Role of the amino-terminal extrahelical region of type I collagen in directing the 4D overlap in fibrillogenesis

Lysyl Oxidases as Targets for Cancer Therapy and Diagnostic Imaging

The understanding of the contribution of the tumour microenvironment to cancer progression and metastasis, in particular the interplay between tumour cells, fibroblasts and the extracellular matrix has grown tremendously over the last years. Lysyl oxidases are increasingly recognised as key players in this context, in addition to their function as drivers of fibrotic diseases. These insights have considerably stimulated drug discovery efforts towards lysyl oxidases as targets over the last decade. This review article summarises the biochemical and structural properties of theses enzymes. Their involvement in tumour progression and metastasis is highlighted from a biochemical point of view, taking into consideration both the extracellular and intracellular action of lysyl oxidases. More recently reported inhibitor compounds are discussed with an emphasis on their discovery, structure-activity relationships and the results of their biological characterisation. Molecular probes developed for imaging of lysyl oxidase activity are reviewed from the perspective of their detection principles, performance and biomedical applications.

Defective chicken skin collagen molecules, hydrolyzed by actinidain protease, assemble to form loosely packed fibrils that promote cell spheroid formation

Cells recognize collagen fibrils as the first step in the process of adherence. Fibrils of chicken skin actinidain-hydrolyzed collagen (low adhesive scaffold collagen, LASCol), in which the telopeptide domains are almost completely removed, cause adhering cells to form spheroids instead of adopting a monolayer morphology. Our goal was to elucidate the ultrastructure of the LASCol fibrils compared with pepsin-hydrolyzed collagen (PepCol) fibrils. At low concentration of 0.2 mg/mL, the time to reach the maximum increasing rate of turbidity for LASCol was all slower than that for PepCol. Differential scanning calorimetry showed that the thermal stability of collagen self-assembly changed significantly between pH 5.5 and pH 6.6 with and without a small number of telopeptides. However, the calorimetric enthalpy change did not vary much in that pH range. The melting temperature of LASCol fibrils at pH 7.3 was 55.1 °C, whereas PepCol fibrils exhibited a peak around 56.9 °C. The D-periodicity of each fibril was the same at 67 nm. Nevertheless, the looseness of molecular packing in LASCol fibrils was demonstrated by circular dichroism measurements and immuno-scanning electron microscopy with a polyclonal antibody against type I collagen. As there is a close relationship between function and structure, loosely packed collagen fibrils would be one factor that promotes cell spheroid formation.

Spatiotemporal regulation of PEDF signaling by type I collagen remodeling

Dynamic remodeling of the extracellular matrix affects many cellular processes, either directly or indirectly, through the regulation of soluble ligands; however, the mechanistic details of this process remain largely unknown. Here we propose that type I collagen remodeling regulates the receptor-binding activity of pigment epithelium-derived factor (PEDF), a widely expressed secreted glycoprotein that has multiple important biological functions in tissue and organ homeostasis. We determined the crystal structure of PEDF in complex with a disulfide cross-linked heterotrimeric collagen peptide, in which the α(I) chain segments-each containing the respective PEDF-binding region (residues 930 to 938)-are assembled with an α2α1α1 staggered configuration. The complex structure revealed that PEDF specifically interacts with a unique amphiphilic sequence, KGHRGFSGL, of the type I collagen α1 chain, with its proposed receptor-binding sites buried extensively. Molecular docking demonstrated that the PEDF-binding surface of type I collagen contains the cross-link-susceptible Lys930 residue of the α1 chain and provides a good foothold for stable docking with the α1(I) N-telopeptide of an adjacent triple helix in the fibril. Therefore, the binding surface is completely inaccessible if intermolecular crosslinking between two crosslink-susceptible lysyl residues, Lys9 in the N-telopeptide and Lys930, is present. These structural analyses demonstrate that PEDF molecules, once sequestered around newly synthesized pericellular collagen fibrils, are gradually liberated as collagen crosslinking increases, making them accessible for interaction with their target cell surface receptors in a spatiotemporally regulated manner.

Detection of nucleic acids and other low abundance components in native bone and osteosarcoma extracellular matrix by isotope enrichment and DNP-enhanced NMR

Sensitivity enhancement by isotope enrichment and DNP NMR enables detection of minor but biologically relevant species in native intact bone, including nucleic acids, choline from phospholipid headgroups, and histidinyl and hydroxylysyl groups. Labelled matrix from the aggressive osteosarcoma K7M2 cell line confirms the assignments of nucleic acid signals arising from purine, pyrimidine, ribose, and deoxyribose species. Detection of these species is an important and necessary step in elucidating the atomic level structural basis of their functions in intact tissue.

Towards elucidating their biological roles in intact tissue, DNP NMR reveals nucleic acids, and other important low abundance biomolecules in a complex biomaterial, bone, and in cancer extracellular matrix.

Evaluation of Fluorine-18-Labeled α1(I)-N-Telopeptide Analogs as Substrate-Based Radiotracers for PET Imaging of Melanoma-Associated Lysyl Oxidase

Accumulating evidence suggests an unequivocal role of lysyl oxidases as key players of tumor progression and metastasis, which renders this enzyme family highly attractive for targeted non-invasive functional imaging of tumors. Considering their function in matrix remodeling, malignant melanoma appears as particularly interesting neoplasia in this respect. For the development of radiotracers that enable PET imaging of the melanoma-associated lysyl oxidase activity, substrates derived from the type I collagen α1 N-telopeptide were labeled with fluorine-18 using N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) as prosthetic reagent. With regards to potential crosslinking to tumor-associated collagen in vivo, their interaction with triple-helical type I collagen was studied by SPR. A mouse model of human melanoma was established on the basis of the A375 cell line, for which the expression of the oncologically relevant lysyl oxidase isoforms LOX and LOXL2 was demonstrated in Western blot and immunohistochemical experiments. The radiopharmacological profiles of the peptidic radiotracers were evaluated in normal rats and A375 melanoma-bearing mice by ex vivo metabolite analysis, whole-body biodistribution studies and dynamic PET imaging. Out of three ¹⁸F-labeled telopeptide analogs, the one with the most favorable substrate properties has shown favorable tumor uptake and tumor-to-muscle ratio. Lysyl oxidase-mediated tumor uptake was proven by pharmacological inhibition using β-aminopropionitrile and by employing negative-control analogs of impeded or abolished targeting capability. The latter were obtained by substituting the lysine residue by ornithine and norleucine, respectively. Comparing the tumor uptake of the lysine-containing peptide with that of the non-functional analogs indicate the feasibility of lysyl oxidase imaging in melanoma using substrate-based radiotracers.

Cyclopeptides containing the DEKS motif as conformationally restricted collagen telopeptide analogues: synthesis and conformational analysis

The collagen telopeptides play an important role for lysyl oxidase-mediated crosslinking, a process which is deregulated during tumour progression. The DEKS motif which is located within the N-terminal telopeptide of the α1 chain of type I collagen has been suggested to adopt a βI-turn conformation upon docking to its triple-helical receptor domain, which seems to be critical for lysyl oxidase-catalysed deamination and subsequent crosslinking by Schiff-base formation. Herein, the design and synthesis of cyclic peptides which constrain the DEKS sequence in a β-turn conformation will be described. Lysine-side chain attachment to 2-chlorotrityl chloride-modified polystyrene resin followed by microwave-assisted solid-phase peptide synthesis and on-resin cyclisation allowed for an efficient access to head-to-tail cyclised DEKS-derived cyclic penta- and hexapeptides. An N(ε)-(4-fluorobenzoyl)lysine residue was included in the cyclopeptides to allow their potential radiolabelling with fluorine-18 for PET imaging of lysyl oxidase. Conformational analysis by (1)H NMR and chiroptical (electronic and vibrational CD) spectroscopy together with MD simulations demonstrated that the concomitant incorporation of a D-proline and an additional lysine for potential radiolabel attachment accounts for a reliable induction of the desired βI-turn structure in the DEKS motif in both DMSO and water as solvents. The stabilised conformation of the cyclohexapeptide is further reflected by its resistance to trypsin-mediated degradation. In addition, the deaminated analogue containing allysine in place of lysine has been synthesised via the corresponding ε-hydroxynorleucine containing cyclohexapeptide. Both ε-hydroxynorleucine and allysine containing cyclic hexapeptides have been subjected to conformational analysis in the same manner as the lysine-based parent structure. Thus, both a conformationally restricted lysyl oxidase substrate and product have been synthetically accessed, which will enable their potential use for molecular imaging of these important enzymes.

Effects of various salts on structural polymorphism of reconstituted type I collagen fibrils

Even though the behavior of collagen monomers self-assembling into fibrils is commonly understood in terms of hydrophobic and electrostatic interactions, the mechanisms that drive their ordered, longitudinal alignment to form a characteristic periodicity are still unclear. By introducing various salts into the collagen fibrillogenesis system, the intermolecular interactions of fibril formation were studied. We found that the pH and ion species play a critical role in forming native fibrils. Turbidity and electron microscopy revealed that collagen self-assembled into fibrils with a native banding pattern in the presence of multivalent ions. The isoelectric point of collagen in 12mM of NaCl is pH 8.9; it shifted to pH 9.4 and pH 7.0 after adding 10mM CaCl2 and Na2SO4, respectively. Native fibrils were reconstituted at pH 7.4 in salts with divalent anions and at pH 9.0 in salts with divalent cations. Circular dichroism spectroscopy showed a loss of helicity in the conditions where fibrillogenesis was unable to be achieved. The multivalent ions not only change the surface charge of collagen, but also facilitate the formation of fibrils with the native D-periodic banding pattern. It is likely that the binding multivalent ions induce the like-charge attraction and facilitate monomers' longitudinal registration to form fibrils with the native banding.

Site-selective radiolabeling of peptides by (18)F-fluorobenzoylation with [(18F)]SFB in solution and on solid phase: a comparative study

Peptides labeled with short-lived positron-emitting radionuclides are of outstanding interest as probes for molecular imaging by positron emission tomography (PET). Herein, the site-selective incorporation of fluorine-18 into lysine-containing peptides using the prosthetic labeling agent N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB) is described. The reaction of [(18)F]SFB with four biologically relevant resin-bound peptides was studied and optimized. For comparison, each peptide was 18F-fluorobenzoylated in solution under different conditions and the product distribution was analyzed confirming the advantages of the solid-phase approach. The method's feasibility for selective radiolabeling either at the N-terminus or at the lysine side chain was demonstrated. Labeling on solid phase with [(18)F]SFB resulted in crude (18)F-fluorobenzoylpeptides whose radiochemical purities were typically greater than 90% and that could be prepared in synthesis times from 65 to 76 min.

In vitro mineralization of dense collagen substrates: a biomimetic approach toward the development of bone-graft materials

Bone is an organic-inorganic composite which has hierarchical structuring that leads to high strength and toughness. The nanostructure of bone consists of nanocrystals of hydroxyapatite embedded and aligned within the interstices of collagen fibrils. This unique nanostructure leads to exceptional properties, both mechanical and biological, making it difficult to emulate bone properties without having a bone-like nanostructured material. A primary goal of our group's work is to use biomimetic processing techniques that lead to bone-like structures. In our prior studies, we demonstrated that intrafibrillar mineralization of porous collagen sponges, leading to a bone-like nanostructure, can be achieved using a polymer-induced liquid precursor (PILP) mineralization process. The objective of this study was to investigate the use of this polymer-directed crystallization process to mineralize dense collagen substrates. To examine collagen scaffolds that truly represent the dense-packed matrix of bone, manatee bone was demineralized to isolate its collagen matrix, consisting of a dense, lamellar osteonal microstructure. This biogenic collagen scaffold was then remineralized using polyaspartate to direct the mineralization process through an amorphous precursor pathway. The various conditions investigated included polymer molecular weight, substrate dimension and mineralization time. Mineral penetration depths of up to 100 μms were achieved using this PILP process, compared to no penetration with only surface precipitates observed for the conventional crystallization process. Electron microscopy, wide-angle X-ray diffraction and thermal analysis were used to characterize the resulting hydroxyapatite/collagen composites. These studies demonstrate that the original interpenetrating bone nanostructure and osteonal microstructure could be recovered in a biogenic matrix using the PILP process.

