Observation of Ultrafast Vibrational Energy Transfer in Fibrinogen and Fibrin Fibers

Myeloperoxidase-induced fibrinogen unfolding and clotting

Due to its unique properties and high biomedical relevance fibrinogen is a promising protein for the development of various matrixes and scaffolds for biotechnological applications. Fibrinogen molecules may form extensive clots either upon specific cleavage by thrombin or in thrombin-free environment, for example, in the presence of different salts. Here, we report the novel type of non-conventional fibrinogen clot formation, which is mediated by myeloperoxidase and takes place even at low fibrinogen concentrations (<0.1 mg/ml). We have revealed fibrillar nature of myeloperoxidase-mediated fibrinogen clots, which differ morphologically from fibrin clots. We have shown that fibrinogen clotting is mediated by direct interaction of myeloperoxidase molecules with the outer globular regions of fibrinogen molecules followed by fibrinogen unfolding from its natural trinodular to a fibrillar structure. We have demonstrated a major role of the Debye screening effect in regulating of myeloperoxidase-induced fibrinogen clotting, which is facilitated by small ionic strength. While fibrinogen in an aqueous solution with myeloperoxidase undergoes changes, the enzymatic activity of myeloperoxidase is not inhibited in excess of fibrinogen. The obtained results open new insights into fibrinogen clotting, give new possibilities for the development of fibrinogen-based functional biomaterials, and provide the novel concepts of protein unfolding.

Influence of Divalent Metal Ions on the Precipitation of the Plasma Protein Fibrinogen

Fibrinogen nanofibers are very attractive biomaterials to mimic the native blood clot architecture. Previously, we reported the self-assembly of fibrinogen nanofibers in the presence of monovalent salts and have now studied how divalent salts influence fibrinogen precipitation. Although the secondary fibrinogen structure was significantly altered with divalent metal ions, morphological analysis revealed exclusively smooth fibrinogen precipitates. In situ monitoring of the surface roughness facilitated predicting the tendency of various salts to form fibrinogen fibers or smooth films. Analysis of the chemical composition revealed that divalent salts were removed from smooth fibrinogen films upon rinsing while monovalent Na⁺ species were still present in fibrinogen fibers. Therefore, we assume that the decisive factor controlling the morphology of fibrinogen precipitates is direct ion-protein contact, which requires disruption of the ion-surrounding hydration shells. We conclude that in fibrinogen aggregates, this mechanism is effective only for monovalent ions, whereas divalent ions are limited to indirect fibrinogen adsorption.

Principles of Fibrinogen Fiber Assembly In Vitro

Fibrinogen nanofibers hold great potential for applications in wound healing and personalized regenerative medicine due to their ability to mimic the native blood clot architecture. Although versatile strategies exist to induce fibrillogenesis of fibrinogen in vitro, little is known about the underlying mechanisms and the associated length scales. Therefore, in this manuscript the current state of research on fibrinogen fibrillogenesis in vitro is reviewed. For the first time, the manifold factors leading to the assembly of fibrinogen molecules into fibers are categorized considering three main groups: substrate interactions, denaturing and non-denaturing buffer conditions. Based on the meta-analysis in the review it is concluded that the assembly of fibrinogen is driven by several mechanisms across different length scales. In these processes, certain buffer conditions, in particular the presence of salts, play a predominant role during fibrinogen self-assembly compared to the surface chemistry of the substrate material. Yet, to tailor fibrous fibrinogen scaffolds with defined structure-function-relationships for future tissue engineering applications, it still needs to be understood which particular role each of these factors plays during fiber assembly. Therefore, the future combination of experimental and simulation studies is proposed to understand the intermolecular interactions of fibrinogen, which induce the assembly of soluble fibrinogen into solid fibers.

Controlling the Multiscale Structure of Nanofibrous Fibrinogen Scaffolds for Wound Healing

As a key player in blood coagulation and tissue repair, fibrinogen has gained increasing attention to develop nanofibrous biomaterial scaffolds for wound healing. Current techniques to prepare protein nanofibers, like electrospinning or extrusion, are known to induce lasting changes in the protein conformation. Often, such secondary changes are associated with amyloid transitions, which can evoke unwanted disease mechanisms. