Biology Reference
In-Depth Information
catalytic to HPX domains could enable the MMP to spread out across collagen
fibrils. Inchworm-like movement could then be possible if the domains were to
approach each other; this could provide a way for an MMP to ratchet its way
processively down the lengthy fibril to scan for sites suitable for hydrolysis (Saffar-
ian et al.
2004
; Overall and Butler
2007
). Understanding of the structural organiza-
tion of the collagen I fibril (Orgel et al.
2006
) led to an important recent model for
initiation of collagenolysis in which proteolytic removal of the C-terminal telopep-
tide from the surface exposes the
3
=
4
to ΒΌ cleavage site (Perumal et al.
2008
). The
resulting groove accommodates the splayed out orientation of the catalytic and
HPX domains upon the exposed and more vulnerable
2 chain (Perumal et al.
2008
). Inchworm-like ratcheting motion of the collagenase can then be envisioned
within the groove.
a
6.4.4 Mapping of Sites of Collagen Triple Helix Binding
in HPX Domain
E200A-inactivated, full-length MMP-1 (having both catalytic and HPX domains)
was incubated with a THP model of the MMP-1 cleavage in type I collagen
designated
1(I) 772-786 THP. Association with this THP protected three seg-
ments of the HPX domain in hydrogen-deuterium exchange mass spectrometry
(HDXMS) of peptide fragments of MMP-1(E200A) (Fig.
6.7
) (Lauer-Fields et al.
2009
). This suggests that the hydrogen bonding of these three epitopes of the
HPX domain was stabilized by direct contact with
a
1(I) 772-786 THP, or alter-
natively by indirect effects of its association. THP-protected Ile287-Met295 lie
within the innermost
a
-propeller
fold of the HPX domain (Fig.
6.7
). THP-protected Arg304-Phe316 form the outer
edge of this first blade. THP-protected Phe439-Leu457 form a large section of
the adjoining fourth blade (Fig.
6.7
). R291A substitution in the first blade impaired
the catalytic efficiency of MMP-1 toward three THP substrates and collagen I,
confirming the participation of that part of the first blade in collagenolysis (Lauer-
Fields et al.
2009
).
In crystals, active MMP-1 had a more wide open arrangement (Fig.
6.7
) than the
latent proform in which the loop of the HPX domain containing Phe308 especially
was rotated toward the catalytic domain, as if it pivoted about Arg300 (Fig.
6.7
)
(Iyer et al.
2006
). Reflecting upon this proposed pivoting, the significant interdo-
main mobility in solution (Bertini et al.
2009b
) and the THP-protected surfaces
across HPX
-hairpin of the first blade of the four-bladed
b
b
-propeller blades 1 and 4 (Lauer-Fields et al.
2009
) raises the question
of whether the catalytic and HPX domains might rotate so as to clamp down upon
an exposed collagen triple helix. Such a rotation and grip would be consistent with
the original hypothesis of the catalytic and HPX domains sandwiching the triple
helix between them (Bode et al.
1994
; Bode
1995
). Such a rotation could be
postulated to be upward and counterclockwise in the view of Fig.
6.7
, about an
b
