Biology Reference
In-Depth Information
substrate. The exact mechanism of collagenolysis by the cathepsin K-C4-S complex
needs to be further investigated.
5.10 Non-vertebrate Collagenases
5.10.1 Crab Collagenase
Certain trypsin-like serine proteinases in hepatopancreas of the decapod crustacea
such as crab, shrimp, krill, crayfish and lobster have the ability to cleave triple
helical collagen I. Those enzymes are called “brachyurins” and they are thought to
be involved in the digestion of foods. There are at least three types in the brachyur-
ins: type Ia with broad specificity with activity similar to trypsin, chymotrypsin and
elastase; type Ib with broad specificity but less trypsin-like activity; and type II with
strictly trypsin-like specificity. Fidler crab collagenases belonging to type Ia and Ib
have been characterized for their activities to cleave collagen types I-V (Welgus
et al.
1982
; Welgus and Grant
1983
). At 25
C, all collagen types were cleaved by
both types of crab collagenase at multiple sites. However, the main sites cleaved in
type I collagen were a few residues in the C-terminal side of the mammalian
collagenase cleavage site (Gln-Arg, Arg-Gly, Leu-Gly in the
a
1(I) chain and
Leu-Gly in the
2(I) chain), suggesting that looser triple helicity makes it more
susceptible to crab collagenase (Tsu et al.
1994
). The Leu
587
-Thr
588
a
1(I)
chain was also cleaved (Tsu et al.
1994
). The 3D structure of crab collagenase has
been solved in complex with the
Escherichia coli
serine proteinase inhibitor ecotin
(Perona et al.
1997
). Ecotin forms a tetrameric complex with the enzyme in a 2:2
molar stoichiometry, and the global structure of crab collagenase is similar to that
of vertebrate trypsins with conservation of the double
in the
a
-barrel core and the catalytic
triad (His
57
, Asp
102
and Ser
195
). However, the specificity of crab collagenase for
collagen substrates is probably due to the subtle modification of several surface
loops creating an extended S7-S4
0
substrate binding site. Molecular modelling
indicates that this structure accommodates the collagen sequence with little or no
arrangement of side chain groups and of main chain atoms (Perona et al.
1997
). An
equivalent structure is not found in other serine proteinases, suggesting that crab
collagenase has adapted the ability to digest collagens. However, it is notable
that collagen concentrations which saturate mammalian MMP-1 failed to saturate
crab collagenase, indicating that the affinity of the crab enzyme for native collagen I
is considerably lower than mammalian collagenases (Welgus et al.
1982
).
b
5.10.2
Clostridium histolyticum
Collagenases
C. histolyticum
is an anaerobic spore-forming bacterium that causes gas gan-
grene. All strains of histotoxic clostridia produce collagenases. There are two
