CHEMICAL BIOLOGY

Collagen Triple Helix, Stability of

Hans Peter Bachinger and Kazunori Mizuno, Shriners Hospital for Children, Portland, Oregon

doi: 10.1002/9780470048672.wecb096

Collagens are the most abundant extracellular matrix proteins in multicellular animals. They all contain a name-giving collagen triple helix, which connects their three chains and varying amounts of other noncollagenous protein domains. To form the triple helix a repeated sequence of -Gly-Xaa-Yaa- is required, where Xaa and Yaa can be any residue. Each chain forms a polyproline-II like left-handed helix. The three chains are staggered by one residue from each other, and form a right-handed helix. Twenty eight types of collagen molecules have been identified in mammals. The stability of the collagen triple helix is based on the length and the amino acid sequence of each polypeptide chain, and also by the presence of interchain cross-links and/or trimerization domains. The 4(R)-hydroxylation of proline residues in the Yaa position significantly increases the stability of the collagen triple helix.

Collagens are proteins that have a typical triple helical higher order structure, and the triple helix is a protein motif that can be found in other proteins. The chemistry of collagen covers more than seven orders of magnitude from subnanometer scale to centimeter scale (from amino acids to a tendon) including a wide range of both noncovalent and covalent interactions. Collagen research covers a wide range of fields such as biochemistry, organic chemistry, and biophysical chemistry.

Biologic Background

Collagen is the major component of the extracellular matrix of multicellular animals. In humans, the collagen molecules are classified into 28 types (Table 1). In addition, other proteins contain collagen-like triple helical domains (Table 2). Most mammals have a similar set of these proteins. Collagens in vertebrates are numbered in the order of their discovery using Roman numerals as type I, type II, type III, and so on. In invertebrates, collagens vary from one species to another including differences in posttranslational modifications. The Caenorhabditis elegans genome has upward of 150 distinct collagen genes (1). If a collagen molecule of an invertebrate is highly homologous to a specific type of the vertebrate collagens, then it is referred to the specific number type of the vertebrate collagen. For example, one of the basement membrane collagens, type IV collagen, is found in both vertebrates and also invertebrates such as Drosophila melanogaster and C. elegans as highly homologous primary structures. In that case, the molecule is called a type IV collagen. In general, the genes of collagen molecules of invertebrates are different from those of vertebrates. Most proteins with a collagen triple helix were found as a component of the extracellular matrix (ECM). However, some proteins contain a transmembrane domain, for example, type XVII collagen molecule is a type II transmembrane protein in hemidesmosomes. In addition, some triple helical proteins are found in the serum as soluble proteins such as collectins and complement C1q. Usually, they are related to natural innate immunity. The most abundant collagen protein in vertebrates is type I collagen. It is distributed ubiquitously in the vertebrate body and forms fibrils with a variety of diameter (25nm-200nm). Type I collagen molecules self assemble at physiologic temperature to fibrils in vitro. When type I collagen molecules are denatured into single polypeptides at higher temperatures, they form a gel at lower temperature (gelatin), which consists of partially refolded molecules. Type I collagen interacts with a wide range of other extracellular matrix molecules to form the specific tissue. The local concentration of the collagen molecules, pH, temperature, the direction of mechanical force, and the order of interactions with other molecules affect the supramolecular organization of the collagen fibrils and the entire ECM. Tendon, skin, and cornea are composed mainly of type I collagen, but their physical properties, such as the length and the diameter of collagen fibrils and the direction of collagen fibrils are different. Cells and ECM interact with each other. The homeostasis of cell-ECM interaction is essential for all biologic phenomena of multicellular animals.

Table 1. Classification of collagen in mammals: Known and estimated information about collagen

FAC IT, fibril associated collagens with interrupted triple helices; dc, discontinuous.

Table 2. Proteins with collagenous domains in mammals

#, Number of human protein is indicated as regular fonts.

Conglutinin, CL-43, and CL-46 are found in bovine. Hibernation proteins are found in Tamias sibiricus.

