2009_Book_FoodChemistry

582 12 Meat
noline content was higher than in extensively fattened
animals.
Histidine can also be involved in cross-linking reactions,
as shown by the detection of histidinohydroxylysino-norleucine:
(12.18)
The amino acid pentosidine was also obtained
from collagen, which indicates the bonding of lysine
and arginine with the participation of a pentose:
(12.19)
The outlined reactions can also occur with hydroxylysine
residues present on collagen fibers.
Of all the compounds mentioned, hydroxylysinonorleucine
and dihydroxylysino-norleucine
have been isolated from collagen in significant
amounts.
In the case of type I, collagen biosynthesis
(Fig. 12.20a–h) involves first the synthesis of
pro-α 1 - and pro-α 2 -precursor chains. The N-terminus
of these precursors contains up to 25%
of extended α 1 -andα 2 -chains (a). Immediately
after the chains are released from polysomes,
hydroxylation of the proline and lysine residues
occurs (cf. reactions under 12.20).
Realignment of the chains follows: two strands of
pro-α 1 and one chain of pro-α 2 are joined to form
a triple-stranded helix (b–d). The extended peptides
at the N-terminus appear to play a distinct
role in these reactions. Disulfide bridging occurs
between the strands at this stage in order to stabilize
the structure. The procollagen thus formed
will cross the membrane of the cell in which it
was synthesized (e). The N-terminal peptides are
removed by limited proteolysis (f) and the pro-
Fig. 12.20. Collagen biosynthesis (according to Bornstein,
1974). a Polysome, b hydroxylation, c chain
straightening, d disulfide bond formation, e cell membrane,
f membrane crossing, g a limited hydrolysis to
tropocollagen, h collagen fiber formation, cross-linking
collagen is converted to tropocollagen (g). Finally,
the tropocollagen is realigned to form collagen
fibers (h). At this stage, collagen maturation,
which coincides with strengthening of collagen
fibers by covalent cross-linking along the
peptide strands, begins. The maturation is initiated
by oxidation of lysine and is followed by the
reactions described above.
Collagen swells but does not solubilize. Enzymatically,
it can be hydrolyzed to various extents with