Matrix metallopeptidase


Matrix metallopeptidases, also known as matrix metalloproteinases or matrixins, are metalloproteinases that are calcium-dependent zinc-containing endopeptidases; other family members are adamalysins, serralysins, and astacins. The MMPs belong to a larger family of proteases known as the metzincin superfamily.
Collectively, these enzymes are capable of degrading all kinds of extracellular matrix proteins, but also can process a number of bioactive molecules. They are known to be involved in the cleavage of cell surface receptors, the release of apoptotic ligands, and chemokine/cytokine inactivation. MMPs are also thought to play a major role in cell behaviors such as cell proliferation, migration, differentiation, angiogenesis, apoptosis, and host defense.
They were first described in vertebrates, including humans, but have since been found in invertebrates and plants. They are distinguished from other endopeptidases by their dependence on metal ions as cofactors, their ability to degrade extracellular matrix, and their specific evolutionary DNA sequence.
Recently matrix metalloproteinases have been proposed as markers of many pathological conditions for their ability to degrade extracellular matrix components and remodel tissues. Herein, MMPs have been reported as one of the main factors of cancer progression and metastasis formation. This led to the emerging research field in biosensors development for the detection of such enzymes.

History

MMPs were described initially by Jerome Gross and Charles Lapiere, who observed enzymatic activity during tadpole tail metamorphosis. Therefore, the enzyme was named interstitial collagenase.
Later, it was purified from human skin, and was recognized to be synthesized as a zymogen.
The "cysteine switch" was described in 1990.

Structure

The MMPs have a common domain structure. The three common domains are the pro-peptide, the catalytic domain, and the haemopexin-like C-terminal domain, which is linked to the catalytic domain by a flexible hinge region.

The pro-peptide

The MMPs are initially synthesized as inactive zymogens with a pro-peptide domain that must be removed before the enzyme is active. The pro-peptide domain is part of the "cysteine switch." This contains a conserved cysteine residue that interacts with the zinc in the active site and prevents binding and cleavage of the substrate, keeping the enzyme in an inactive form. In the majority of the MMPs, the cysteine residue is in the conserved sequence PRCGxPD. Some MMPs have a prohormone convertase cleavage site as part of this domain, which, when cleaved, activates the enzyme. MMP-23A and MMP-23B include a transmembrane segment in this domain.

The catalytic domain

structures of several MMP catalytic domains have shown that this domain is an oblate sphere measuring 35 x 30 x 30 Å. The active site is a 20 Å groove that runs across the catalytic domain. In the part of the catalytic domain forming the active site there is a catalytically important Zn2+ ion, which is bound by three histidine residues found in the conserved sequence HExxHxxGxxH. Hence, this sequence is a zinc-binding motif.
The gelatinases, such as MMP-2, incorporate Fibronectin type II modules inserted immediately before in the zinc-binding motif in the catalytic domain.

The hinge region

The catalytic domain is connected to the C-terminal domain by a flexible hinge or linker region. This is up to 75 amino acids long, and has no determinable structure.

The hemopexin-like C-terminal domain

The C-terminal domain has structural similarities to the serum protein hemopexin. It has a four-bladed β-propeller structure. β-Propeller structures provide a large flat surface that is thought to be involved in protein–protein interactions. This determines substrate specificity and is the site for interaction with TIMP's. The hemopexin-like domain is absent in MMP-7, MMP-23, MMP-26, and the plant and nematode. The membrane-bound MMPs are anchored to the plasma membrane via a transmembrane or a GPI-anchoring domain.

Catalytic mechanism

There are three catalytic mechanisms published.
The MMPs can be subdivided in different ways.

Evolutionary

Use of bioinformatic methods to compare the primary sequences of the MMPs suggest the following evolutionary groupings of the MMPs:
Analysis of the catalytic domains in isolation suggests that the catalytic domains evolved further once the major groups had differentiated, as is also indicated by the substrate specificities of the enzymes.

Functional

The most commonly used groupings are based partly on historical assessment of the substrate specificity of the MMP and partly on the cellular localization of the MMP. These groups are the collagenases, the gelatinases, the stromelysins, and the membrane-type MMPs.
However, it is becoming increasingly clear that these divisions are somewhat artificial as there are a number of MMPs that do not fit into any of the traditional groups.

Genes

Matrix metalloproteinases combines with the metal binding protein, metallothionine; thus helping in metal binding mechanism.

Function

The MMPs play an important role in tissue remodeling associated with various physiological or pathological processes such as morphogenesis, angiogenesis, tissue repair, cirrhosis, arthritis, and metastasis. MMP-2 and MMP-9 are thought to be important in metastasis. MMP-1 is thought to be important in rheumatoid arthritis and osteoarthritis. Recent data suggests active role of MMPs in the pathogenesis of Aortic Aneurysm. Excess MMPs degrade the structural proteins of the aortic wall. Disregulation of the balance between MMPs and TIMPs is also a characteristic of acute and chronic cardiovascular diseases.

Activation

All MMPs are synthesized in the latent form. They are secreted as proenzymes and require extracellular activation. They can be activated in vitro by many mechanisms including organomercurials, chaotropic agents, and other proteases.

Inhibitors

The MMPs are inhibited by specific endogenous tissue inhibitor of metalloproteinases, which comprise a family of four protease inhibitors: TIMP-1, TIMP-2, TIMP-3, and TIMP-4.
Synthetic inhibitors generally contain a chelating group that binds the catalytic zinc atom at the MMP active site tightly. Common chelating groups include hydroxamates, carboxylates, thiols, and phosphinyls. Hydroxymates are particularly potent inhibitors of MMPs and other zinc-dependent enzymes, due to their bidentate chelation of the zinc atom. Other substituents of these inhibitors are usually designed to interact with various binding pockets on the MMP of interest, making the inhibitor more or less specific for given MMPs.

Pharmacology

, at subantimicrobial doses, inhibits MMP activity, and has been used in various experimental systems for this purpose, such as for recalcitrant recurrent corneal erosions. It is used clinically for the treatment of periodontal disease and is the only MMP inhibitor that is widely available clinically. It is sold under the trade name Periostat by the company CollaGenex. Minocycline, another tetracycline antibiotic, has also been shown to inhibit MMP activity.
A number of rationally designed MMP inhibitors have shown some promise in the treatment of pathologies that MMPs are suspected to be involved in. However, most of these, such as marimastat, a broad-spectrum MMP inhibitor, and cipemastat, an MMP-1 selective inhibitor, have performed poorly in clinical trials. The failure of Marimastat was partially responsible for the folding of British Biotech, which developed it. The failure of these drugs has been due largely to toxicity and failure to show expected results. The reasons behind the largely disappointing clinical results of MMP inhibitors is unclear, especially in light of their activity in animal models.