Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution

Direct NMR measurement of folding kinetics of a trimeric peptide

Direct NMR measurements of the folding kinetics are performed on a collagen-like triple helical peptide. The triple helical peptide was designed to model a biologically important region of collagen and has the sequence (POG)3ITGARGLAG(POG)4. Triple helical peptides were synthesized with specifically labeled 15N amino acid residues in key positions, and the kinetics of folding of the individual residues were monitored directly by measuring the loss of monomer intensity and the increase in trimer intensity. The residues at the terminal ends and central region could be followed independently and quantitated directly. Residues located at the terminal ends have rates and kinetics of folding that are distinct from residues in the central region of the peptide. This allows the monitoring of different steps in the folding mechanism and the postulation of the existence of a kinetic intermediate. The NMR data are consistent with a mechanism of association/nucleation and propagation. Hereditary connective tissue diseases are associated with mutations that result in abnormal folding of collagen, and the NMR folding experiments on a collagen-like peptide provide a basis for characterizing the molecular defect in folding mutations.

The effect of deamination and/or blocking arginine residues on the molecular assembly of acid-extracted rat tail tendon collagen

We describe the effect of deamination of lysine and blocking of arginine residues on the assembly of collagen into native fibrils and SLS aggregates. Treatment of collagen solutions with one or both of these procedures does not prevent the formation of fibrils or SLS aggregates but reduces their ability to form assemblies with accurate longitudinal registration. These observations provide direct confirmation that hydrophobic interactions are important in collagen assembly. Unbanded fibrils were formed within the first 24 h at 4 degrees C from both acidic and neutralized deaminated and from neutralized control collagen solutions, transversely banded fibrils appearing later. This is compatible with the suggestion that initially, collagen fibrils are assembled by lyotropic liquid crystallization and with other observations which suggest that collagen molecules are initially free to move laterally within the fibril before being locked into place. Fibrils assembled from deaminated collagen solution show two variant longitudinal registration patterns which grade into one another. This suggests that, with a reduction in positively charged side chains, the thermodynamic energy minima responsible for longitudinal registration are less sharp compared with control collagen solutions. Reduction of positive charge by chemical modification helps to explain why the chemical modifications reduce swelling of collagen fibres. It also helps to explain why fibrils form spontaneously at 4 degrees C in both arginine-blocked and deaminated collagen solutions. Thus chemical modifications of rat tail tendon provides new insight into the mechanisms in collagen assembly.

Mineral changes in a mouse model of osteogenesis imperfecta detected by Fourier transform infrared microscopy

Osteogenesis imperfecta (OI) is a heritable disease characterized by skeletal deformities and brittle bones. In the current study, the nature of the mineral in long bones of a mouse model of OI (oim/oim, a mutant which produces an alpha 1(I) collagen homotrimer) was examined by Fourier transform infrared microscopy. The mineral:matrix ratio of oim/oim cortical bone was greater than that of the heterozygous oim/+ and of the normal +/+ bones, probably as a result of reduced collagen content. The molecular environments of the apatitic phosphates differed among the oim/oim and the oim/+ and the +/+ bones. This was attributable to several factors, including dissimilar mineral-matrix interactions and differences in the chemical composition of the mineral. It was concluded from these data that the defective collagen matrix leads to abnormal mineral formation at the molecular level and thus results in tissues with reduced mechanical properties.

NMR and x-ray studies of collagen model peptides

The main structural component in collagen is the triple helix which is generally composed of the amino acid sequence repeat (X-Y-Gly)n with proline and hydroxyproline often present at positions X and Y. Non-globular, fibrillar proteins like most collagens are difficult to work with from a structural perspective. An alternative approach to collagen structural elucidation is to study considerably shorter fragments of the triple helix. To date, various triple helical model peptides such as (Pro-Pro-Gly)n and (Pro-Hyp-Gly)n have been investigated by various physical and spectroscopic techniques. The advent of easy solid phase peptide synthetic methodology and the development of multi-dimensional heteronuclear and high field NMR technologies have promoted significant advances in the structure elucidation of a number of triple helix peptides. Here, the main focus is to review and to address the current state of knowledge in the field of NMR and x-ray analysis of triple helical model peptides.

