12.3 Muscle Tissue: Composition and Function 583(12.20)a series of collagenases from different sources
and with different specificities. A vertebrate an-
imal collagenase, which is a metal proteinase,
splits a special bond in native collagen while the
collagenase fromClostridium histolyticum,also
a metal proteinase, cleaves collagen preferentially
at glycine residues, forming tripeptides:
↓↓
−Pro−X−Gly−Pro−X−Gly−Pro−
(12.21)Collagenase enzymes which are serine pro-
teinases are also known.
Denatured collagen, as formed post-mortem by
the action of lactic acid, can also be cleaved by
lysosomal enzymes, e. g., lysosomal collagenase
and cysteine proteinase cathepsin B 1. Thermally
denatured collagen is attacked by pepsin and
trypsin.
One characteristic of the intact collagen fiber is
that it shrinks when heated (cooking or roast-
ing). The shrinkage temperature (Ts)isdiffer-
ent for different species. For fish collagen, the
Tsis 45◦C and for mammals, 60–65◦C. When
native or intact collagen is heated to T>Ts,
the triple-stranded helix is destroyed to a great
extent, depending on the cross-links. The dis-
rupted structure now exists as random coils which
are soluble in water and are called gelatin. De-
pending on the concentration of the gelatin so-
lution and of the temperature gradient, a transi-
tion into organized structures occurs during cool-
ing. Figure 12.21 schematically summarizes these
transitions. At low concentrations, intramolecular
back-pleating occurs preferentially with single-
strands. At higher concentrations and slow rates
of cooling, a structure is rebuilt which resem-
bles the original native structure. At even higher
concentrations and rapid cooling, structures are
obtained in which the helical segments alternate
with randomly coiled portions of the strand. All
these structures can immobilize a large amount of
water and form gelatin gels.Fig. 12.21.Collagen conversion into gelatin. (accord-
ing toTraubandPiez, 1971). Ts: shrinkage tempera-
ture, T: temperature, c: concentration; (see text)