino acid sequences, typically leading to the formation of antiparallel
β-pleated sheets (Marsh et al., 1955a; Takahashi et al., 1999). In the crystal-
line region, these polypeptides chains, aligned along the fi bre axis, are held
together by a close network of interchain hydrogen bonds between the >CO
and >NH groups within the β sheets while the β-pleated sheets are held

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