Modified platelet deposition on matrix metalloproteinase 13 digested collagen I.

BACKGROUND: Atherothrombosis underlies acute coronary syndromes, including unstable angina and acute myocardial infarction. Within the unstable plaque, monocytes express collagenolytic matrix metalloproteinases (MMPs), including MMP-13, which degrades fibrous collagen. Following rupture, vessel wall components including degraded collagen are exposed to circulating platelets. Platelet receptors then mediate the recruitment and activation of platelets to form a thrombus, blocking blood flow and resulting in myocardial infarction and sudden death. OBJECTIVES: Here we aim to provide information on the effects of collagen degradation on platelet adhesion and thrombus formation. METHODS: Using increasing concentrations of MMP-13, we induced progressive degradation of fibrous and monomeric collagen I, visualized by electrophoresis, and then investigated the capacity of the resulting fragments to support static platelet adhesion and thrombus formation in whole flowing blood. RESULTS: Both integrin and glycoprotein VI-dependent interactions with fibrous collagen underpin high levels of platelet adhesion under both conditions, with little obvious effect of MMP-13 treatment. Static platelet adhesion to monomeric collagen was strongly α2ß1-dependent regardless of degradation status. Under flow conditions, partially degraded monomeric collagen supported increased thrombus deposition at 10 µg mL(-1) MMP-13, falling close to background when collagen degradation was complete (100 µg mL(-1) MMP-13). CONCLUSIONS: New binding activities come into play after partial digestion of collagen monomers, and net platelet-reactivity through all axes is abolished as degradation becomes more complete.

Role of newly synthesized fibronectin in vascular smooth muscle cell migration on matrix-metalloproteinase-degraded collagen.

The migration of vascular smooth muscle cells (VSMC) is known to be a key process in the development of a number of vascular lesions, although the precise mechanisms involved have still to be elucidated. In the present study, the production of endogenous fibronectins by VSMC migrating across intact and matrix-metalloproteinase-degraded collagen type I has been explored. Cellular fibronectin seems to play a role in the enhanced migration seen when VSMC are exposed to degraded collagen and platelet-derived growth factor-BB. VSMC were found to synthesize both exon IIIA-containing fibronectin (which predominated) and exon IIIB-containing fibronectin. When these cells were exposed to substrates consisting of recombinant exon IIIA- or exon IIIB-containing fibronectin, rates of migration were not elevated above those seen with undegraded collagen. Endogenous fibronectin production may thus be necessary, but not sufficient, for VSMC migration over degraded collagenous substrates.

Characterization of the role of the "MT-loop": an eight-amino acid insertion specific to progelatinase A (MMP2) activating membrane-type matrix metalloproteinases.

Progelatinase A (proGLA) activation is thought to be initiated almost exclusively by the type I transmembrane members of the membrane type matrix metalloproteinase family (MT-MMP): MT1, -2, -3, and -5-MMP (MMP14, -15, -16, and -24). One difference between these enzymes and the other MMP family members is the insertion of eight amino acids between strands betaII and III in the catalytic domain. In MT1-MMP, the best characterized of these enzymes to date, these residues consist of (163)PYAYIREG(170). To investigate the role of this region of MT1-MMP on its catalytic activities, we have made a variety of mutations and deletions in both soluble and membrane-bound forms of the enzyme. Characterization of the activity of the soluble forms toward peptides and fibrinogen revealed that neither mutation nor deletion of residues 163-170 significantly impaired catalytic function, suggesting these residues have little influence on conformation of the active site cleft. Equally none of the mutants showed significant differences in K(I)(app) for the N-terminal inhibitory domain of TIMP2, again indicating that mutation or deletion of resides 163-170 has no major effect on the overall topology of the active site of MT1-MMP. However, characterization of the kinetics of activation of proGLA with and without its gelatin binding region by the mutants generated have shown that efficient activation of proGLA is, at least in part, through an interaction with residues 163-170 of MT1-MMP. The expression, localization, and processing from the 63- to the 60/45-kDa forms of wild-type and key mutant forms of MT1-MMP were also examined by transient transfection in Chinese hamster ovary cells, but no differences were observed. Processing and activation of proGLA was also examined in transiently transfected cells. All the mutants examined were able process proGLA but, as found with the soluble forms, were kinetically impaired when compared with wild-type MT1-MMP.