Development of bone-like composites via the polymer-induced liquid-precursor (PILP) process. Part 1: influence of polymer molecular weight

Bone is an organic-inorganic composite consisting primarily of collagen fibrils and hydroxyapatite crystals intricately interlocked to provide skeletal and metabolic functions. Non-collagenous proteins (NCPs) are also present, and although only a minor component, the NCPs are thought to play an important role in modulating the mineralization process. During secondary bone formation, an interpenetrating structure is created by intrafibrillar mineralization of the collagen matrix. Many researchers have tried to develop bone-like collagen-hydroxyapatite (HA) composites via the conventional crystallization process of nucleation and growth. While those methods have been successful in inducing heterogeneous nucleation of HA on the surface of collagen scaffolds, they have failed to produce a composite with the interpenetrating nanostructured architecture of bone. Our group has shown that intrafibrillar mineralization of type I collagen can be achieved using a polymer-induced liquid-precursor (PILP) process. In this process, acidic polypeptides are included in the mineralization solution to mimic the function of the acidic NCPs, and in vitro studies have found that acidic peptides such as polyaspartate induce a liquid-phase amorphous mineral precursor. Using this PILP process, we have been able to prepare collagen-HA composites with the fundamental nanostructure of bone, wherein HA nanocrystals are embedded within the collagen fibrils. This study shows that through further optimization a very high degree of mineralization can be achieved, with compositions matching that of bone. Synthetic collagen sponges were mineralized with calcium phosphate while analyzing various parameters of the reaction, with the focus of this report on the molecular weight of the polymeric process-directing agent. In order to determine whether intrafibrillar mineralization was achieved, an in-depth characterization of the mineralized composites was performed, including wide-angle X-ray diffraction, electron microscopy and thermogravimetric analyses. The results of this work lead us closer to the development of bone-like collagen-HA composites that could become the next generation of synthetic bone grafts.

Actinidain-hydrolyzed type I collagen reveals a crucial amino acid sequence in fibril formation

We investigated the ability of type I collagen telopeptides to bind neighboring collagen molecules, which is thought to be the initial event in fibrillogenesis. Limited hydrolysis by actinidain protease produced monomeric collagen, which consisted almost entirely of alpha1 and alpha2 chains. As seen with ultrahigh resolution scanning electron microscopy, actinidain-hydrolyzed collagen exhibited unique self-assembly, as if at an intermediate stage, and formed a novel suprastructure characterized by poor fibrillogenesis. Then, the N- and C-terminal sequences of chicken type I collagen hydrolyzed by actinidain or pepsin were determined by Edman degradation and de novo sequence analysis with matrix-assisted laser desorption ionization-tandem time-of-flight mass spectrometry, respectively. In the C-telopeptide region of the alpha1 chain, pepsin cleaved between Asp(1035) and Phe(1036), and actinidain between Gly(1032) and Gly(1033). Thus, the actinidain-hydrolyzed alpha1 chain is shorter at the C terminus by three residues, Gly(1033), Phe(1034), and Asp(1035). In the alpha2 chain, both proteases cleaved between Glu(1030) and Val(1031). We demonstrated that a synthetic nonapeptide mimicking the alpha1 C-terminal sequence including GFD weakly inhibited the self-assembly of pepsin-hydrolyzed collagen, whereas it remarkably accelerated that of actinidain-hydrolyzed collagen. We conclude that the specific GFD sequence of the C-telopeptide of the alpha1 chain plays a crucial role in stipulating collagen suprastructure and in subsequent fibril formation.

Temperature dependence of photochemical fluorescence fading in Skh-1 hairless mouse collagen

BACKGROUND

Type I mammalian collagens have several photolabile fluorescent moieties that absorb UV rays capable of reaching the dermis. We studied the temperature dependence of fluorescence fading as a marker of photochemical damage.

METHODS

Collagen solutions were exposed to radiation from 0 to 240 min from either a UVG-11 hand lamp, total dose=1.173 x 10(3) J/m(2); a UVL-21 hand lamp total dose=2.030 x 10(3) J/m(2); or the fluorometer, at 325+/-5 nm, total dose=0.156 x 10(3) J/m(2). We recorded intensities at excitation/emission wavelengths 270/300, 270/330, 270/360, 270/400, 325/400, and 370/450 nm at T=9.0-59.3 degrees C.

RESULTS

Results indicated simultaneous forward and reverse reactions. However, the 270/360 nm fluorophore could be analyzed as a second-order reaction. The Arrhenius curve showed two straight lines intersecting near the denaturation temperature, with helix activation energy E(a) approximately 0 and coil E(a)=7.6+/-0.6 kcal/mol (31.7+/-2.5 kJ/mol).

DISCUSSION

Collagen-bound fluorophores are not just passive markers of oxidative stress and age-related damage. Their photolability to wavelengths reaching the dermis may result in pathological conditions, particularly at elevated body temperatures.

Catalytic properties and structural components of lysyl oxidase

Key aspects of the biosynthesis and catalytic specificity of lysyl oxidase (LO) have been explored. Oxidation of peptidyl lysine in synthetic oligopeptides is markedly sensitive to the presence of vicinal dicarboxylic ami/no acid residues. Optimal activity is obtained with the -Glu-Lys- sequence within a polyglycine 11-mer, whereas the -Lys-Glu- sequence is much less efficiently oxidized. The -Asp-Glu-Lys- sequence is a very poor substrate, although this sequence is oxidized in type I collagen fibrils. These results are considered in the light of a model requiring collagen to be assembled as fibrils prior to oxidation by LO. An in vitro system for the expression of catalytically active LO has been devised. Deletion or inclusion of the cDNA coding for the propeptide region in the expressed construct results in apparently identical, catalytically active enzyme products, indicating the lack of essentiality of this region for active enzyme production. These effects are considered with respect to the conservation of the amino acid sequence of LO produced by different species.

Collagen degradation products modulate matrix metalloproteinase expression in cultured articular chondrocytes

Destruction of collagen within osteoarthritic cartilage depends in part on collagen-degrading matrix metalloproteases (MMP). Degradative fragments of type II collagen (Col II) occur in normal and in osteoarthritic cartilage, and may contribute to regulation of matrix turnover by interfering with normal cell-matrix communication pathways. Therefore, the effects of different types of collagen fragments on mRNA and protein levels of MMP-2, MMP-3, MMP-9, and MMP-13 in cultured bovine articular knee chondrocytes and explants were examined. Primary chondrocytes and explants were incubated with fragments from whole cartilage collagen matrix (Colf) and from purified type II collagen (Col2f), or with a synthetic 29-mer peptide representing the amino-terminal domain of type II collagen (Ntelo). Gelatin zymography revealed increases of proMMP-2, a shift towards active MMP-2 and increases in proMMP-9, depending on the type of fragment. In situ hybridization of cartilage sections displayed MMP-3 mRNA in virtually all cells. Moderate to strong increases in MMP-2, MMP-3, MMP-9, and MMP-13 mRNA levels were detected by quantitative PCR. The results demonstrate stimulating effects of collagen fragments on both mRNA and/or protein from MMP -2, -3, -9, and -13, and suggest a novel mechanism of MMP induction and activation that includes a particular role for N-telo in controlling catabolic pathways of matrix turnover.

