Biology Reference
In-Depth Information
it is accelerated during the process of tissue remodelling and wound healing. For
microbes to invade into host animal tissues, fibrillar collagens are barriers. Effec-
tive degradation of native collagens requires enzymes that cleave their triple
helical regions, but only a limited number of collagenases are found in vertebrates,
e.g., collagenolytic MMPs and cathepsinKinmammals.Innon-vertebrates,
including microbes, there are a number of proteinases reported to have collageno-
lytic activity, and the bacterial collagenases and crab collagenases have been well
studied. One common feature of these enzymes, regardless of the mechanisms of
peptide bond hydrolysis, is that triple helicase activity is assisted by the non-
catalytic ancillary domains, as shown for mammalian MMPs and bacterial
collagenases. Cathepsin K also needs to be periodically assembled on a chondroi-
tin 4-sulphate glycosaminoglycan chain to be an effective collagenase. Similarly,
dimerization or oligomerization of the cell surface-bound MMP-14 is important to
be a functional collagenase. Another key aspect of collagenolysis is that collagens
need to be partially unwound to be cleaved by those proteinases, as triple helical
strands cannot access the active site of the enzyme in the native collagen structure.
An exception is crab collagenase, which accommodates of the triple helix in the
active site (Perona et al.
1997
). While there is a debate as to whether collagenases
unwind triple helical collagen before they cleave triple helical chains or a partial
unfolding of collagen is sufficient, the currently available data with MMPs suggest
that at least vertebrate collagenases appear to have an ability to unwind triple
helical structure in full-length forms. The next challenge is to elucidate how these
collagenolytic enzymes unwind native collagens.
Thermal stability of collagens is an important factor in collagenolysis. When
collagen is completely unwound, many non-specific proteinases can cleave the
denatured collagen. Collagenases prefer triple helical structure compared with
heat-denatured gelatins. At 37
C native collagen I is much more readily cleaved
by MMP-1 than gelatin. However, below 10
C, little collagenolytic activity is
detected, whereas gelatinolytic activity can be detected (Chung et al.
2004
). The
activation energy of human MMP-1 calculated for human type I collagen was
49 kcal mol
1
and that for gelatin was 14 kcal mol
1
, suggesting that heat makes
an important contribution to collagenolysis (Welgus et al.
1981b
). Therefore, the
induction of a correct secondary and tertiary structure in the substrate is contributed
by cooperation between the heat and the full-length collagenase-collagen interac-
tion. Understanding the exact molecular mechanism by which collagenases unwind
triple helical collagen requires investigations of where collagen binds to the
enzymes and how the
2(I) chain fits into the active site of the enzyme at different
temperatures using the biophysical and structural analyses along with mutagenesis
studies.
a
Acknowledgements We thank Dr. Joseph P.R.O. Orgel for provision of atomic coordinates of
microfibillar structure of rat tail tendon collagen. This work was supported by grants from the
Wellcome Trust and Arthritis Research, UK.