Specific collagenolysis by gelatinase A, MMP-2, is determined by the hemopexin domain and not the fibronectin-like domain.

In view of the essential role of the hemopexin domain of the traditional interstitial collagenases, MMP-1, -8, -13 and MT1-MMP (MMP-14), in determining specific collagen cleavage we have studied the function of this domain in MMP-2, relative to that of the fibronectin-like domain that promotes gelatinolysis. Although the fibronectin-like domain promotes avid binding to collagen, our data demonstrate that the catalytic and hemopexin domains of MMP-2 are sufficient to effect the critical step in cleavage of rat type I collagen into 3/4 and 1/4 fragments. The mechanism of MMP-2 cleavage of collagen proceeds in two phases, the first resembling that of the interstitial collagenases, followed by gelatinolysis, promoted by the fibronectin-like domain.

Differential expression and localization of TIMP-1 and TIMP-4 in human gliomas.

Studies have suggested that an imbalance of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) may contribute to the malignant phenotype of gliomas. In this study, we have undertaken a detailed analysis of expression of the TIMP family in normal human brain and malignant gliomas at both the mRNA and protein level. Reverse transcription-PCR (RT-PCR) analyses of total RNA from surgical tumour specimens revealed unique expression patterns for the 4 members of the TIMP family, with TIMP-1 and -4 showing positive and negative correlations, respectively, with glioma malignancy. By RT-PCR, TIMP-2 and TIMP-3 expression did not change with tumour grade. In situ hybridization localized TIMP-1 to glial tumour cells and also to the surrounding tumour vasculature. TIMP-4 transcripts were predominantly localized to tumour cells, though minor expression was found in vessels. Recombinant TIMP-4 reduced invasion of U251 glioma cells through Matrigel, and U87 clones overexpressing TIMP-4 showed reduced invasive capacity in vitro. TIMP-4, but not TIMP-1, blocked Membrane Type-1-MMP-mediated progelatinase-A (MMP-2) activation in human umbilical vein endothelial cells. The differential expression and localization of individual TIMPs may contribute to the pathophysiology of human malignant gliomas, particularly with regard to tumour vascularization.

Activation of pro-(matrix metalloproteinase-2) (pro-MMP-2) by thrombin is membrane-type-MMP-dependent in human umbilical vein endothelial cells and generates a distinct 63 kDa active species.

Thrombin, a critical enzyme in the coagulation cascade, has also been associated with angiogenesis and activation of the zymogen form of matrix metalloproteinase-2 (MMP-2 or gelatinase-A). We show that thrombin activated pro-MMP-2 in a dose- and time-dependent manner in cultured human umbilical-vein endothelial cells (HUVECs) to generate a catalytically active 63 kDa protein that accumulated as the predominant form in the conditioned medium. This 63 kDa thrombin-activated MMP-2 is distinct from the 62 kDa species found following concanavalin A or PMA stimulated pro-MMP-2 activation. Hirudin and leupeptin blocked thrombin-induced pro-MMP-2 activation, demonstrating that the proteolytic activity of thrombin is essential. However, activation was also dependent upon membrane-type-MMP (MT-MMP) action, since it was blocked by EDTA, o-phenanthroline, hydroxamate metalloproteinase inhibitors, tissue inhibitor of metalloproteinase-2 (TIMP-2) and TIMP-4, but not TIMP-1. Thrombin inefficiently cleaved recombinant 72 kDa pro-MMP-2, but efficiently cleaved the 64 kDa MT-MMP-processed intermediate form in the presence of cells. Thrombin also rapidly (within 1 h) increased cellular MT-MMP activity, and at longer time points (>6 h) it increased expression of MT1-MMP mRNA and protein. Thus signalling via proteinase-activated receptors (PARs) may play a role in thrombin-induced MMP-2 activation, though this does not appear to involve PAR1, PAR2, or PAR4 in HUVECs. These results indicate that in HUVECs the activation of pro-MMP-2 by thrombin involves increased MT-MMP activity and preferential cleavage of the MT-MMP-processed 64 kDa MMP-2 form in the presence of cells. The integration of these proteinase systems in the vascular endothelium may be important during thrombogenesis and tissue remodelling associated with neovascularization.

Tyrosine 36 plays a critical role in the interaction of the AB loop of tissue inhibitor of metalloproteinases-2 with matrix metalloproteinase-14.