Mutations Near Amino End of α1(I) Collagen Cause Combined Osteogenesis Imperfecta/Ehlers-Danlos Syndrome by Interference with N-propeptide Processing

Patients with OI/EDS form a distinct subset of osteogenesis imperfecta (OI) patients. In addition to skeletal fragility, they have characteristics of Ehlers-Danlos syndrome (EDS). We identified 7 children with types III or IV OI, plus severe large and small joint laxity and early progressive scoliosis. In each child with OI/EDS, we identified a mutation in the first 90 residues of the helical region of alpha1(I) collagen. These mutations prevent or delay removal of the procollagen N-propeptide by purified N-proteinase (ADAMTS-2) in vitro and in pericellular assays. The mutant pN-collagen which results is efficiently incorporated into matrix by cultured fibroblasts and osteoblasts and is prominently present in newly incorporated and immaturely cross-linked collagen. Dermal collagen fibrils have significantly reduced cross-sectional diameters, corroborating incorporation of pN-collagen into fibrils in vivo. Differential scanning calorimetry revealed that these mutant collagens are less stable than the corresponding procollagens, which is not seen with other type I collagen helical mutations. These mutations disrupt a distinct folding region of high thermal stability in the first 90 residues at the amino end of type I collagen and alter the secondary structure of the adjacent N-proteinase cleavage site. Thus, these OI/EDS collagen mutations are directly responsible for the bone fragility of OI and indirectly responsible for EDS symptoms, by interference with N-propeptide removal.

Collagen fibril form and function

The majority of collagen in the extracellular matrix is found in a fibrillar form, with long slender filaments each displaying a characteristic approximately 67?nm D-repeat. Here they provide the stiff resilient part of many tissues, where the inherent strength of the collagen triple helix is translated through a number of hierarchical levels to endow that tissue with its specific mechanical properties. A number of collagen types have important structural roles, either comprising the core of the fibril or decorating the fibril surface to give enhanced functionality. The architecture of subfibrillar and suprafibrillar structures (such as microfibrils), lateral crystalline and liquid crystal ordering, interfibrillar interactions, and fibril bundles is described. The fibril surface is recognized as an area that contains a number of intimate interactions between different collagen types and other molecular species, especially the proteoglycans. The interplay between molecular forms at the fibril surface is discussed in terms of their contribution to the regulation of fibril diameter and their role in interfibrillar interactions.

Type I collagen N-Telopeptides adopt an ordered structure when docked to their helix receptor during fibrillogenesis*

The in vitro rate and specificity of fibrillogenesis in type I collagen depends on the integrity of the amino (N)-telopeptide domain. In vivo an intact N-telopeptide domain is also required for normal fibril assembly. Although Chou-Fasman predictions and NMR studies suggested that a type I beta-turn could be induced in alpha1(I) N-telopeptide chains, computer modeling did not identify ordered structures. Nevertheless, X-ray analysis and electron tomography studies have shown that the N-telopeptide is in one of the most highly ordered fibril domains. This study was undertaken to determine if the docking of the N-telopeptide to its helix receptor domain could induce the telopeptides to take up a specific conformation. With use of molecular modeling suite of programs, a (Gly-Pro-Pro)(n) triple-helical structure was built on the basis of high-resolution X-ray crystallographic coordinates and then replaced with the actual bovine collagen residues 924-938, the triple-helical alpha1(I)-N-telopeptide-receptor sequences. Energy minimization produced a modified triple-helical conformation. The bovine alpha1(I) N-telopeptide sequence was similarly minimized and docked to this receptor. The docking induced an ordered conformation with a stabilizing hydrogen bond in the N-telopeptide and, importantly, a reciprocal reordering of the triple-helical conformation in the binding domain. This docked structure placed Lys residues in both telopeptide and helix in the correct locations for cross-link formation. The modeling has been extended to the three-chain N-telopeptide domain and finally to the construction of the Hulmes-Miller quasi-hexagonal packing structure. Each N-telopeptide domain can form linkages with two adjacent, aligned helix receptor domains. The telopeptides and the order of staggering of the three chains in the helix play crucial roles in the packing and intrafibrillar cross-linking patterns and the relative azimuthal orientations of adjacent molecules in the fibril. The models confirm the high order in the N-telopeptide 4D overlap zone.

The effects of collagen fragments on the extracellular matrix metabolism of bovine and human chondrocytes

Cartilage matrix degradation generates collagen type II fragments. The objective of this study is to explore the possibility that these collagen fragments may be part of an endogenous metabolic feedback. Initially, collagen fragments were extracted from normal or osteoarthritic cartilage, as part of a matrix fragment preparation. Later, collagen fragments were generated by digestion of bovine collagen type II with bacterial collagenase (col2f). These fragments were added to cultures of isolated chondrocytes (bovine and human) and cartilage explants (human). In a dose-dependent manner, col2f caused inhibition of cell attachment to collagen, inhibition of collagen synthesis, and induction of matrix degradation. In addition, when col2f were added to human cartilage explants, an induction of gelatinase activity was detected in the media. These data sets present first evidence that degradation products of collagen may be directly involved in the regulation of cartilage homeostasis.

Processing of type II procollagen amino propeptide by matrix metalloproteinases

In many embryonic tissues, type IIA procollagen is synthesized and deposited into the extracellular matrix containing the NH(2)-propeptide, the cysteine-rich domain of which binds to bone morphogenic proteins. To investigate whether matrix metalloproteinases (MMPs) synthesized during development and disease can cleave the NH(2) terminus of type II procollagens, we tested eight types of enzymes. Recombinant trimeric type IIA collagen NH(2)-propeptide encoded by exons 1-8 fused to the lectin domain of rat surfactant protein D was used as a substrate. The latter allowed trimerization of the propeptide domain and permitted isolation by saccharide affinity chromatography. Although MMPs 1, 2, and 8 did not show cleavage, MMPs 3, 7, 9, 13, and 14 cleaved the recombinant protein both at the telopeptide region and at the procollagen N-proteinase cleavage site. MMPs 7 and 13 demonstrated other cleavage sites in the type II collagen-specific region of the N-propeptide; MMP-7 had another cleavage site close to the COOH terminus of the cysteine-rich domain. To prove that an MMP can cleave the native type IIA procollagen in situ, we demonstrated that MMP-7 removes the NH(2)-propeptide from collagen fibrils in the extracellular matrix of fetal cartilage and identified the cleavage products. Because the N-proteinase and telopeptidase cleavage sites are present in both type IIA and type IIB procollagens and the telopeptide cleavage site is retained in the mature collagen fibril, this processing could be important to type IIB procollagen and to mature collagen fibrils as well.