Structure of the Collagen Triple Helix

The general sequence of the collagen triple helix requires a repeat of -Gly-Xaa-Yaa- where the Xaa and the Yaa residues can be any amino acid. Glycine in every third residue is required to form the triple helix. Figure 1 shows a model of the collagen triple helix. In the triple helix, all residues in the Xaa and the Yaa positions are exposed to the molecular surface. Each polypeptide forms a polyproline-II like left-handed helix. Three chains are staggered by one residue, and they form a right-handed superhelix (Fig. 1). The carbonyl group of the collagen helix is almost perpendicular to the molecular axis. This orientation is different from the alpha-helix, in which the carbonyl groups are almost parallel to the helical axis.

Figure 1. Collagen triple helix structure. (a) Side view of the single peptide of collagen helix. (b) Side view of collagen triple helix (PDB: 1V4F, crystal structure of collagen model peptides with the repeated -Pro-4(R)Hyp-Gly-sequence). The carboxy-terminal is on the top, and the amino-terminal is on bottom. (c) Top view of the collagen triple helix (PDB: 1V4F, crystal structure of collagen model peptides with the repeated -Pro-4(R)Hyp-Gly-sequence). The carboxy-terminal is in the front, and the amino-terminal is in the back. Only the C alpha atoms are shown. Each polypeptide is a left-handed helix, and the three-chains form right-handed super helix. Glycine residues are located near the center of the helix.

In the analysis of crystals of collagen model peptides, the carbonyl oxygen of the Yaa position residue has two hydrogen bonds with water molecules and that of Gly has one, because the other site for a hydrogen bond is hindered stereochemically by the neighboring peptide chain. In addition, the carbonyl oxygen in the Xaa position of Pro is directed to the center of the triple helix and participates in a direct hydrogen bond with the NH of Gly in the neighboring chain. This interchain Gly(NH... OC)Xaa hydrogen bond is almost perpendicular to the molecular axis. Based on the diffraction data of crystals, the left-handed 7/2-helical structure (Fig. 1) with a 20 A axial repeat is obtained when both the Xaa and the Yaa positions are occupied by imino acids (2). (Here, left-handed 7/2-helical symmetry means the seven Gly-Xaa-Yaa- tripeptide units from three chains make two left-handed turns in an axial repeat). Each peptide strand forms a right-handed 7/1-helix in which seven tripeptide units and one helical unit turn in a 60 A axial repeat. Therefore, the tripeptide unit twist and the tripeptide unit height of the strand are 51.4 (= 360/7) and 8.57 A (= 60/7), respectively. The type I collagen molecule has a major triple helical domain with 1,014 amino acid residues of repeated -Gly-Xaa-Yaa- sequence. The pitch of each residue is 2.85 A along the molecular axis. Therefore, the length of type I collagen molecule is about 300 nm. In collagen fibrils or the crystals of collagen model peptides, the distance between each triple helix is almost 1.5 nm.

The triple helical domain of collagen has a unique amino acid composition. Glycine accounts for one third of the total amino acid content. In homothermal animals, proline and 4(K)-hydroxyproline (Hyp) accounts for almost 10% each. These three residues comprise more than half of the amino acids in collagen. Glycine and proline are usually regarded as two exceptional residues in proteins. Glycine is the only nonchiral amino acid, and it has only a hydrogen atom as side chain; usually, the composition is less than the other amino acid residues in globular proteins. Because of the smallest side chains, the dihedral angles of glycine are the most conformationally flexible among the 20 amino acids. In contrast, proline is the least conformationally flexible because of the five-membered pyrrolidine ring structure that restrict the phi angle. Proline is the only imino acid among the 20 coded amino acids. The high content of Gly and Pro/4(R)Hyp give the collagen triple helix unique properties not found in globular proteins. The content of hydrophobic residues in the triple helix is much lower than in globular proteins. In the collagen triple helix, more positively charged residues exist (Lys + Arg) than negatively charged residues (Glu + Asp). Because of the highly extended rod-like structure of the collagen triple helix, the accessible surface area per residue is much larger than that of globular proteins. All residues in the Xaa and the Yaa position are exposed to the molecular surface in a radial pattern.