The tissue inhibitor of metalloproteinases-2 (TIMP-2) is potentially an important inhibitor of all known matrix metalloproteinases (MMPs). However, it has been shown to undergo specific interactions with both MMP-2 (gelatinase A) and MMP-14 (MT1-MMP), and it has been proposed that these three proteins function as a cell surface-based activation cascade for matrix metalloproteinases and as a focus of proteolytic activity. In this study, we have carried out mutagenesis and kinetic analyses to examine the unique interactions between the AB loop of TIMP-2 and MMP-14. The results demonstrate that the major binding contribution of the AB loop is due solely to residue Tyr-36 at the tip of the hairpin. From this work, we propose that TIMP-2 may be engineered to abrogate MMP-14 binding, whereas its binding properties for other MMPs, including MMP-2, are maintained. Mutants of TIMP-2 with more directed specificity may be of use in gene therapeutic approaches to human disease.

The role of exon 5 in fibroblast collagenase (MMP-1) substrate specificity and inhibitor selectivity.

Interstitial collagen is degraded by members of the matrix metalloproteinase (MMP) family, including MMP-1. Previous work has shown that the region of MMP-1 coded for by exon 5 is implicated both in substrate specificity and inhibitor selectivity. We have constructed a chimeric enzyme, the exon 5 chimera, consisting primarily of MMP-1, with the region coded for by exon 5 replaced with the equivalent region of MMP-3, a noncollagenolytic MMP. Unlike MMP-3, the exon 5 chimera is capable of cleaving type I collagen, but the activity is only 2.2% of trypsin-activated MMP-1. 'Superactivation' of the chimera has no discernible effect, suggesting that the salt bridge formed in 'superactive' MMP-1 is not present. The kinetics for exon 5 chimera cleavage of two synthetic substrates display an MMP-3 phenotype, however, cleavage of gelatin is slightly impaired as compared to the parent enzymes. The K(iapp) values for the exon 5 chimera complexed with synthetic inhibitors and N-terminal TIMP-2 also show a more MMP-3-like behaviour. However, the k(on) values for N-terminal TIMP-1 and N-terminal TIMP-2 are more comparable to those for MMP-1. These data show that the region of MMP-1 coded for by exon 5 is involved in both substrate specificity and inhibitor selectivity and the structural basis for our findings is discussed.

Catalytic activities of membrane-type 6 matrix metalloproteinase (MMP25).

This study describes the biochemical characterisation of the catalytic domain of membrane-type 6 matrix metalloproteinase (MT6-MMP, MMP25, leukolysin). Its activity towards synthetic peptide substrates, components of the extracellular matrix and inhibitors of MMPs was studied and compared with MT1-MMP, MT4-MMP and stromelysin-1. We have found that MT6-MMP is closer in function to stromelysin-1 than MT1 and MT4-MMP in terms of substrate and inhibitor specificity, being able to cleave type-IV collagen, gelatin, fibronectin and fibrin. However, it differs from stromelysin-1 and MT1-MMP in its inability to cleave laminin-I, and unlike stromelysin-1 cannot activate progelatinase B. Our findings suggest that MT6-MMP could play a role in cellular migration and invasion of the extracellular matrix and basement membranes and its activity may be tightly regulated by all members of the TIMP family.

Full-length and N-TIMP-3 display equal inhibitory activities toward TNF-alpha convertase.

We previously reported that tumor necrosis factor-alpha converting enzyme (TACE) was specifically inhibited by TIMP-3 but not TIMP-1, -2, and -4. Further mutagenesis studies showed that the N-terminal domain of TIMP-3 (N-TIMP-3) retained full inhibitory activity towards TACE. Full-length TIMP-3 and N-TIMP-3 exhibited indistinguishable values for the association rate constant and inhibitory affinity constant for the active catalytic domain of TACE (k(on) approximately 10(5) M(-1) s(-1) and K(app)(i) approximately 0.20 nM). Moreover, their k(on) (approximately 10(4) M(-1) s(-1)) and K(app)(i) (approximately 1.0 nM) values with a longer form of TACE (which encompasses the complete ectodomain including disintegrin, EGF and Crambin-like domains) were also shown to be similar. Detailed kinetic analyses indicated that TIMP-3 associated more quickly and with tighter final binding with TACE devoid of these C-terminal domains. We conclude that, unlike the interaction between many MMPs and TIMPs, the C-terminal domains of TIMP-3 and TACE are not essential in the formation of a tight binary complex.

