Food Chemistry

582 12 Meat

noline content was higher than in extensively fat-
tened animals.
Histidine can also be involved in cross-linking re-
actions, as shown by the detection of histidino-
hydroxylysino-norleucine:

(12.18)

The amino acid pentosidine was also obtained
from collagen, which indicates the bonding of ly-
sine and arginine with the participation of a pen-
tose:

(12.19)

The outlined reactions can also occur with hy-
droxylysine residues present on collagen fibers.
Of all the compounds mentioned, hydroxylysino-
norleucine 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-ter-
minus of these precursors contains up to 25%
of extendedα^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 pep-
tides 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 sta-
bilize 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 toBorn-

stein, 1974).aPolysome,bhydroxylation,cchain

straightening,ddisulfide bond formation,ecell mem-

brane,fmembrane crossing,ga limited hydrolysis to

tropocollagen,hcollagen fiber formation, cross-linking

collagen is converted to tropocollagen (g). Fi-

nally, the tropocollagen is realigned to form col-

lagen fibers (h). At this stage, collagen matura-

tion, which coincides with strengthening of col-

lagen fibers by covalent cross-linking along the

peptide strands, begins. The maturation is initi-

ated by oxidation of lysine and is followed by the

reactions described above.

Collagen swells but does not solubilize. Enzymat-

ically, it can be hydrolyzed to various extents with