4. Representación gráfica y definición constructiva y de los sistemas de la vivienda
4.2. Definición constructiva y de los sistemas de la vivienda
All MMP family members have conserved structural and functional features (Das et al., 2003). These include a catalytic domain that possesses the zinc binding site; an N-terminal pro-peptide domain; and a hemopexin-like
C
-terminal domain that is linked to the catalytic domain by a hinge region (Westermarck and Kahari, 1999, Massova et al., 1998). The overall structure and conserved subunits are described in Figure 1.2.Figure 1-2 Structure of MMP. A) Crystallographic image of basic MMP structure showing propeptide region, catalytic domain and haemopexin -like domain. The orange sphere in the catalytic domain represents catalytic zinc whereas the blue spheres in the catalytic and in the haemopexin like domains represent calcium ions. Green chain represents propeptide region.
Hinge region that links the two main domains catalytic and haemopexin-like is represented in white. B) Illustrative figures of the MMPs. The catalytic domain is represented in pink and in case of MMP-2 and MMP-9, this domain has gelatin binding domain unlike all other MMPs (Massova et al., 1998).
The main function of the pro-peptide region is to preserve the latency of the MMPs until they needed to be activated (Das et al., 2003). The pro-peptide domain is comprised of 80–90 amino acids holding a ‘’cysteine switch’’ motif, a conserved unique PRCG(V/N)PD sequence (Nagase and Woessner, 1999). The thiol group in the cysteine residue interacts with the zinc atom in the catalytic domain making the MMP inactive (Massova et al., 1998, Becker et al., 1995).
The catalytic domain is approximately 170 amino acids in length (Evrosimovska et al., 2011).
This domain contains a conserved zinc-binding motif, HEXXHXXGXX (H/D), and a methionine residue at the centre of a conserved loop referred to as the Met-turn (Gomis-Ruth, 2009, Cerda-Costa and Gomis-Ruth, 2014). Activity of the MMP is dependent upon chelated ions, namely of two Zinc ions and at least one calcium ion, which are coordinated to various residues within the catalytic domain (Massova et al., 1998). One of the two zinc atoms is located in the active site of the enzyme, where it functions in the proteolytic activity of MMPs (Bode et al., 1994). The other zinc atom (also called the structural zinc atom) and the calcium atom are present in the catalytic domain approximately 12 Å away from the catalytic zinc (Massova et al., 1998). Not a lot is known about the function role of the structural zinc or the calcium atom, but their highly conserved presence in the majority MMPs suggests an important, as yet undiscovered, function (Bode et al., 1994).
The final major domain within MMPs is the haemopexin-like domain, which consists of approximately 210 amino acids (Evrosimovska et al., 2011). The sequence of haemopexin-like domain is similar to the plasma protein haemopexin (Massova et al., 1998). It is responsible for binding of either substrate molecules or the tissue inhibitors of metalloproteinases (TIMPs), which are endogenous inhibitors of MMPs (Borden and Heller, 1997, Massova et al., 1998). Even though only the catalytic domain has the proteolytic activity, this haemopexin domain is a prerequisite for MMP functionality, allowing these
collagenases to break down the triple helical interstitial collagens (Nagase and Woessner, 1999). In addition, this domain is required for pro-MMP-2 activation by MT1- MMP (Nagase and Woessner, 1999). While the haemopexin-like domain is considered an obligate characteristic for all MMPs, it is worth noting that two MMP family members are exceptions to this rule: MMP-23 has cysteine-rich, proline- rich and IL-1 receptor-like regions instead of the haemopexin domain (Nagase and Woessner, 1999); the smallest MMP, matrilysin (MMP-7), lacks any form of haemopexin-like domain (Westermarck and Kahari, 1999).
As shown in Figure 1.2, a hinge region exists between the haemopixin-like domain and the catalytic domain. The classic length of this hinge region is 16 amino acid residues, and it is rich in proline residues (Evrosimovska et al., 2011). Although the reason for this proline rich nature is not clear, it is probably necessary to add the correct level of twist in the protein secondary structure, creating the hinge (Nagase and Woessner, 1999).
In contrast to the conservation of these three major domains among all MMP family members, there are a number of subsidiary motifs that are only found in selected MMPS (Figure 1.2). For example, the gelatinases MMP-2 and MMP-9 contain three repeats of a fibronectin type-II like motif within the catalytic domain, which has been reported to be implicated in the binding of gelatin within the active site (Murphy et al., 1994). This domain is therefore commonly referred to as the gelatin binding domain, or fibronectin type-II like domain. Its presence is specific for gelatinases (Massova et al., 1998). Moreover, MT-MMPs except MT4-MMP have a trans-membrane domain that is comprised of hydrophobic acids in the C-terminal end of the hemopexin domain. It is approximately 25 amino acids in length and acts to anchor these MMP into the plasma membrane. In addition, MT-MMPs possess a recognition motif (RXKR) for furin-like convertases at the end of the pro-peptide domain, increasing its substrate repertoire (Puente et al., 1996, Sato et al., 1996). Stromelysin 3
(MMP-11) is similar to MT-MMPs, being activated intracellularly by furin due to its furin recognition motif (Nagase and Woessner, 1999, Massova et al., 1998).
In spite of the diversity that MMPs show in their structural domains and substrate specificity, the MMP have common characteristics. First, they are all thought to be zinc dependant.
Secondly, the inhibition of MMP is by tissue inhibitors of metalloproteinase, the natural inhibitors, which form inactive 1:1 complexes with these enzymes. Lastly, the MMPs are synthesized in inactive form that requires activation to enable their function (Van Wart and Birkedal-Hansen, 1990).