Identification and enzymatic characterization of two diverging murine counterparts of human interstitial collagenase (MMP-1) expressed at sites of embryo implantation.

Remodeling of fibrillar collagen in mouse tissues has been widely attributed to the activity of collagenase-3 (matrix metalloproteinase-13 (MMP-13)), the main collagenase identified in this species. This proposal has been largely based on the repeatedly unproductive attempts to detect the presence in murine tissues of interstitial collagenase (MMP-1), a major collagenase in many species, including humans. In this work, we have performed an extensive screening of murine genomic and cDNA libraries using as probe the full-length cDNA for human MMP-1. We report the identification of two novel members of the MMP gene family which are contained within the cluster of MMP genes located at murine chromosome 9. The isolated cDNAs contain open reading frames of 464 and 463 amino acids and are 82% identical, displaying all structural features characteristic of archetypal MMPs. Comparison for sequence similarities revealed that the highest percentage of identities was found with human interstitial collagenase (MMP-1). The new proteins were tentatively called Mcol-A and Mcol-B (Murine collagenase-like A and B). Analysis of the enzymatic activity of the recombinant proteins revealed that both are catalytically autoactivable but only Mcol-A is able to degrade synthetic peptides and type I and II fibrillar collagen. Both Mcol-A and Mcol-B genes are located in the A1-A2 region of mouse chromosome 9, Mcol-A occupying a position syntenic to the human MMP-1 locus at 11q22. Analysis of the expression of these novel MMPs in murine tissues revealed their predominant presence during mouse embryogenesis, particularly in mouse trophoblast giant cells. According to their structural and functional characteristics, we propose that at least one of these novel members of the MMP family, Mcol-A, may play roles as interstitial collagenase in murine tissues and could represent a true orthologue of human MMP-1.

Localization of the death domain of tissue inhibitor of metalloproteinase-3 to the N terminus. Metalloproteinase inhibition is associated with proapoptotic activity.

The tissue inhibitors of metalloproteinases (TIMPs) are a family of four secreted inhibitors of matrix metalloproteinases (MMPs). Recently, additional functions have been attributed to the TIMPs, including cell growth and inhibition of angiogenesis. In particular, we demonstrated that TIMP-3 overexpression using gene transfer induces apoptosis in a variety of cell types and can inhibit vascular neointima formation in vivo. However, little is know about the mechanisms underlying TIMP-3-mediated apoptosis. Here, using both purified recombinant proteins and novel adenoviral vectors we demonstrate that the prodeath domain of TIMP-3 is located within the N-terminal three loops of TIMP-3. Although both wild type and N-terminal TIMP-3 proteins promoted apoptosis, a T-2/T-3 chimera, in which the N-terminal three loops of TIMP-3 are replaced by those of TIMP-2, failed to induce cell death. Furthermore, a point mutation at residue 1 of TIMP-3 totally abolished MMP-inhibitory activity of TIMP-3 and also failed to promote apoptosis. This study demonstrates, using multiple apoptosis assays, that the prodeath function of TIMP-3 is located within the N-terminal three loops and the presence of functional metalloproteinase-inhibitory activity is associated with the induction of apoptosis.

Matrix metalloproteinases 19 and 20 cleave aggrecan and cartilage oligomeric matrix protein (COMP).

Matrix metalloproteinase (MMP)-19 and MMP-20 (enamelysin) are two recently discovered members of the MMP family. These enzymes are involved in the degradation of the various components of the extracellular matrix (ECM) during development, haemostasis and pathological conditions. Whereas MMP-19 mRNA is found widely expressed in body tissues, including the synovium of normal and rheumatoid arthritic patients, MMP-20 expression is restricted to the enamel organ. In this study we investigated the ability of MMP-19 and MMP-20 to cleave two of the macromolecules characterising the cartilage ECM, namely aggrecan and the cartilage oligomeric matrix protein (COMP). Both MMPs hydrolysed aggrecan efficiently at the well-described MMP cleavage site between residues Asn(341) and Phe(342), as shown by Western blotting using neo-epitope antibodies. Furthermore, the two enzymes cleaved COMP in a distinctive manner, generating a major proteolytic product of 60 kDa. Our results suggest that MMP-19 may participate in the degradation of aggrecan and COMP in arthritic disease, whereas MMP-20, due to its unique expression pattern, may primarily be involved in the turnover of these molecules during tooth development.