Effects of singlet oxygen on the extracellular matrix protein collagen: oxidation of the collagen crosslink histidinohydroxylysinonorleucine and histidine

The reaction of singlet oxygen, a putative agent of skin photodamage, with the dermal collagen crosslink histidinohydroxylysinonorleucine (HHL) and its precursor histidine is reported. Reaction studies were performed with both purified HHL and bovine dermal tissue. We demonstrate that singlet oxygen can selectively oxidize HHL and histidine amino acid residues in dermal tissue and that intermediate oxidation products of histidine lead to new crosslink products. A novel mechanism for crosslink formation was proposed to involve nucleophilic addition to a transient imidazolone intermediate formed from singlet oxygen oxidation of the histidine imidazole moiety. The implication for such adduct formation and histidine oxidation in collagen proteins is the expression of aberrant collagen crosslinks, perturbation of the dermal collagen function, and hence an altered dermal state.

A model for type II collagen fibrils: distinctive D-band patterns in native and reconstituted fibrils compared with sequence data for helix and telopeptide domains

The periodical D-band pattern is generally considered a unique ultrastructural feature shared by all fibril-forming collagens, which correlates with the intrafibril, paracrystalline array of tropocollagen monomers. Distinct band patterns have been reported, however, for collagen stained long-spacing (SLS) crystallites of genetic types I, II, and III. Moreover, D-band patterns of negatively stained, native type II collagen fibrils were found to be not identical to those of type I in our previous research. Because of (a) these distinctive features, (b) tropocollagen heterotrimeric conditions (type I) vs homotrimeric conditions (type II), and (c) different lengths and poor homology between extrahelical telopeptides, the molecular array or telopeptide conformation within the extensively studied type I collagen fibrils could be not the same as those in the very much less intensively studied type II collagen fibrils. In this investigation, a distinctive positive-staining D-band pattern was found for type II collagen fibrils obtained from human cartilages. A fibril model was developed by analyzing actual D-band patterns, and matching them against simulated patterns based on the primary structure of extrahelical and helical domains in human type II tropocollagen. In particular, a more prominent b(1) band was apparent in native type II collagen fibrils than in type I. This distinctive feature was also observed for native-type collagen fibrils reconstituted from purified type II collagen, i.e., free from associated minor type XI collagen. On modeling possible monomer arrays, the best fit between microdensitograms and simulation traces was found for 234 amino acid staggering, as is also the case for type I collagen fibrils. On comparing this model with an analogous one for type I collagen fibrils, there was a higher intraband distribution of charged residues for band b(1), consistent with the higher electrondensity observed for this band in type II collagen fibrils. N- and C-telopeptide displacement in the model corresponded to D-locations of a c(2) subband, which we named c(2.0), and band a(3), respectively. In simulation profiles, c(2.0) -like and a(3) -like peaks mimicked the corresponding peaks in microdensitograms when molecular reversals were adopted at positions 10N-12N, 12C-14C, and 17C-19C for N- and C-telopeptides. Hydrophobic interactions and algorithmic predictions of protein secondary structure, according to Chou and Fasman and Rost and Sander criteria, were consistent with these conformational models, and suggest that an additional molecular reversal may occur at positions 3N-5N. These telopeptide "S-fold" conformations, interpreted as axial projections of tridimensional conformation, may represent starting points for further investigation into the still unresolved tridimensional conformation of telopeptides in monomers arrayed within type II collagen fibrils.

The in situ conformation and axial location of the intermolecular cross-linked non-helical telopeptides of type I collagen

BACKGROUND

Type I collagen contains specific lysine and hydroxylysine residues that are critical in the formation of intermolecular cross-links crucial for the normal configuration and stability of the 67 nm axial repeat of collagen fibrils in the extracellular matrix. The major cross-linkage sites are believed to occur between the non-helical terminal regions (telopeptides) and helical segments of adjacent collagen molecules. In this X-ray fibre diffraction study the tissue has been maintained in the hydrated fibrillar state, whilst detailed structural information was obtained using highly collimated synchrotron radiation.

RESULTS

The axial component of the X-ray diffraction patterns extends more than twice as far in reciprocal space than that of any already published. The structure-factor phases were calculated using the multiple isomorphous addition method, avoiding model-based approaches, and produced an electron-density profile of the molecular arrangement projected on to the fibre axis to 0.54 nm resolution. This corresponds to the phasing of 124 orders of the meridional diffraction pattern.

CONCLUSIONS

The axially projected electron-density profile and the electron-density difference maps showed that both the N- and C-terminal telopeptides are contracted structures. This profile puts narrow constraints on the possible conformations of the C-terminal telopeptide; the best fit to the electron-density profile is when the alpha1 chains adopt a folded conformation with a sharp hairpin turn around residues 13 and 14 of the 25-residue telopeptide. Our results reveal for the first time the location, parallel to the fibril axis, of the intermolecular cross-links in normal hydrated tissue. These cross-links are essential for the biological function of the tissue.

Recombinant procollagen II: Deletion of D period segments identifies sequences that are required for helix stabilization and generates a temperature-sensitive N-proteinase cleavage site

A cDNA cassette system was used to synthesize recombinant versions of procollagen II in which one of the four blocks of 234 amino acids that define a repeating D periods of the collagen triple helix were deleted. All the proteins were triple helical and all underwent a helix-to-coil transition between 25 and 42 degreesC as assayed by circular dichroism. However, the details of the melting curves varied. The procollagen lacking the D1 period unfolded 3 degreesC lower than a full-length molecule. With the procollagen lacking the D4 period, the first 25% of unfolding occurred at a lower temperature than the full-length molecule, but the rest of the structure unfolded at the same temperature. With the procollagen lacking the terminal D0.4 period, the protein unfolded 3 degreesC lower than the full-length molecule and a smaller fraction of the protein was secreted by stably transfected clones than with the other recombinant procollagens. The results confirmed previous suggestions that the collagen triple helix contains regions of varying stability and they demonstrated that the two D periods at the end of the molecule contain sequences that serve as clamps for folding and for stabilizing the triple helix. Reaction of the recombinant procollagens with procollagen N-proteinase indicated that in the procollagen lacking the sequences, the D1 period assumed an unusual temperature-sensitive conformation at 35 degreesC that allowed cleavage at an otherwise resistant Gly-Ala bond between residues 394 and 395 of the alpha1(II) chain.

