Surface Modification of Cellulose Acetate with Cutinase and ...

Chapter 1
(Drenth et al., 1968; Kamphuis et al., 1984). Papain is a 212 amino acids protein with a
molecular weight of 23.4 kDa. The enzyme has three internal disulphide bridges and an
isoelectric point of 8.75.
The optimal activity of papain occurs at pH 5.8 - 7.0 and at temperature 50 - 57 ºC when
casein is used as the substrate (Light et al., 1964; Kamphuis et al., 1984). The general
mechanism of papain action has been very well studied. The catalytic triad is formed by
Cys25, His159 and Asn175 residues. Asn175 is important for orientation of the
imidazolium ring of the histidine in the catalytic cleft. The reactive thiol group of the
enzyme has to be in the reduced form for catalytic activity. Thus, the cysteine proteases
require a rather reducing and acidic environment to be active (Theodorou et al., 2007).
Generally, papain can cleave various peptide bonds and possesses therefore fairly broad
specificity.
8.1 Degumming of silk
Papain is reported to be used for boiling off cocoons and degumming of silk. Raw silk
must be degummed to remove sericin, a proteinaceous substance that covers the silk
fibre. Degumming is usually performed in an alkaline solution containing soap, a harsh
treatment that also attacks fibrin structure. Several alkaline, acidic and neutral proteases
have been studied as degumming agents since they can dissolve sericin but are unable to
affect silk fibre protein. Alkaline proteases have a better performance removing sericin
and improving silk surface properties like handle, shining and smoothness (Freddi et al.,
2003; Arami et al., 2007), but there is no commercial application yet on this field.
In the past, papain was also used to ‘shrink-proof’ wool. A successful method involved
the partial hydrolysis of the scale tips. This method also gave wool a silky lustre and
added to its value. The method was abandoned a few years ago for economic reasons.
9. Transglutaminases (TGs)
Transglutaminases are a group of thiol enzymes that catalyse the post-translational
modification of proteins mainly by protein to protein cross-linking, but also through the
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