Proline ring puckering

The pyrrolidine ring of (hydroxy)proline is not planar. The analyses of model collagen peptides at high resolution have revealed that there is distinct propensity of ring puckering in the Xaa and the Yaa position. In most cases, the imino acid residues in the Xaa position have the Cy-endo (down) puckering (Fig. 2). In contrast, most of imino acid residues in the Yaa position have the Cy-exo (up) puckering conformation.

Figure 2. Proline ring pucker. Ball and stick model of the proline ring structures (a) Cy-endo (down) puckering of Pro and (b) Cy-exo (up) puckering of 4(R)-hydroxyproline from PDB:1V4F

Posttranslational Modifications

Collagen molecules undergo extensive posttranslational modifications (Fig. 3) in the rough endoplasmic reticulum during biosynthesis (3). These modifications include 4-hydroxylation of proline to 4(R)-hydroxyproline (enzymes: prolyl-4- hydroxylases, P4H 1, 2 & 3 EC 1.14.11.2) at most -Gly-Xaa-Pro-Gly- sequences, 3-hydroxylation of proline to 3(S)-hydroxyproline (prolyl 3-hydroxylases P3H 1, 2, and 3; EC 1.14.11.7) in some -Gly-Pro-4Hyp-Gly- sequences, and 5-hydroxylation of lysine to 5-hydroxylysine (5-lysylhydroxylases, ly- syl hydroxylases (LHs), procollagen-lysine 2-oxoglutarate 5-dioxygenases LH 1, 2a/2b & 3: EC 1.14.11.4) in some Gly-Xaa-Lys-Gly- sequences. In almost all mammals and avians, prolyl 4-hydroxylation occurs in almost all Pro residues in the Yaa position; however, prolyl 3-hydroxylation is much less frequent. Each type I collagen polypeptide chain has one 3-hydroxyproline. About a quarter of Lys in the Yaa position is modified to 5-hydroxylysine. Sometimes hydroxylysine is modified even more to O-β- galactosylhydroxylysine (enzyme: galactosyltransferase, EC 2.4.1.50) and 2-O-α-D-glucosyl-O-β-D-galactosylhydroxylysine (enzyme: galactosylhydroxylysyl glucosyltransferase, EC 2.4.1.66). This type of glycosylation has been found only in the collagen triple helical domain. LH3 also possesses relatively low levels of collagen glucosyltransferase activity and very low levels of collagen galactosyltransferase activity in addition to the lysyl hydroxylase activity. The posttranslational modifications of invertebrate collagens are more complex than in vertebrates. The cuticle collagen of the sea hydrothermal vent tube worm Riftia pachyptila contains glycosylated threonine (4) residues in the Yaa position and very few proline or hydroxyproline residues. Some species have 4-hydroxyproline in the Xaa position that is not found in vertebrates.

After the secretion into the extracellular space, collagen molecules may be modified chemically even more. Some collagen processing enzymes cleave the procollagen molecules to mature tissue type molecules. Lysine and 5-hydroxylysine residues can be modified enzymatically to allysine (alpha-aminoadipic-acid delta-semialdehyde) and hydroxyallysine (delta-hydroxy, alpha-aminoadipic acid delta-semialdehyde), respectively, by lysyl oxidase (protein-lysine 6-oxidase) (EC 1.4.3.13). The aldehyde group then interacts with the amino group of an adjacent lysine residue to form a Schiff base. Many intermolecular cross-links between collagen triple helices are found in the tissues (Fig. 4) (5). Some cross-links are necessary for the tensile strength of tissues. In contrast, some are the result of chemical deterioration of tissues (6, 7) and are related to senescence and the various kinds of diseases such as diabetes mellitus. The half-life of human skin collagen is estimated to be 15 years by the racemization of aspartic acid, which indicates that collagen molecules have potentially more chances to be modified chemically in vivo.