Inhibition of the self-assembly of collagen I into fibrils with synthetic peptides. Demonstration that assembly is driven by specific binding sites on the monomers

A series of experiments were carried out to test the hypothesis that the self-assembly of collagen I monomers into fibrils depends on the interactions of specific binding sites in different regions of the monomer. Six synthetic peptides were prepared with sequences found either in the collagen triple helix or in the N- or C-telopeptides of collagen I. The four peptides with sequences found in the telopeptides were found to inhibit self-assembly of collagen I in a purified in vitro system. At concentrations of 2.5 mM, peptides with sequences in the C-telopeptides of the alpha1(I) and alpha2(I) chain inhibited assembly at about 95%. The addition of the peptide with the alpha2-telopeptide sequence was effective in inhibiting assembly if added during the lag phase and early propagation phase but not later in the assembly process. Experiments with biotinylated peptides indicated that both the N- and C-telopeptides bound to a region between amino acid 776 and 822 of the alpha(I) chain. A fragment of nine amino acids with sequences in the alpha2-telopeptide was effective in inhibiting fibril assembly. Mutating two aspartates in the 9-mer peptide to serine had no effect on inhibition of fibril assembly, but mutating two tyrosine residues and one phenylalanine residue abolished the inhibitory action. Molecular modeling of the binding sites demonstrated favorable hydrophobic and electrostatic interactions between the alpha2telopeptide and residues 781-794 of the alpha(I) chain.

Endoplasmic reticulum protein Hsp47 binds specifically to the N-terminal globular domain of the amino-propeptide of the procollagen I alpha 1 (I)-chain

Hsp47, an endoplasmic reticulum-resident heat shock protein in fibroblasts, has gelatin-binding properties. It had been hypothesized that it functions as a chaperone regulating procollagen chain folding and/or assembly, but the mechanism of the hsp47-procollagen I interaction was not clear. Hsp47 could bind to both denatured and native procollagen I. A series of competition studies were carried out in which various collagens and collagen domain peptides were incubated with 35[S]-methionine-labeled murine 3T6 cell lysates prior to mixing with gelatin-Sepharose 4B beads. The gelatin-bound proteins were collected and analyzed by gel electrophoresis and autoradiography. Collagenase digested procollagen I had the same effect as denatured intact procollagen, indicating that the propeptides were the major interaction sites. The addition of intact pro alpha 1(I)-N-propeptide at 25 micrograms/ml completely inhibited hsp47 binding to the gelatin-Sepharose. Even the pentapeptide VPTDE, residues 86-90 of the pro alpha 1(I)-N-propeptide, inhibits hsp47-gelatin binding. These data implicating the pro alpha 1(I)-N-propeptide domain were confirmed by examination of polysome-associated pro alpha chains. The nascent pro alpha 1(I)-chains with intact N-propeptide regions could be precipitated by monoclonal hsp47 antibody 11D10, but could not be precipitated by monoclonal anti-pro alpha 1 (I)-N-propeptide antibody SP1.D8 unless dissociated from the hsp47. GST-fusion protein constructs of residues 23-108 (NP1), 23-151 (NP2), and 23-178 (NP3) within the pro alpha 1 (I)- N-propeptide were coupled to Sepharose 4B and used as affinity beads for collection of hsp47 from 3T6 cell lysates. NP1 and NP2 both showed strong specific binding for lysate hsp47. Finally, the interaction was studied in membrane-free in vitro cotranslation systems in which the complete pro alpha 1(I)- and pro alpha 2(I)-chain RNAs were translated alone and in mixtures with each other and with hsp47 RNA. There was no interaction evident between pro alpha 2(I)-chains and hsp47, whereas there was strong interaction between pro alpha 1(I)-chains and nascent hsp47. SP1.D8 could not precipitate pro alpha 1(I)-chains from the translation mix if nascent hsp47 was present. These data all suggest that if hsp47 has a "chaperone" role during procollagen chain processing and folding it performs this specific role via its preferential interaction with the pro alpha 1 (I) chain, and the pro alpha 1(I) amino-propeptide region in particular.

Photochemistry of type I acid-soluble calf skin collagen: dependence on excitation wavelength

Although previous studies have demonstrated that the predominant photochemistry of type I collagen under 254 nm irradiation may be attributed either to direct absorption by tyrosine/phenylalanine or to peptide bonds, direct collagen photochemistry via solar UV wavelengths is much more likely to involve several age- and tissue-related photolabile collagen fluorophores that absorb in the latter region. In this study, we compare and contrast results obtained from irradiation of a commercial preparation of acid-soluble calf skin type I collagen in solution with UVC (primarily 254 nm), UVA (335-400 nm) and broad-band solar-simulating radiation (SSR; 290-400 nm). Excitation spectroscopy and analysis of photochemically induced disappearance of fluorescence (fluorescence fading) indicates that this preparation has at least four photolabile fluorescent chromophores. In addition to tyrosine and L-3,4-dihydroxyphenylalanine, our sample contains two other fluorophores. Chromophore I, with emission maximum at 360 nm, appears to be derived from interacting aromatic moieties in close mutual proximity. Chromophore II, with broad emission at 430-435 nm, may be composed of one or more age-related molecules. Collagen fluorescence fading kinetics are sensitive to excitation wavelength and to conformation. Under UVC, chromophore I fluorescence disappears with second-order kinetics, indicating a reaction between two proximal like molecules. Adherence to second-order kinetics is abrogated by prior denaturation of the collagen sample. A new broad, weak fluorescence band at 400-420 nm, attributable to dityrosine, forms under UVC, but not under solar radiation. This band is photolabile to UVA and UVB wavelengths.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibody to the aminotelopeptide of type II collagen: loss of the epitope after stromelysin digestion

A monoclonal antibody was prepared to the aminotelopeptide of type II collagen after immunization of DBA/1 mice with lathyritic type II collagen and subsequent screening for antibodies that recognize lathyritic but not pepsin-digested type II collagen. One antibody (called 5B2) was identified that recognized a short peptide sequence in the aminotelopeptide of chicken type II collagen but did not recognize other collagen types. Further characterization of the epitope was achieved using a Multipin system and the epitope was localized to a short linear sequence of six amino acids. The antibody recognized type II collagen from a variety of species including man and mouse. The epitope for 5B2 was found to be susceptible to cleavage with recombinant stromelysin without cleavage of the major collagen triple helix. Comparison was made between MAb 5B2 and two other antibodies (called MAb 2B1 and MAb 6B3) that recognize separate epitopes located along the triple helix of the type II collagen molecule.

