Biology Reference
In-Depth Information
5.5 The Triple Helicase Activity of Collagenases
When the 3D structures of the catalytic domains of MMP-1 and MMP-8 were
reported in 1994 and the full-length pig MMP-1 in 1995 (Li et al. 1995 ), it became
apparent that the active site of the enzyme cannot accommodate the triple helical
collagens because the entrance of the active site is only 5 ˚ wide, and the diameter
of the collagen triple helix is 15 ˚ (Bode 1995 ). Simple docking of a triple helical
peptide based on type III collagen (Kramer et al. 2001 ) covering the C-terminal of
the collagenase-cleavage site does not allow the peptide bond to access the catalytic
zinc ion. The closest peptide bond is
7 ˚ away from the catalytic zinc atom. In
addition, due to the spatial orientation of three left-handed
a
chain in the native collagen does not engage into the subsite pockets of the enzyme.
Effective peptide hydrolysis by MMPs utilizes extended substrate binding subsites
(Nagase 2001 ) and favours a
a
chains, each single
-strand-like structure which forms numerous hydro-
gen bonds with the enzyme (Grams et al. 1995 ). It has therefore been postulated that
either the active site of collagenase undergoes large conformational changes to
accommodate triple helical collagen or the collagen needs to be unwound for a
single
b
chain to be presented to the active site to initiate collagenolysis. A number
of hypotheses have been proposed for the latter (Bode 1995 ; de Souza et al. 1996 ;
Gomis-R
a
uth et al. 1996 ; Ottl et al. 2000 ; Overall 2002 ). This includes the “proline
zipper” model (de Souza et al. 1996 ) in which the proline-rich linker of collage-
nases interacts with collagen and unwinds the triple helical collagen, and the
“collagen-trapping” model in which the Hpx folds over the Cat domain sandwich-
ing collagen (Gomis-Ruth et al. 1996 ). Other possibilities are that collagen may
unwind spontaneously around the cleavage site or unwinding may be induced by
collagenases. These two models are currently being considered.
5.5.1 The Collagenase-Induced Collagen Unwinding Model
The evidence that collagenase unwinds triple helical collagen before hydrolyzing
the peptide bond was presented by Chung et al. ( 2004 ) using the MMP-1 and guinea
pig collagen I system at 25 C as a model. They postulated that if MMP-1 actively
unwinds triple helical collagen, the MMP-1 mutant that lacks the ability to cleave
peptide bonds should retain collagen-unwinding activity. If so, collagen I interact-
ing with such a mutant becomes susceptible to non-collagenolytic proteases. The
mutation of the catalytically essential Glu200 of MMP-1 to Ala [MMP-1(E200A)]
inactivated its peptidolytic activity. The catalytic domain of MMP-1 (MMP-1Cat),
MMP-3Cat and a serine proteinase, neutrophil elastase does not cleave collagen I
unless triple helical chains are unfolded and could be used as a cutter enzyme of
unwound collagen. Their studies showed that, in the presence of MMP-1(E200A),
non-collagenolytic proteinases cleaved collagen I into typical
and ¼ fragments
in a dose- and time-dependent manner. The study has also suggested that MMP-1
¾
Search WWH ::




Custom Search