An analysis of two refolding routes for a C-terminally truncated human collagenase-3 expressed in Escherichia coli.

We describe here the expression of a C-terminally truncated form of human procollagenase-3 in Escherichia coli. The protein was found almost exclusively in inclusion bodies that were solubilized and refolded by two separate methods and then purified on Ni-NTA agarose. The purified proenzyme could be activated with either trypsin or APMA and active enzyme could be purified on a peptidic hydroxamate affinity column. Competitive elution from the affinity matrix yielded a highly purified preparation.

Membrane type 4 matrix metalloproteinase (MMP17) has tumor necrosis factor-alpha convertase activity but does not activate pro-MMP2.

Membrane type 4 matrix metalloproteinase (MT4-MMP) shows the least sequence homology to the other MT-MMPs, suggesting a distinct function for this protein. We have isolated a complete cDNA corresponding to the mouse homologue which includes the signal peptide and a complete pro-domain, features that were lacking from the human form originally isolated. Mouse MT4-MMP (mMT4-MMP) expressed in COS-7 cells is located at the cell surface but does not show ability to activate pro-MMP2. The pro-catalytic domain was expressed in Escherichia coli as insoluble inclusions and active enzyme recovered after refolding. Activity of the isolated catalytic domain against synthetic peptides commonly used for MMP enzyme assays could be inhibited by TIMP1, -2, and -3. The recombinant mMT4-MMP catalytic domain was also unable to activate pro-MMP2 and was very poor at hydrolyzing components of the extracellular matrix with the exception of fibrinogen and fibrin. mMT4-MMP was able to hydrolyze efficiently a peptide consisting of the pro-tumor necrosis factor alpha (TNFalpha) cleavage site, a glutathione S-transferase-pro-TNFalpha fusion protein, and was found to shed pro-TNFalpha when co-transfected in COS-7 cells. MT4-MMP was detected by Western blot in monocyte/macrophage cell lines which in combination with its fibrinolytic and TNFalpha-converting activity suggests a role in inflammation.

The in vitro activity of ADAM-10 is inhibited by TIMP-1 and TIMP-3.

A recombinant soluble form of the catalytic domain of human ADAM-10 was expressed as an Fc fusion protein from myeloma cells. The ADAM-10 was catalytically active, cleaving myelin basic protein and peptides based on the previously described 'metallosheddase' cleavage sites of tumour necrosis factor alpha, CD40 ligand and amyloid precursor protein. The myelin basic protein degradation assay was used to demonstrate that hydroxamate inhibitors of matrix metalloproteinases (MMPs) were also inhibitors of ADAM-10. The natural MMP inhibitors, TIMP-2 and TIMP-4 were unable to inhibit ADAM-10, but TIMP-1 and TIMP-3 were inhibitory. Using a quenched fluorescent substrate assay and ADAM-10 we obtained approximate apparent inhibition constants of 0.1 nM (TIMP-1) and 0.9 nM (TIMP-3). The TIMP-1 inhibition of ADAM-10 could therefore prove useful in distinguishing its activity from that of TACE, which is only inhibited by TIMP-3, in cell based assays.

Collagen degradation and platelet-derived growth factor stimulate the migration of vascular smooth muscle cells.

Cell migration is a key event in many biological processes and depends on signals from both extracellular matrix and soluble motogenic factors. During atherosclerotic plaque development, vascular smooth muscle cells migrate from the tunica media to the intima through a basement membrane and interstitial collagenous matrix and proliferate to form a neointima. Matrix metalloproteinases have previously been implicated in neointimal formation and in this study smooth muscle cell adhesion and migration on degraded collagen have been evaluated. Vascular smooth muscle cells adhered to native intact collagen type I and to its first degradation by-product, 3/4 fragment (generated by collagenase-3 cleavage), unwound at 35 degrees C to mimic physiological conditions. PDGF-BB pre-treatment induced a fourfold stimulation of smooth muscle cell motility on the collagen 3/4 fragment whereas no increase in smooth muscle cell motility on collagen type I was observed. Cell migration on collagen type I was mediated by alpha2 integrin, whereas PDGF-BB-stimulated migration on the 3/4 collagen fragment was dependent on alphavbeta3 integrin. alphavbeta3 integrin was organised in clusters concentrated at the leading and trailing edges of the cells and was only expressed when cells were exposed to the 3/4 collagen fragment. Tyrphostin A9, an inhibitor of PDGF receptor-beta tyrosine kinase activity, resulted in complete abolition of migration of PDGF-BB treated cells on collagen type I and 3/4 fragment. These results strongly support the hypothesis that the cellular migratory response to soluble motogens can be regulated by proteolytic modification of the extracellular matrix.