Conformational analysis of the type II and type III collagen alpha-1 chain N-telopeptides by 1H-NMR spectroscopy and restrained molecular mechanics calculations

The type II and type III collagen alpha-1 chain N-telopeptides are a nonadecamer with the sequence pEMAGGFDEKAGGAQLGVMQ-NH2 and a tetradecamer with the sequence pEYEAYDVKSGVAGG-NH2, respectively. Their conformations have been studied in CD3OH/H2O (60/40) solution by means of two-dimensional proton nmr spectroscopy. Based on double quantum filtered correlation spectroscopy, total correlation spectroscopy, rotating frame nuclear Overhauser enhancement (ROE) spectroscopy, and nuclear Overhauser enhancement (NOE) spectroscopy experiments, all resonances were assigned and the conformational properties were analyzed in terms of vicinal NH-H alpha coupling constants, sequential and medium-range NOEs (ROEs), and amide proton temperature coefficients. The NOE distance constraints as well as dihedral constraints based on the vicinal NH-H alpha coupling constants were used as input parameters for restrained molecular mechanics, consisting of restrained molecular dynamics and restrained energy minimization calculations. The type II N-telopeptide's conformation is dominated by a fused beta gamma-turn between Phe6 and Ala10, stabilized by three hydrogen bonds and a salt bridge between the side-chain end groups of Glu8 and Lys9. The first 5 amino acids are extended with a much higher degree of conformational freedom. The 2 Gly residues following the turns were found to be highly flexible (hinge-like), leaving the spatial position of the second half of the molecule relative to the fused beta gamma-turn undefined. In the type III telopeptide, a series of sequential NH(i)-NH(i + 1) ROEs were observed between the amino acids Tyr2 and Ser9, indicating that a fraction of the conformational space is helical. However, the absence of medium-range ROEs and the lack of regularity of the effects associated with alpha-helices suggest the presence of a nascent rather than a complete helix.

Early assembly pathways of type I collagen

A method was developed for computing the free energy (delta Fi) of aggregates of type I collagen. The method was based on a treatment of Matheson and Flory describing phase equilibria of rigid rod polymers. It included a polymer-solvent interaction term that depended on near neighbor transfer energies. Extrahelical portions of the molecule were assigned local interaction energies differing from that assigned to the helix. Free energies of reaction for successive steps along assembly pathways (delta Fi-i+1) were computed. When allowance was made for specific pairing between extrahelical and helical domains, the so-called D-staggered (D = 670 A) alignment of molecules was preferred, as opposed to a nonstaggered, or nematic, alignment. Based on delta Fi-i+1 alone, it appeared that 1D-staggered oligomers arise first in assembly, followed later by addition of molecules in 4D alignment. Neither 4D dimers nor 4D-8D trimers were predicted to be major intermediates in assembly. This result is contrary to previous hypotheses. When energies of activation were included in the analysis, the prediction was less certain, and specific circumstances were identified in which 4D dimers and 4D-8D trimers were the earliest aggregated species in assembly.

An energetic evaluation of a "Smith" collagen microfibril model

An energy minimized three-dimensional structure of a collagen microfibril template was constructed based on the five-stranded model of Smith (1968), using molecular modeling methods and Kollman force fields (Weiner and Kollman, 1981). For this model, individual molecules were constructed with three identical polypeptide chains [Gly-Pro-Pro)n, (Gly-Prop-Hyp)n, or (Gly-Ala-Ala)n, where n = 4, 12, and 16) coiled into a right-handed triple-helical structure. The axial distance between adjacent amino acid residues is about 0.29 nm per polypeptide chain, and the pitch of each chain is approximately 3.3 residues. The microfibril model consists of five parallel triple helices packed so that a left-handed superhelical twist exists. The structural characteristics of the computed microfibril are consistent with those obtained for collagen by X-ray diffraction and electron microscopy. The energy minimized Smith microfibril model for (Gly-Pro-Pro)12 has an axial length of about 10.2 nm (for a 36 amino acid residue chain), which gives an estimated D-spacing (234 amino acids per chain) of approximately 66.2 nm. Studies of the microfibril models (Gly-Pro-Pro)12, (Gly-Pro-Hyp)12, and (Gly-Ala-Ala)12 show that nonbonded van der Waals interactions are important for microfibril formation, while electrostatic interactions contribute to the stability of the microfibril structure and determine the specificity by which collagen molecules pack within the microfibril.

Morphology of sheet-like assemblies of pN-collagen, pC-collagen and procollagen studied by scanning transmission electron microscopy mass measurements

At high concentrations, type I pN-collagen, pC-collagen and procollagen (the first 2 generated from procollagen by enzymic cleavage of C-propeptides and N-propeptides, respectively) can all be made to assemble in vitro into thin D-periodic sheets or tapes. Scanning transmission electron microscopy mass measurements show that the pN-collagen sheets and procollagen tapes have a mass per unit area corresponding to that of approximately 6.8 monolayers of close-packed molecules. pN-collagen sheets are extensive and remarkably uniform in mass thickness (fractional S.D. 0.035); procollagen tapes are neither as extensive nor as uniform in thickness. The mean thickness of pC-collagen tapes is less and the variability is greater. In pN-collagen sheets, the overlap: gap mass contrast in a D-period is increased from 5:4 (the ratio in a native collagen fibril) to 6:4, showing that the N-propeptides do not project into the gap but are folded back over the overlap zone. Assuming all N-propeptides to be constrained to the two surfaces of a sheet, their surface density can be found from the mass thickness of the sheet. In a lateral direction (i.e. normal to the axial direction where the spacing is D-periodic), the N-propeptide domains are calculated to be spaced, centre to centre, by 2.23 (+/- 0.1) nm on both surfaces. This value (approx. 1.5 x the triple-helix diameter) implies close-packing laterally with adjacent domains in contact. Sheet formation and the "surface-seeking" behaviour of propeptides can be understood in terms of the dual character of the molecules, evident from solubility data, with propeptides possessing interaction properties very different from those displayed by the rest of the molecule. The form and stability of sheets (and of first-formed fibrils assembling in vivo) could, it is suggested, depend on the partially fluid-like nature of lateral contacts between collagen molecules.

FTIRS in H2O demonstrates that collagen monomers undergo a conformational transition prior to thermal self-assembly in vitro

The assembly of type I collagen molecules into native fibrils can be accomplished in vitro in solutions at physiological ionic strength and pH by raising the temperature above 30 degrees C. The thermal self-assembly reaction exhibits a distinct lag phase. This lag phase has been proposed to be evidence for a conformational transition in the monomer. Fourier transform infrared spectroscopy (FTIRS) is a very sensitive probe of the H-bonded states within the triple helix. The carbonyl group spectrum (amide I, 1700-1600 cm-1) has been investigated in collagen/H2O solutions at 1 mg/mL under self-assembly conditions from 4 to 34 degrees C and, in the same range, at a higher ionic strength where self-assembly does not occur. The deconvoluted spectra show three very clear bands at approximately 1660, 1644, and 1630 cm-1. These bands vary in both frequency maxima and relative intensity over the temperature range examined. Spectra were also obtained in the amide II and III regions. Spectral changes were evident in the 22-26 degrees C range, under fibril-forming conditions, which lead to the hypothesis that the triple helix of the semiflexible collagen molecule is actually perfected during the lag phase, facilitating nucleation and intermolecular interaction. Further spectral changes after fibrils do form show that the molecules are once again distorted as they are bent to fit within the fibrils.

