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The ECM is made of glycoproteins, collagens, proteoglycans and glycosaminoglycans, and its structural details and function differ between different tissues and organs. More than 80 genes code for different ECM components including 34 genes for the collagen family of proteins, which are the most abundant proteins constituting 30% of total body proteins.The ECM is particularly abundant in connective tissues such as bones, ligaments, tendons and cartilage.
The ECM has many functions. It is the glue that supports and keeps tissues and organs together. It regulates cell behavior and can tell cells what to do. It can play a permissive or instructive and morphogenetic role in growth and development. It can be good, bad and ugly. This is most dramatically illustrated by a variety of devastating genetic diseases and developmental disorders affecting the skeleton, blood vessels, skin and many other organ systems. A large number of such diseases are caused by gene mutations for collagens and other ECM components. How can this happen, how can genetically altered mutant ECM components cause these diseases? Is there anything common in the molecular pathogenesis of these diseases? What can be done to treat patients suffering from these diseases? What is the function of the ECM in normal development and health anyway?
Collagenases (MMP1, MMP8 and MMP13) are the only enzymes that can cleave fibrillar collagens such as collagen types I, II, III, VII and X.
Gelatinases A and B, also known as 72 kDa and 92 kDa type IV collagenases (MMP2 and MMP9), digest denatured collagen (gelatin) and other ECM components including basement membranes, a special type of ECM of endothelial, epithelial, fat, muscle and peripheral nerve cells.
Stromelysins (MMP3, MMP10 and MMP11) are wide spectrum enzymes, and stromelysin-1 (MMP3) degrades almost any ECM component including cartilage proteoglycans. Also, stromelysin-1 proteolytically activates other MMPs such as interstitial collagenase (MMP1), matrilysin (MMP7), collagenase-2 (MMP8), 92 kDa type IV collagenase (MMP9) and collagenase-3 (MMP13) suggesting that stromelysin-1 plays a special "upstream" role in ECM degradation and tissue remodeling.
MT-MMPs represent a recently discovered novel class of MMPs. Unlike other MMPs, they are not secreted but remain anchored to the cell surface via their carboxyl-terminal transmembrane domain. At present, four different MT-MMPs have been cloned and characterized. MT-MMPs digest ECM components and proteolytically activate other MMPs such as 72 kDa type IV collagenase and therefore, may regulate pericellular matrix degradation at the cell surface.
Many MMPs are expressed widely during embryogenesis, but not in adult life, often in a highly cell and tissue-specific manner suggesting distinct roles for different MMPs in growth, development and tissue remodeling. In adult life, MMPs are expressed in rapidly remodeling tissues such as the term placenta, menstrual endometrium, involuting mammary glands, and during wound healing and inflammation.
MMPs are also thought to play a critical role in tumor growth and metastasis, and in the progression of other diseases such as arthritis, atherosclerosis and aneurysm.
A catalytic domain (CD) of about 180 amino acids contains a highly conserved sequence HE-GH-LGL-H that provides three binding sites (His-residues) for the catalytic metal ion Zn(2+). The Cys-residue in the propeptide of inactive MMPs provides the fourth binding site for Zn(2+). In activated MMPs, this site is taken up by a water molecule, which is also hydrogen bonded to the conserved glutamic acid (E). This presumably destabilizes and "activates" the bound water molecule, a process that facilitates hydrolysis of a peptide bond of the target substrate.
Another conserved sequence DD--GIQ has a catalytic Asp-residue. Mutation of this aspartic acid (underlined) to glycine in neutrophil collagenase-2 (MMP8) completely abolished its proteolytic activity on casein and native fibrillar collagen. This aspartyl-residue probably has a conformation stabilizing role and, in the crystal structure of collagenase-2, it forms hydrogen bonds to the so-called Met-turn residues Leu214 and Met215. Met-turn is a characteristic sequence Ala-Leu-Met-Tyr (with an invariant methionine) of zinc-dependent metalloproteinases including MMPs. The catalytic domain is a compact structure made of three alpha-helices and a twisted five-stranded beta-sheet.
A linker peptide (LP) of 17-72 amino acids, an unstructured random coil, links the catalytic domain to a C-terminal domain of MMPs. LP is rich in proline and basic amino acids (Arg, Lys), and its only conserved feature is that it begins with glycine and ends with proline. LP, which is the most variable domain, is involved in the substrate selection of MMPs.
The C-terminal domain (VD) of about 200 amino acids is composed of four tandem repeats of a motif found in vitronectin and pexin-family of proteins. The C-terminal domain is a compact structure that looks like a four-bladed propeller. Each blade of the MMP propeller is made of an antiparallel four-stranded beta-sheet and the adjacent blades are arranged perpendicularly around a funnel-shaped tunnel. The C-terminal domain is important for protein-protein interactions, it binds to TIMPs and is involved in the substrate selection of MMPs.
Several MMPs have extra domains. Gelatinase A and gelatinase B (MMP2 and MMP9) contain three tandem repeats (each encoded by a separate exon) of a motif found in fibronectin that form a 176-amino acid-long fibronectin-like domain (FD). MT-MMPs contain a C-terminal transmembrane (TM) domain of 24 amino acids followed by a cytoplasmic tail domain (TD) of 20 amino acids.
Some MMPs lack domains. Matrilysin (MMP7) lacks the linker peptide and C-terminal vitronectin-like domain altogether, and XMMP, a novel MMP from Xenopus laevis, has no linker peptide.
MMPs are extremely slow enzymes. One stromelysin-1 (MMP3) molecule cuts one substrate molecule in 8 minutes. In comparison, carbonic anhydraze, a Zn(2+)-dependent metalloenzyme and the fastest enzyme there is, speeds up the reaction H2O + CO2 = HCO3(-) + H(+) 10 million-fold. It catalyzes the reaction 100,000 times per second.
August 23, 1997