Biology Reference
In-Depth Information
activated forms of MMP-9 (Rosenblum et al.
2007
). Almost immediately upon
association, a fifth ligand joined in coordination of the zinc, probably the carboxyl
group of the key glutamate in the active site. By a quarter of a second after
association, the cysteine
thiolate
coordination of the zinc by the prodomain was
lost (Rosenblum et al.
2007
). Simulation suggested that this should allow water to
penetrate and coordinate the zinc and allow the presumed glutamate carboxylate
ligand to dissociate a little more than a second after association with kallikrein.
The evidence for adjustable depth of the helix floor of the active site and adjustable
breadth of the cleft suggested the dimensions of the active site to be malleable
(Bhaskaran et al.
2007
). This would provide a context where changes in zinc coor-
dination and distances are plausible. The association of physiological activating
partners then appears to have the potential to trigger allosteric and dynamic changes
in the active site that could initiate activation of MMPs (Rosenblum et al.
2007
).
6.1.2 Activation of Pro-MMP-7 by Glycosaminoglycans
An important means of anchoring MMP-7 (matrilysin) to epithelial cell surfaces
appears to be its affinity for GAG such as heparin sulfate (HS) (Yu and Woessner
2000
) and HS-containing proteoglycan CD44 (Yu et al.
2002
). Heparin from mast
cells is more sulfated than HS and affects the activity of several proteases and
inhibitors from the blood coagulation cascade (Capila and Linhardt
2002
). The
autolytic activation and activity of pro-MMP-1 and -2 were found to be enhanced
by heparin (Crabbe et al.
1993
,
1994
). Heparin tightly bound MMP-7 and activated
rat pro-MMP-7 (which it bound even more tightly) by greater than 15-fold in vitro
while leaving the prodomain intact (Yu and Woessner
2000
,
2001
). A recent study
found that association with heparin, chondroitin-4,6-sulfate (~500 nM
K
d
values),
and dermatan sulfate accelerated the intermolecular autolytic activation of pro-
MMP-7 even more dramatically, as well as accelerating activation of MMP-7's
processing of pro-
-defensins to their mature antibacterial form (Ra et al.
2009
).
Chondroitin-4,6-sulfate is a component of serglycin. Although activated MMP-7
retained high affinity (150 nM
K
d
) for heparin, its affinity for chondroitin-4,
6-sulfate (60
a
M
K
d
) was much weakened by the absence of the prodomain (Ra et al.
2009
). Since activation of a pro-MMP requires disruption of the bond between
catalytic Zn
2+
and conserved cysteine thiolate from the prodomain (Rosenblum
et al.
2007
), an important question is how does GAG binding promote this
activation.
Binding of GAGs was predicted to occur around the swath of basic residues that
cradles the catalytic domain of MMP-7 (Yu and Woessner
2000
). These lysines and
arginines wrap around 60% of the circumference of the catalytic domain most
distant from the active site (Fig.
6.1
). They are distributed in the amphipathic
helices A and C,
m
-strand II, and six loops (Fig.
6.1
). The zymogen form having
enhanced affinity for heparin (Yu and Woessner
2000
) and 100-fold higher affinity
for chondroitin-4,6-sulfate (Ra et al.
2009
) suggested that these GAGs interact with
b