Perspectives on the synthesis and application of triple-helical, collagen-model peptides

Collagens can be distinguished from other proteins based on their triple-helical structure. Synthetic peptide models have been developed to better understand the triple helix structurally and to evaluate the triple helix as a recognition element for biological processes. Associated triple-helical peptides were first designed and assembled by solid-phase methodology in the late 1960s. Such peptides were used for triple-helical structural characterization by CD, nmr, and ir spectroscopies, and x-ray crystallography, and for studying the structural preferences of hydroxylases. In the late 1970s, methods were developed for covalently linking the three strands of triple-helical peptides. One benefit of "branched" peptides was the enhancement of triple-helical thermal stability. The incorporation of specific collagen sequences into thermally stable synthetic triple helices in the early 1990s has allowed for the mechanistic investigation of collagen-mediated cell adhesion and platelet aggregation. In time, discriminatory therapeutics may result from the continued exploration and further understanding of the biological effects of collagen primary, secondary, and tertiary structures via triple-helical peptide models.

Inductive Effects on the Energetics of Prolyl Peptide Bond Isomerization: Implications for Collagen Folding and Stability

The hydroxylation of proline residues in collagen enhances the stability of the collagen triple helix. Previous X-ray diffraction analyses had demonstrated that the presence of an electron-withdrawing substituent on the pyrrolidine ring of proline residues has significant structural consequences [Panasik, N., Jr.; Eberhardt, E. S.; Edison, A. S.; Powell, D. R.; Raines, R. T. Int. J. Pept. Protein Res.1994, 44, 262-269]. Here, NMR and FTIR spectroscopy were used to ascertain kinetic and thermodynamic properties of N-acetyl-[β,γ-(13)C]D,L-proline methylester (1); N-acetyl-4(R)-hydroxy-L-proline [(13)C]methylester (2); and N-acetyl-4(R)-fluoro-L-proline methylester (3). The pK(a)'s of the nitrogen atom in the parent amino acids decrease in the order: proline (10.8) > 4(R)-hydroxy-L-proline (9.68) > 4(R)-fluoro-L-proline (9.23). In water or dioxane, amide I vibrational modes decrease in the order: 1 > 2 > 3. At 37 °C in dioxane, the rate constants for amide bond isomerization are greater for 3 than 1. Each of these results is consistent with the traditional picture of amide resonance coupled with an inductive effect that results in a higher bond order in the amide C=O bond and a lower bond order in the amide C-N bond. Further, at 37 °C in water or dioxane equilibrium concentrations of the trans isomer increase in the order: 1 < 2 < 3. Inductive effects may therefore have a significant impact on the folding and stability of collagen, which has a preponderance of hydroxyproline residues, all with peptide bonds in the trans conformation.

Disrupted collagen architecture in the crystal structure of a triple-helical peptide with a Gly-->Ala substitution

The crystal structure of the collagen-based peptide (Pro-Hyp-Gly)4-Pro-Hyp-Ala-(Pro-Hyp-Gly)5 has provided for the first time a highly detailed picture of the architectural elements that come into play in the collagen triple helix. The center of the molecule, which harbors a Gly-->Ala substitution, shows subtle conformational changes that result in a local untwisting of the triple helix. The characteristic hydrogen bonding pattern of collagen triple helices is replaced by interstitial water bridges. These effects may be relevant to the diseased states derived from Gly-->X mutations in collagens. The possible implications of this disrupted architecture for collagen assemblies are discussed.

Localization of the site on the complement component C1q required for the stimulation of neutrophil superoxide production

C1q, the recognition subunit of the classical complement pathway, interacts with specific cell surface molecules via its collagen-like region (C1q-CLR). This binding of C1q to neutrophils triggers the generation of toxic oxygen species. To identify the site on C1q that interacts with the neutrophil C1q receptor, C1q was isolated, digested with pepsin to produce C1q-CLR, and further cleaved with either trypsin or endoproteinase Lys-C. The resulting fragments were separated by gel filtration chromatography and analyzed functionally (activation of the respiratory burst in neutrophils) and structurally. Cleavage of C1q-CLR with endoproteinase Lys-C did not alter its ability to trigger neutrophil superoxide production. However, when C1q-CLR was incubated with trypsin under conditions permitting optimal cleavage, the ability of C1q-CLR to stimulate superoxide production in neutrophils was completely abrogated. Fractionation of the digests obtained with the two enzymes and identification by amino acid sequencing permitted localization of the receptor interaction site to a specific region of the C1q-CLR. Circular dichroism analyses demonstrated that cleavage by trypsin does not denature the remaining uncleaved collagen-like structure, suggesting that after trypsin treatment, the loss of activity was not due to a loss of secondary structure of the molecule. However, irreversible heat denaturation of C1q-CLR also abrogated all activity. Thus, a specific conformation conferred by the collagen triple helix constitutes the functional receptor interaction site. These data should direct the design of future specific therapeutic reagents to selectively modulate this response.

