Influence of Lipidation on the Folding and Stability of Collagen Triple Helices-An Experimental and Theoretical Study.

The folding of triple-helical collagen, the most abundant protein in nature, relies on the nucleation and propagation along the strands. Hydrophobic moieties are crucial for the folding and stability of numerous proteins. Instead, nature uses for collagen a trimerization domain and cis-trans prolyl isomerases to facilitate and accelerate triple helix formation. Yet, pendant hydrophobic moieties endow triple-helical collagen with hyperstability and accelerate the cis-trans isomerization to an extent that thermally induced unfolding and folding of collagen triple helices take place at the same speed. Here, we systematically explored the effect of pendant fatty acids on the folding and stability of collagen triple helices. Thermal denaturation and kinetic studies with a series of collagen mimetic peptides (CMPs) bearing saturated and unsaturated fatty acids with different lengths revealed that longer and more flexible fatty acid appendages increase the stability and the folding rate of collagen triple helices. Molecular dynamics simulations combined with experimental data indicate that the hydrophobic appendages stabilize the triple helix by interaction with the grooves of the collagen triple helix and accelerate the folding and unfolding process by creating a molten globule-like intermediate.

Alkylation of γ-Azaproline Creates Conformationally Adaptable Proline Derivatives for pH-Responsive Collagen Triple Helices.

Cγ -substituted proline derivatives are valuable tools for developing functionalized collagen peptides for biological and materials investigations, yet the stereochemistry at Cγ can produce undesired steric or stereoelectronic constraints. Alkylated γ-azaproline (γ-azPro) derivatives are proline mimetics that lack a stereogenic center at the γ-position of the ring and can thus utilize the invertibility of nitrogen to adapt their conformation. NMR spectroscopic analyses and DFT calculations highlighted how alkylated γ-azPro derivatives are conformationally dynamic and adopt conformational preferences through ring pucker flip along with nitrogen inversion. Lastly, incorporation of alkylated γ-azPro into collagen peptides produced functionalized pH-responsive triple helices with similar thermal stabilities, regardless of their placement in the Xaa or Yaa position within the characteristic Xaa-Yaa-Gly repeating unit of collagen peptides.

Hydrophobic Moieties Bestow Fast-Folding and Hyperstability on Collagen Triple Helices.

Trans amide bonds and fast cis- trans isomerization of Xaa-Pro bonds are crucial for the stability and folding rate of collagen, the most abundant protein in mammals. Here, we explored the effect of pendant hydrophobic moieties on the folding and stability of collagen triple helices. Kinetic studies with a series of collagen model peptides showed that a local hydrophobic environment accelerates cis- trans isomerization to an extent that thermally induced unfolding and folding of the collagen triple helix take place at the same speed. Thermal denaturation studies revealed that the hydrophobic appendages provide hyperstable collagen triple helices ( Tm = 70 °C).

γ-Azaproline Confers pH†Responsiveness and Functionalizability on Collagen Triple Helices.

Proline derivatives bearing substituents at Cγ are valuable tools for biological and materials investigations. However, the stereochemistry at Cγ can produce undesired steric or stereoelectronic interactions. Here, we introduce γ-azaproline (γ-azPro), which lacks a stereogenic center at Cγ, as a pH-responsive and functionalizable proline analogue that can adapt to its environment. Conformational analyses by NMR spectroscopy and DFT calculations revealed that the imidazolidine ring of γ-azPro is flexible. Incorporation of γ-azPro into collagen model peptides (CMPs) produced pH-responsive triples helices and triple helices that can be easily functionalized.

pH-Responsive Aminoproline-Containing Collagen Triple Helices.

(4S)- and (4R)-configured aminoproline (Amp) residues were used as pH-responsive probes to tune the thermal stability of collagen triple helices in acidic and basic environments. The different steric and stereoelectronic properties of amino versus ammonium groups lead to a switch of the ring pucker of Amp upon changing the pH. The choice of the position of Amp within collagen model peptides (CMPs) as well as the absolute configuration at C(4) of the pH-responsive probe allows for tuning of the stability of Amp-containing collagen triple helices over a broad range. Comparative quantum chemical calculations on the steric and stereoelectronic effects of amino and ammonium groups versus fluorine, hydroxy, chlorine, and methyl substituents support the experimental findings. The research also shows that substitution of the naturally occurring hydroxy group in collagen by electron-withdrawing groups with a larger hydration shell than that of the hydroxy group is not tolerated.

Why Proline? Influence of Ring-Size on the Collagen Triple Helix.

The effect of four- and six-membered ring-size analogs (azetidine- and piperidine-2-carboxylic acid, H-Aze-OH and H-Pip-OH) of proline on the stability of the collagen triple helix was examined. Computational and nuclear magnetic resonance spectroscopic studies with model compounds and thermal denaturation experiments with collagen peptides showed that the ring-size analogs destabilize the triple helix to a similar extent by either mismatching backbone dihedral angles ϕ and ψ (Pip) or by an unfavorable trans/cis amide bond ratio (Aze).

Temperature-controlled electrospray ionization mass spectrometry as a tool to study collagen homo- and heterotrimers.

Collagen model peptides are useful for understanding the assembly and structure of collagen triple helices. The design of self-assembling heterotrimeric helices is particularly challenging and often affords mixtures of non-covalent assemblies that are difficult to characterize by conventional NMR and CD spectroscopic techniques. This can render a detailed understanding of the factors that control heterotrimer formation difficult and restrict rational design. Here, we present a novel method based on electrospray ionization mass spectrometry to investigate homo- and heterotrimeric collagen model peptides. Under native conditions, the high resolving power of mass spectrometry was used to access the stoichiometric composition of different triple helices in complex mixtures. A temperature-controlled electrospray ionization source was built to perform thermal denaturation experiments and provided melting temperatures of triple helices. These were found to be in good agreement with values obtained from CD spectroscopic measurements. Importantly, for mixtures of coexisting homo- and heterotrimers, which are difficult to analyze by conventional methods, our technique allowed for the identification and monitoring of the unfolding of each individual species. Their respective melting temperatures could easily be accessed in a single experiment, using small amounts of sample.

Effect of N- and C-terminal functional groups on the stability of collagen triple helices.

The effect of charged versus neutral N- and C-termini on the stability of the collagen triple helix was examined. Thermal denaturation studies at different pH with collagen model peptides showed that an ammonium group at the N-terminus destabilizes the triple helix more than a carboxylate at the C-terminus. A neutral carboxylic acid stabilizes the triple helix more than an amido moiety at the C-terminus.