Figure 3. Posttranslational modifications in collagen. (a) 3(S)-hydroxyproline. (b) 4(R)-hydroxyproline. (c) 5-hydroxylysine. (d) O-p-galactosyl-5-hydroxylysine. (e) 2-O-a-D-glucosyl-O-p-D-galactosyl-5-hydroxylysine.

Figure 4. Structure of collagen cross-links. The 6-amino group of (hydroxy)lysinie is modified to the oxidative deamination allysine Hydroxyallysine.

Stability of the Collagen Triple Helix

The temperature where one half of the polypeptide exists as a triple helix is termed the melting temperature (Tm), and is specific for different collagens from different species. The Tm of soluble collagen at neutral pH physiologic salt solution is around the body temperature of the organism from which it is isolated. The thermal unfolding of interstitial fibrillar collagens, type I, II, and III, occurs in a very narrow temperature interval of less than 3 degrees in passing from a 90% native to a 90% denatured conformation. Also, the guanidinium chloride induced transition of collagens is very sharp and occurs after increasing the concentration of the denaturant by only 0.2 M. The denaturation of these types of collagen is highly cooperative. The enthalpy change of the triple helix ⇔ coil transition for type I collagen was determined to be ∆H = 15-18 kJ/mole tripeptide unit. The enthalpy change per residue of triple helix is significantly greater than that of globular proteins. The source of this greater enthalpy change is still controversial. Another unique thermodynamic property of collagen molecules is the absence of a change in heat capacity after the denaturation.

The stability of the collagen triple helix depends on the sequence and the length of the three polypeptide chains. The thermal stability of collagen from different animal species is correlated with their highest environmental temperature. In general, the thermal stability of collagen is increased with the content of imino acid, praline, and 4(R)-hydroxyproline. Especially, the contribution of 4(R)-hydroxyproline in the Yaa position is important. An additional stabilization of the collagen triple helix occurs on the formation of fibrils. The melting temperature of the collagen triple helix in fibrils is around 55 ° C.

To clarify the mechanism of the stability of the triple helix, synthetic collagen model peptides have been used. The effect of 20 types of amino acid residues on the stability of the triple helix was analyzed in the context of host-guest system, that is, acetyl-(Gly-Pro-4(R)Hyp)3-Gly-Xaa-4(R)Hyp-(Gly-Pro-4(R) Hyp)4-Gly-Gly-NH2 and acetyl-(Gly-Pro-4(R)Hyp)3-Gly-Pro-Yaa-(Gly-Pro-4(R)Hyp)4-Gly-Gly-NH2 (8).

Some residues, such as Gly and aromatic residues significantly destabilize the triple helix. Galactosylations of threonine (9) and that of the lysine (10) increases the stability of the triple helix.Regarding the stabilizing effect of 4(R)-hydroxyproline on the triple helix, studies with model peptides showed that the order of the stability of the triple helix is -Gly-Pro-4(R)Flp- > -Gly-Pro-4(R)Hyp- > Gly-Pro-Pro-, where Flp is fluoroproline. Fluorine is the most electronegative atom, and organic fluorine forms only weak hydrogen bonds. Neither -Gly-4(R)Hyp-Pro nor -Gly-Pro-4(S)Hyp- form a triple helix. These properties can be explained by a stereoelectronic effect of the 4-substitution (11). The inductive effect, the gauche effect of the pyrrolidine ring (4-subsitution and the amide group), and an n → π* interaction. These effects adjust the optimum phi and psi angles of both the Xaa and the Yaa positions of collagen peptides and the puckering of the pyrrolidine ring.

Solvent affects the stability of collagen molecules and the triple helix of collagen model peptides. Polyols, such as glycerol, 1,2- and 1,3-propanediol, polyethylene glycol, sugars, and glycosaminoglycans increase the melting temperature of the triple helix. Trimethylamine N-oxide (TMAO) also stabilizes the triple helix of collagen model peptides. Urea and guani- dinium hydrochloride decrease the Tm of collagen concentration linearly with increasing concentrations.