Substitutions of aspartic acid for glycine-220 and of arginine for glycine-664 in the triple helix of the pro alpha 1(I) chain of type I procollagen produce lethal osteogenesis imperfecta and disrupt the ability of collagen fibrils to incorporate crystalline hydroxyapatite

We identified two infants with lethal (type II) osteogenesis imperfecta (OI) who were heterozygous for mutations in the COL1A1 gene that resulted in substitutions of aspartic acid for glycine at position 220 and arginine for glycine at position 664 in the product of one COL1A1 allele in each individual. In normal age- and site-matched bone, approximately 70% (by number) of the collagen fibrils were encrusted with plate-like crystallites of hydroxyapatite. In contrast, approximately 5% (by number) of the collagen fibrils in the probands' bone contained crystallites. In contrast with normal bone, the c-axes of hydroxyapatite crystallites were sometimes poorly aligned with the long axis of fibrils obtained from OI bone. Chemical analysis showed that the OI samples contained normal amounts of calcium. The probands' bone samples contained type I collagen, overmodified type I collagen and elevated levels of type III and V collagens. On the basis of biochemical and morphological data, the fibrils in the OI samples were co-polymers of normal and mutant collagen. The results are consistent with a model of fibril mineralization in which the presence of abnormal type I collagen prevents normal collagen in the same fibril from incorporating hydroxyapatite crystallites.

Effect of protamine on the solubilization of collagen-tailed acetylcholinesterase: potential heparin-binding consensus sequences in the tail of the enzyme

Asymmetric acetylcholinesterase (AChE) contains three tetrameric sets of catalytic subunits disulfide-linked to structural subunits of a collagenic tail. This form is localized in the basement membrane zone of the neuromuscular junction, where it interacts with proteoglycans. It has been described that heparin-binding domains of many proteins contains clusters of basic residues. Here we show that protamine--a highly basic protein--specifically solubilizes asymmetric AChE from the rat neuromuscular junction, starting at 25 micrograms/ml and reaching a plateau at 250 micrograms/ml protamine. We also show that protamine was able to displace AChE bound to heparin-agarose. Two synthetic peptides corresponding to the sequence of the collagenic tail polypeptide also release the enzyme. Finally, we propose that two heparin-binding consensus sequences (-B-B-X-B-) are present in the tail of AChE. Our results indicate that clusters of basic residues are responsible for the interaction of the collagen-tailed AChE with proteoglycans.

Hydration structure of a collagen peptide

BACKGROUND

The collagen triple helix is a unique protein motif defined by the supercoiling of three polypeptide chains in a polyproline II conformation. It is a major domain of all collagen proteins and is also reported to exist in proteins with host defense function and in several membrane proteins. The triple-helical domain has distinctive properties. Collagen requires a high proportion of the post-translationally modified imino acid 4-hydroxyproline and water to stabilize its conformation and assembly. The crystal structure of a collagen-like peptide determined to 1.85 Angstrum showed that these two features may be related.

RESULTS

A detailed analysis of the hydration structure of the collagen-like peptide is presented. The water molecules around the carbonyl and hydroxyprolyl groups show distinctive geometries. There are repetitive patterns of water bridges that link oxygen atoms within a single peptide chain, between different chains and between different triple helices. Overall, the water molecules are organized in a semi-clathrate-like structure that surrounds and interconnects triple helices in the crystal lattice. Hydroxyprolyl groups play a crucial role in the assembly.

CONCLUSIONS

The roles of hydroxyproline and hydration are strongly interrelated in the structure of the collagen triple helix. The specific, repetitive water bridges observed in this structure buttress the triple-helical conformation. The extensively ordered hydration structure offers a good model for the interpretation of the experimental results on collagen stability and assembly.