Biochemical characterization of the catalytic domain of human matrix metalloproteinase 19. Evidence for a role as a potent basement membrane degrading enzyme.

We have recently cloned MMP-19, a novel matrix metalloproteinase, which, due to unique structural features, was proposed to represent the first member of a new MMP subfamily (Pendás, A. M., Knäuper, V. , Puente, X. S., Llano, E., Mattei, M. G., Apte, S., Murphy, G., and López-Otin, C. (1997) J. Biol. Chem. 272, 4281-4286). A recombinant COOH-terminal deletion mutant of MMP-19 (proDelta(260-508)MMP-19), comprising the propeptide and the catalytic domain, was expressed in Escherichia coli, refolded, and purified. Interestingly, we found that proDelta(260-508)MMP-19 has the tendency to autoactivate, whereby the Lys(97)-Tyr(98) peptide bond is hydrolyzed, resulting in free catalytic domain. Mutation of two residues (Glu(88) --> Pro and Pro(90) --> Val) within the propeptide latency motif did not prevent autoactivation but the autolysis rate was somewhat reduced. Analysis of the substrate specificity revealed that the catalytic domain of MMP-19 was able to hydrolyze the general MMP substrate Mca-Pro-Leu-Gly-Dpa-Ala-Arg-NH(2) and, with higher efficiency, the stromelysin substrate Mca-Pro-Leu-Ala-Nva-Dpa-Ala-Arg-NH(2). Kinetic analysis of the interactions of the catalytic domain of MMP-19 with the natural MMP inhibitors, the tissue inhibitors of metalloproteinases (TIMPs), showed strong inhibition using TIMP-2, TIMP-3, and TIMP-4, while TIMP-1 was less efficient. We also demonstrated that synthetic hydroxamic acid-based compounds efficiently inhibited the enzyme. The catalytic domain of MMP-19 was able to hydrolyze the basement membrane components type IV collagen, laminin, and nidogen, as well as the large tenascin-C isoform, fibronectin, and type I gelatin in vitro, suggesting that MMP-19 is a potent proteinase capable of hydrolyzing a broad range of extracellular matrix components. Neither the catalytic domain nor the full-length MMP-19 was able to degrade triple-helical collagen. Finally, and in contrast to studies with other MMPs, MMP-19 catalytic domain was not able to activate any of the latent MMPs tested in vitro.

Recognition and catabolism of synthetic heterotrimeric collagen peptides by matrix metalloproteinases.

BACKGROUND: The general consensus is that interstitial collagens are digested by collagenases and denatured collagen by gelatinases, although processing of fibrillar and acetic-acid-soluble collagen by gelatinase A has also been reported. One of the main difficulties in studying the mechanism of action of these matrix metalloproteinases (MMPs) derives from the physicochemical properties of the natural triple-helical collagen, which makes it difficult to handle. RESULTS: Synthetic heterotrimeric collagenous peptides that contain the collagenase cleavage site of human collagen type I and differ in the thermal stability of the triple-helical fold were used to mimic natural collagen and gelatin, respectively. Results from digestion of these substrates by fibroblast and neutrophil collagenases (MMP-1 and MMP-8), as well as by gelatinase A (MMP-2), confirmed that the two classes of enzymes operate within the context of strong conformational dependency of the substrates. It was also found that gelatinases and collagenases exhibit two distinct proteolytic mechanisms: gelatinase digests the gelatin-like heterotrimer rapidly in individual steps with intermediate releases of partially processed substrate into the medium, whereas collagenases degrade the triple-helical heterotrimer by trapping it until scission through all three alpha chains is achieved. CONCLUSIONS: The results confirm the usefulness of synthetic heterotrimeric collagenous peptides in the folded and unfolded state as mimics of the natural substrates collagen and gelatin, respectively, to gain a better a insight into the proteolytic mechanisms of matrix metalloproteinases.