The stability of the triple helix is not affected as much by pH as globular proteins. There is a slight destabilization of the triple helix at acid pH. The effect of salt ions on the stability is similar to the Hofmeister series (12).

Chemical Tools and Techniques for Collagen Research

Collagen is one of the most popular proteins for biophysical and biochemical analysis because of its unique shape, ease of acquisition, and stability. The structure of the collagen triple helix seems more homogeneous and simpler than that of globular proteins. However, the difficulties of collagen preparations for biochemical and biophysical analysis are the heterogeneity of the sample by posttranslational modifications, intermolecular covalent cross linking, the large molecular weight, the aggregation properties at physiologic temperatures, and the hysteresis of the folding-refolding reaction. Almost all biochemical and biophysical techniques have been applied to collagen such as circular dichroism (CD), analytical ultracentrifugation, differential scanning calorimetry (DSC), isothermal titration calorimetry, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, scanning tunneling microscopy, second harmonic generation microscopy, laser scattering, electric birefringence, X-ray analysis, IR and NMR.

The collagen triple helix has a unique CD spectrum. The spectrum shows a positive peak at 220-225 nm, and a negative peak at 195-200 nm. In contrast, the polyproline II helix has a positive peak at 228 nm, and a negative peak at 206 nm. The polyproline II like poly-4-hydroxyproline helix has a positive peak at 219 nm and a negative peak at 205 nm (13). The thermal stability of the triple helix can be monitored easily by the CD signal at 221 nm. The CD signal monitored as a function of temperature shows a sigmoidal denaturation curve.

NMR is used for the characterization of the triple helical structure and also for the folding kinetics of the triple helix. NMR studies with synthetic model peptides of the triple helix are difficult because of overlapping resonances of the repeated sequence and by peak broadening from the shape. Isotopic labeling is used to observe specific residues using heteronuclear NMR techniques. Hydrogen exchange studies are used to show the Gly amide exchange. The exchange is faster in the imino acid-poor regions of a synthetic peptide compared with the Gly-Pro-4(R)Hyp region.

DSC was used to characterize the thermodynamic properties of the collagen triple helix to coil transition in individual molecules and in fibrils. Many microscopy techniques were used to visualize individual collagen molecules and their supramolecular assemblies.

Collagen model peptide synthesis

Because the natural collagen molecules are large, heterogeneous, and difficult to purify, collagen model peptides are used for many analyses. Recent developments in solid-phase peptide synthesis techniques have made the synthesis of longer peptides easier than before. The commercial availability of Fmoc-derivatives of 4(R)-hydroxyproline, 4(R)-fluoroproline and 3(S)-hydroxyproline, the synthesis of glycosylated-Fmoc threonine and glycosylated-Fmoc-hydroxylysine, and other unusual amino acids have extended the range of collagen-like peptides that have been synthesized. Overlapping cysteine linked peptides of type III collagen have been made, which covers the whole triple helical region of type III collagen (1029 amino acids) (14). Several cross-links were introduced to stabilize the three chains of the triple helix (see below).

Figure 5. Oligomerization of collagenous model peptides.

Trimerized triple helical model design

In vivo, collagen molecules have domains that initiate trimerization of the synthesized alpha chains. Usually, globular domains at the carboxyl terminal end of the molecule have this role.