Acid destabilization of a triple-helical peptide model of the macrophage scavenger receptor

Electrostatic interactions were studied in a triple-helical peptide, (POG)3PKGQKGEKG(POG)4, which contains a lysine-rich 9 residue sequence from the collagen-like domain of the macrophage scavenger receptor (MSR). This peptide adopts a stable triple-helical conformation only when the pH is higher than 4.5, corresponding to ionization of the Glu side chain. Modeling shows Glu forms ion pairs with one of the Lys residues, stabilizing the structure. Previously studied collagen-like peptides show relatively small contributions of electrostatic interactions to stability. The large magnitude of the pH mediated structural changes seen for this peptide suggests that specific placement of charged residues in the triple-helix conformation can generate strong electrostatic interactions.

Three-dimensional energy-minimized model of human type II "Smith" collagen microfibril

A procedure is described for constructing a three-dimensional model of fibril-forming human type II collagen based on the "Smith" microfibril model. This model is a complex of five individual collagen triple-helical molecules, and is based on known structural parameters for collagen. Both experimental and theoretical data were used as constraints to guide the modeling. The resulting fibril model for type II collagen is in agreement with both physical and chemical characteristics produced by experimental staining patterns of type II fibrils. Some advantages of the type II model are that the stereochemistry of all the sidechain groups is accounted for, and specific atomic interactions can now be studied. This model is useful for: development of therapeutics for collagen related diseases; development of synthetic collagen tissues; design of chemical reagents (i.e., tanning agents) to treat collagen-related products; and study of the structural and functional aspects of type II collagen. Described is the procedure by which the Smith microfibril of type II collagen was developed using molecular modeling tools, validation of the model by comparison to electron-microscopic images of fibril staining patterns, and some applications of this microfibril model.

Atypical Gly-X-Y sequences surround interruptions in the repeating tripeptide pattern of basement membrane collagen

The triple-helical domains of type IV collagen chains have more than 20 sites at which the repeating (Gly-X-Y)n pattern is interrupted. Analysis of alpha 1 (IV) and alpha 2 (IV) chains indicates the residues in the three Gly-X-Y triplets preceding or following interruptions differ statistically from the rest of the chain. Unusually high frequencies of charged residues are seen at a number of X and Y sites, with the charge density being particularly high C-terminal to the interruption site. Analyses were carried out on individual categories of interruptions, classified as insertions or deletions in the Y position. All of the residues in the X and Y positions of the triplets flanking insertion sites are atypical, with a high concentration of charged residues. Triplets flanking sites where there has been a deletion in the Y position show unusually high frequencies of charged residues at some sites, hydrophobic residues at other sites, and an invariant imino acid N-terminal to the interruption. The presence of atypical sequences surrounding interruptions could be important at a molecular level, related to triple-helix stability, or at a supramolecular level, related to the association of molecules to form networks in basement membranes.

Two heparin-binding domains are present on the collagenic tail of asymmetric acetylcholinesterase

The collagen-tailed form of acetylcholinesterase (AChE) binds to heparin and heparan sulfate proteoglycans. We have employed synthetic peptides corresponding to the central collagenic region of the tail of AChE, to identify the heparin-binding domains of the tail of asymmetric AChE. Two putative heparin-binding consensus sequences were localized in the collagenic tail. Peptides containing such sequences (P-(145-159) and P-(249-262)) were able to release asymmetric AChE bound to heparin-agarose. A triple mutation, Asn-Asp-Gly-Gly instead of Arg-His-Gly-Arg, completely abolishes the capacity of the peptide P-(145-159) to elute AChE from the heparin column. Our results suggest that the interaction between the collagen-tailed AChE and proteoglycans is mediated by clusters of basic residues that form two belts around the triple helix of the collagenic tail.

Temperature-favoured assembly of collagen is driven by hydrophilic not hydrophobic interactions

It has become almost axiomatic that protein folding and assembly are dominated by the hydrophobic effect. The contributions from this, and other, hydrophilic interactions can now be better distinguished by direct measurement of forces between proteins. Here we report the measurement of forces between triple helices of type I collagen at different temperatures, pH and solute concentrations. We separate repulsive and attractive components of the net force and analyze the origin of the attraction responsible for the collagen self-assembly. In this case the role of the hydrophobic effect appears to be negligible. Instead, water-mediated hydrogen bonding between polar residues is the most consistent explanation.

Trimeric C-type lectin domains in host defence

Recent crystal structures, established for fragments of human and rat mannose-binding proteins, indicate that the triple-stranded alpha-helical coiled coil present in these collectins is responsible for the trimeric orientation of C-type lectin domains.