Page 273 - Advances in Textile Biotechnology

254 Advances in textile biotechnology

in most connective tissues, blood vessels and in basement membranes in all
organs where they participate in the cellular matrix structure and function.
Animals such as cattle, pigs and horses as well as marine animals such as
fi sh, sponges and the mussel byssus can being considered as sources of col-
lagen (Olsen et al., 2003).
Collagens are composed of three polypeptide chains (α-chains) with a
characteristic sequence of (Gly–X–Y), where X and Y are usually proline
and 4-hydroxyproline, respectively. The three α-chains twist around a
common axis into a right-handed triple helix and the amino-acid sequences
of three α chains can be the same or different. To date, 27 types of human

collagen have been identified, differing in primary sequence chemistry. Of
them, depending on morphology and function, collagens are classifi ed into
eight groups, fibril-forming collagens (types I, II, III) being the most exten-

sively documented (Huang et al., 2007).
The key to producing thermally stable collagens, with a melting tem-
perature 39–40 °C (the normal melting temperature of human collagen) and
with a proper triple helical conformation, in recombinant expression systems
relies on the ability to effect appropiate post-translational processing of the
recombinant collagen proteins. Recombinant collagens have been produced
by transfected mammalian cells, insect cells, yeast, E. coli, transgenic tobacco,
mice and silkworms. Of them, only mammalian cells transfected with a col-
lagen gene and not with the enzyme prolyl-4-hydroxylase (PH4) genes
expressed hydroxylated full-length collagens. In the other expression
systems, overexpression of PH4 was required for the production of fully
hydroxylated collagen (Olsen et al., 2003).
Byssus threads of marine mussels are interesting elastomeric fi bers with
a great capacity for absorbing and dissipating energy that have been
reported to have three distinct collagenous proteins in the thread, preCol
P, preCol D and preCol NG, the last being a central collagen domain (stiff

segment), and the first two being flanking domains having distinctive struc-

tural properties: preCol P has elastin-like flanking domains (soft segment)

and preCol D has stiffer silk fibroin-like domains (hard segment) (Vaccaro

and Waite, 2001). In common with other protein elastomers such as elastin,
resiline and abductine, byssus threads are quite tough. Their Young’s
modulus is low but the extensibility can be as high as 200%. Byssus threads
have been reported where up to 70% of the total absorbed energy can be
dissipated. Byssal threads are at least five times more extensible and fi ve

times tougher than an Achilles tendon. Indeed, unlike the tendon, byssal
threads have a non-periodic microstructure and shrinkage and melting
temperatures in excess of 90 °C (Qin et al., 1997).

Purified collagens are capable of undergoing spontaneous alignment to
form fibrils that have defined features characteristic of collagen fi bers

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