BLBS102-c20 BLBS102-Simpson March 21, 2012 13:35 Trim: 276mm X 219mm Printer Name: Yet to Come
20 Fish Collagen 367and XI are built up of three chains and all are composed of the
continuous triple-helical structure (Lee et al. 2001). Types I, II,
III, and V are also called as fibril-forming collagens and have
large sections of homologous sequences independent of species
(Timpl 1984). Foegeding et al. (1996) reported that types IV and
III collagens contain oxidizable cysteine residues and is rich in
hydroxyproline and hydroxylysine. In type IV collagen (base-
ment membrane), the triple-helical conformation is interrupted
with large nonhelical domains as well as with the short nonheli-
cal peptide interruption (Lee et al. 2001). Like type IV collagen,
type V collagen is rich in hydroxyproline and hydroxylysine but
contains no cysteine. Type VI is microfibrillar collagen, and type
VII is anchoring fibril collagens (Samuel et al. 1998).
Based on its macromolecular structures, collagen can be di-
vided into three major groups: (a) striated fibrous collagen,
which includes types I, II, and III collagen, (b) nonfibrous col-
lagen, which contains type IV or basement membrane collagen,
and (c) microfibrillar collagen, which encompasses types VI and
VII (the matrix microfibrils), types V, IX, and X (the pericellu-
lar collagen), and types VIII and XI, which are yet unclassified
(Xiong 1997). Fibril-associated collagens (types IX, XI, XII,
and XIV) have small chains, which contain some nonhelical
domains (Lee et al. 2001). Each is characterized by having the
triple-helical motif as part of its structure (Ramshaw et al. 2009).
In the extracellular space, collagen molecules align them-
selves into microfibrils in quarter stagger array. Cross-linking
is initiated and larger diameter fibrils are formed either by
the addition of microfibrils or by association with other fibrils
(McCormick 1994). During maturation or aging, collagen fibers
can be strengthened and are stabilized primarily by covalent
cross-linkages (Belitz et al. 2004). Thus, the cross-links confer
mechanical strength to collagen fiber. Belitz et al. (2004)
proposed that cross-link formation involves (i) enzymatic
oxidation of lysine and hydroxylysine to the corresponding
ω-aldehydes, (ii) conversion of these aldehydes to aldols and
aldimines, and (iii) stabilization of these primary products by
additional reduction or oxidation reactions. The cross-linking
stereochemistry derives from the reaction of specific peptidyl
aldehydes, in the NH 2 - and COOH-terminal nonhelical peptide,
with vicinalε-amino groups of specific peptidyl residues of
Lys and Hyl, located in the triple-helical regions of molecules
(Mechanic et al. 1987). Intermolecular cross-links are confined
to the end-overlap region involving a lysine aldehyde in the
telopeptide of one chain and a hydroxylysine of an adjacent
chain (Foegeding et al. 1996). Furthermore, Avery and Bailey
(2005) proposed that the slow turnover of mature collagen
subsequently allows accumulation of the products of the
adventitious nonenzymic reaction of glucose with the lysines
in the triple helix to form glucosyl lysine and its Amadori
product. These products are subsequently oxidized to a complex
series of advanced glycation end-products; some of which are
intermolecular cross-links between the triple helices, rendering
the fiber too stiff for optimal functioning of the collagen fibers,
and, consequently, of the particular tissue involved.
The number of cross-links in collagen increase with increas-
ing age of the animal (Zayas 1997, Belitz et al. 2004). Most
connective tissue in fish is renewed annually, and highly cross-linked protein is not generally found in fish (Foegeding et al.
1996). Collagenous tissue from older animals with more cross-
linkages is more resistant to swelling and has a lower water-
holding capacity (Zayas 1997). The steady increase in mature
collagen cross-linking is due to progressive and ongoing cross-
linking reactions that occur within fibrillar collagen and with the
slowing of collagen synthesis rates as animals reach maturity
(McCormick 1994). Collagen from young animals is more eas-
ily solubilized and produces structures with low tensile strength.
In contrast, collagen from old animals is difficult to solubilize
and produces a structure with high tensile strength (Miller et al.
1983). In starving fish, the sarcoplasmic and myofibrillar pro-
teins undergo gradual degradation, while the connective tissues
are not utilized, or extra collagen is even deposited in the my-
ocommata and in the skin (Sikorski et al. 1990). Thus, the tough-
ening process in fish seems to be much more reversible than that
of higher animals, where the amount of cross-linking increases
with age (Regenstein and Regenstein 1991). Foegeding et al.
(1996) reported that starving fish produce more collagen, espe-
cially collagen with a greater degree of cross-linking, than the
fish that are well fed. Furthermore, collagens in myocommata
are thickened with more intermolecular cross-links of collagen
during starvation (Love et al. 1976).
Glycine represents nearly one-third of the total residues, and
it is distributed uniformly at every third position throughout
most of the first collagen molecule. The repetitive occurrence
of glycine is absent in the first 14 amino acid residues fromN-
terminus and the first ten residues from theC-terminus, in which
these end portions are termed “telopeptides” (Foegeding et al.
1996). The presence of glycine at every three residues is a criti-
cal requirement for the collagen superhelix structure (Regenstein
and Zhou 2007). Glycine contains no side chain, which allows
it to come into the center of the superhelix without any steric
hindrance to form a close packing structure (Te Nijenhuis 1997).
The triple helix of collagen assembled from specific polypeptide
chain (α-chain) with the Gly-X-Y repeat and contain frequent
occurrence of proline (Pro) and hydroxyproline (Hyp) in theX-
andY-positions, respectively (Johnston-Banks 1990, Xu et al.
2002). Hydroxyproline is located atY-position, as is hydroxyly-
sine, while proline can be found in either theX-orY-positions
(Johnston-Banks 1990). The distribution of polar and nonpolar
residues in theX-position determines the ordered aggregation
of molecules into fibrils (Xiong 1997). The segments of the
polypeptide chain consisting of repeating triplets with imino
acid residues are the nonpolar regions, and the segments con-
taining Gly-X-Y triplets without imino acids are mostly polar
(Wong 1989). Collagen is a hydrophilic protein because of the
greater content of acidic, basic, and hydroxylated amino acid
residues than lipophilic residues. Therefore, it swells in polar
liquids (Johnston-Banks 1990).
The amino acid composition of collagen is unique. The col-
lagens are rich in glycine, proline and alanine, and contained
low or no cysteine and tryptophan (Table 20.2). Relatively low
contents of tyrosine and histidine are reported in collagen. Two
amino acids that are not commonly present in other proteins
include hydroxyproline and hydroxylysine (Wong 1989). The
amino acid composition of collagen is nutritionally unbalanced,