D-periodic assemblies of type I procollagen

The solubility limit of purified chick type I procollagen, incubated at 37 degrees C in phosphate-buffered saline, was found to be in the range 1 to 1.5 mg/ml. At higher concentrations large aggregates formed. These comprised: (1) D-periodic assemblies; (2) narrow filaments with no apparent periodicity; and (3) segment-long-spacing-like aggregates. The D-periodic assemblies, which predominated at high concentrations, were separated from the other types of aggregate and found to be ribbon-like. Ribbons were uniform in thickness (approximately 8 nm) and up to 1 micron wide. Staining patterns showed features similar to those in native-type collagen fibrils. Immunolabelling indicated that the carboxyl-terminal propeptide domains were close to the carboxyl-terminal gap-overlap junction, and that the amino-terminal propeptide domains were folded over into the amino-terminal side of the overlap zone. Both propeptide domains appeared to be located on the surface of the assemblies. These observations show that intact propeptide domains hinder, but do not prevent, the formation of D-periodic assemblies. The presence of the propeptide domains on the surface of a growing assembly could restrict its lateral growth and limit its final thickness.

The collagen fibril--a model system for studying the staining and fixation of a protein

A collagen fibril is made up of long rod-like molecules regularly D-staggered with respect to one another. This means that (i) its axially projected fine structure, resolvable to approximately 2 nm in electron micrographs, repeats D-periodically (D = 67 nm), and (ii) the amino acid residues contributing to each element of the fine structure can be inferred from sequence data. Electron-optical data from a fibril D-period can can therefore be correlated directly with chemical data. Such correlations confirm the electrostatic nature of the staining reaction when a fibril is positively stained. After negative staining, the principal factor determining the small-scale distribution of stain is local exclusion by 'bulky' amino acid side-chains. ('Bulkiness' is the average cross-sectional area, or 'plumpness', of a side-chain.) A small superimposed positive staining contribution can also be detected. Fixation of collagen by aldehydes and diimidoesters occurs via an initial reaction with lysyl (and hydroxylsyl) side-chains and alpha-amino groups, followed by secondary cross-linking reactions that differ from fixative to fixative. These secondary reactions determine the nature and abundance of the cross-links and the extent to which they influence subsequent staining behaviour.

The role of extrahelical peptides in stabilization of collagen fibrils

A quantitative model for fibril assembly of type I collagen was extended to include the explicit effect of extrahelical peptides. The collagen molecule was simulated by rod-like sequences to which short, rigid tails were connected by "nondimensional" flexible joints. Three collagen structures were studied: (1) intact collagen, simulated by a rod of axial ratio 200 (The axial ratio x was taken as a segment length) with two tails of length x = 1 and x = 2, respectively, appended to each end; (2) pepsin-digested collagen, simulated by one rigid segment of length 200 and one tail of length 1; and (3) pronase-digested collagen, by a single rigid segment of length x = 200. Phase equilibria of such structures were calculated, using a lattice theory of Matheson and Flory, and the relation of the polymer-solvent interaction parameter chi to the equilibrium solubility was determined. The chi for each collagen species was then related to temperature (T) and ionic strength (I), based on the approximation that local (per segment) stabilization of collagen fibrils was due to hydrophobic and electrostatic forces only. Solubility vs temperature curves for all three collagen species were computed and compared to published experimental data. From the chi factors for each species, the composite chi was resolved into components representing energetic contributions of the extrahelical peptides relative to the helix, which were interpreted in terms of hydrophobic or electrostatic interactions stabilizing the collagen fibril.

Solution conformation of the type I collagen alpha-1 chain N-telopeptide studied by 1H NMR spectroscopy

The solution conformation of the type I collagen alpha-1 chain N-telopeptide has been studied by CD and 1H NMR spectroscopy at 600 MHz in CD3OH/H2O (60/40 v/v) and H2O solutions. The 19 amino acids form the N-terminal end of the alpha-1 polypeptide chain. By the combined application of several two-dimensional, phase-sensitive NMR techniques (COSY, RELAY, ROESY), a complete assignment of all proton resonances was achieved, and the conformation of the backbone could be established on the basis of the coupling constant and NOE data. In CD3OH/H2O solutions the spectroscopic evidence clearly indicates that two sections of the molecule (pE1-Y6 and T11-M19) are extended and that the D7-S10 segment forms a beta-turn, stabilized by a hydrogen bond between NH(S10) and CO(D7). The data suggest that the turn is of the type I kind (minor) and that it coexists with an extended structure (major conformer). Interactions between the two extended parts of the peptide were not observed, thus excluding the existence of a beta-sheet. In H2O solution the conformation is significantly different, with no beta-turn, but a completely extended structure is observed.

Collagen fibril surface structures: freeze-etching data and computer modelling

In order to investigate the correlation between primary structure and surface profile of collagen, freeze-fractured and deep-etched (dehydrated) collagen fibrils have been compared with a fibril model built directly from the amino acid sequence. The model appeared to be almost identical to the freeze-etched collagen fibril with respect to the most relevant features, such as the gap-overlap ratio and the location of the two main intraperiod ridges (X3 and X2). However, it did not show the minor ridges observed in the gap zone (X1 and Y1), thus suggesting that these structures, although consistently present in collagen fibrils, may in fact be due to post-translational modifications or to non-collagenic material adhering to the fibril.

Phasing the meridional diffraction pattern of type I collagen using isomorphous derivatives

The meridional X-ray diffraction pattern of wet rat tail tendon contains information about the one-dimensional structure, or axial projection of electron density distribution of the type I collagen fibril. Using synchrotron radiation we have determined the intensities of the first 50 meridional X-ray diffraction reflections. The approach of isomorphous addition with reagents, selected using criteria of chemical reactivity, which label at fewer sites than the stains used in previous studies was applied to phase these 50 reflections to produce a one-dimensional electron density distribution map of a single D-repeat of the collagen fibril. This method is not model-dependent and thus constitutes the first unambiguous determination of the meridional phases of type I collagen.

Reactivity of the allysine aldehyde group

Allysine is a very important crosslink precursor in connective tissue proteins. We describe the reactions of Phthalyl-Allysine-p-Nitrobenzylester, which is a suitable compound to investigate the reactions of the aldehyde group in vitro. Crosslinked compounds are synthesized by mixing suitable stoichiometric amounts of Phthalyl-Allysine-p-Nitrobenzylester and nucleophiles in aqueous organic solvents. The results are compared with the reaction pathways which have been proposed for crosslink synthesis in vivo.