Once this domain is removed, refolding of the three collagen polypeptides to the correctly staggered molecules is very difficult or impossible. The folding reaction from three polypeptides into a triple helix is affected by the concentration of the peptides. For the kinetic refolding analysis of triple helical model peptides, several techniques have been developed to link the three polypeptides (Fig. 5). These are

a The type III collagen homotrimeric disulfide knot (-Gly-Pro-Cys-Cys- sequence)

b Regioselective artifical cystine knot for heterotrimers (15)

c The foldon domain of T4 phage fibritin (16)

d A di-lysine scaffold (17)

e cis,cis -1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid (Kemp tri acid) (18)

f TREN (Tris(2-aminoethyl)amine) succnic acid (19)

g 1,2,3-propane carboxylic acid (20)

h A coordination of Fe(II) ions to built-in bipyridine ligands (21)

i Monoalykyl chains (22)

j 18-membered cyclic hydropyran oligolide

References

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4. Mann K, Mechling DE, Bachinger HP, Eckerskorn C, Gaill F, Timpl, R. Glycosylated threonine but not 4-hydroxyproline dominates the triple helix stabilizing positions in the sequence of a hydrothermal vent worm cuticle collagen. J. Mol. Biol. 1996; 261:255-266.

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11. Raines RT. 2005 Emil Thomas Kaiser award. Protein. Sci. 2006; 15:1219-1225.

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14. Raynal N, Hamala SW, Siljander PRM, Maddox B, Peachey AR, Fernandez R, Foley LJ, Slatter DA, Jarvis GE, Farndale RW. Use of synthetic peptides to locate novel integrin alpha1beta1-binding motifs in human collagen III. J. Biol. Chem. 2006; 281:3821-3831.

15. Ottl J, Battistuta R, Pieper M, Tschesche H, Bode W, Kuhn K, Moroder L. Design and synthesis of heterotrimeric collagen peptides with a built-in cystine-knot. Models for collagen catabolism by matrix-metalloproteases. FEBS. Lett. 1996; 398:31-36.

16. Frank S, Kammerer RA, Mechling D, Schulthess T, Landwehr R, Bann J, Guo Y, Lustig A, Bachinger HP, Engel J. Stabilization of short collagen-like triple helices by protein engineering. J. Mol. Biol. 2001; 308:1081-1089.

17. Fields CG, Mickelson DJ, Drake SL, McCarthy JB, Fields GB. Melanoma cell adhesion and spreading activities of a synthetic 124-residue triple-helical “mini-collagen”. J. Biol. Chem. 1993; 268:14153-14160.

18. Kemp DS, Petrakis KS. Synthesis and conformational analysis of cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid. J. Org. Chem. 1981; 46:5140-5143.

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22. Yu YC, Tirrell M, Fields GB. Minimal lipidation stabilizes protein-like molecular architecture. J Am. Chem. Soc. 1998; 120:9979-9987.

Further Reading

Bachinger HP, Engel J. The thermodynamics and kinetics of collagen folding. In: Protein Folding Handbook. Buchner J, Kiefhaber T, eds. 2005. Wiley, New York.

Brinckmann J, Notbohm H, Muller PK, eds. Collagen Primer in Structure, Processing and Assembly (Series: Topics in Current Chemistry, vol. 247). 2005. Springer, Berlin.

Brodsky B, Persikov AV. Molecular structure of the collagen triple helix. Adv. Protein Chem. 2005; 70:301-339.

Kielty CM, Grant ME. The collagen family: structure, assembly, and organization in the extracellular matrix. In: Connective Tissue and its Heritable Disorders. 2nd edition. Royce PM, Steinmann B, eds. 2002. Wiley-Liss, New York.

Jenkins CL, Raines RT. Insights on the conformational stability of collagen. Nat. Prod. Rep. 2002; 19:49-59.

Okuyama K, Xu X, Iguchi M, Noguchi K. Revision of collagen molecular structure. Biopolymers 2006; 84:181-191.

Richard-Blum S, van der Rest M, Dublet B. Unconventional Collagens: Types VI, VII, VIII, IX, X, XII, XIV, XVI, and XIX. Protein Profile Series. 2000. Oxford University Press, New York.

Wess TJ. Collagen fibril form and function. Adv. Protein Chem. 2005; 70:341-374.

See Also

Protein Folding: Energetics of

Peptide Synthesis

Calorimetry: Techniques for Proteins