Structural Basis of Extracellular Matrix Interactions with Matrix Metalloproteinases - Extracellular Matrix Degradation - page 131

functional importance in specificity of the exosite nearest to the active site, and
(c) unwinding of the triple helix for cleavage (Bhaskaran et al. 2008 ). The hypoth-
esis of bending-induced unwinding of the triple helix was likened to transcription
factors' bending of DNA that promotes underwinding of the duplex and strand
separation to open a transcription bubble (Bhaskaran et al. 2008 ).
Most consequential to triple-helical peptidase activity was the introduction of a
mere hydroxyl group by the F185Y substitution of MMP-12 (in the IV-V loop) that
knocked down the catalytic efficiency of 19-fold toward the

1(V) 436-447 fTHP
and seven-fold toward fluorescently labeled collagen IV (Palmier et al. 2010 ). Also
supporting triple peptidase activity are locations in neighboring loops such as the
S-shaped III-IV loop (G178 and I180 in MMP-12), the B-C loop (G227), and the
S1 0 specificity loop (T239 and K241) (Palmier et al. 2010 ). Howmuch these sequence
positions might support the triple-helical peptidase activities of collagenolytic MMPs
more generally may prove interesting and relevant for targeting of diagnostic and
therapeutic agents.

a

6.3 Fibronectin-Like Modules: Ties That Bind, Distort,
and Unwind

Identification of exosites in MMPs has usually instead referred to their presence
in domains other than the catalytic domain, such as in the insertion of modules
resembling fibronectin II (FnII) or the C-terminal HPX domain (Overall 2001 ,
2002 ; Tam et al. 2004 ; Gioia et al. 2009 ). MMP-2 and -9 (72 and 92 kDa type IV
collagenases or gelatinases A and B) contain three FnII-like modules of 58 residues
each inserted at the aforementioned V-B loop of the catalytic domain. MMP-2 and -9
apparently required their inserts of FnII-like modules to hydrolyze elastin (Shipley
et al. 1996 ), collagen (Collier et al. 1992 ; O'farrell and Pourmotabbed 1998 ; Tam
et al. 2004 ), and THP mimics of collagens (Lauer-Fields et al. 2008 ). The insert of
FnII-like modules from MMP-2 is often called its collagen-binding domain (CBD)
(Steffensen et al. 1995 ; Gioia et al. 2009 ; Xu et al. 2009 ).

6.3.1 Exosites That Interact with Intact and Denatured
Collagen Triple Helices

Early work of Goldberg's group localized a gelatin-binding exosite in the second
FnII-like module of MMP-9 (Collier et al. 1992 ); this maps in crystal structures
to the cyan-colored residues at top in the view of Fig. 6.4 . Compared with MMP-9,
the CBD domain of MMP-2 generally binds protein fibrils more tightly with
dissociation constants appearing to be in the low

M range for both intact and
denatured forms of collagens I, IV, and V, as well as elastin (Steffensen et al. 1995 ).

m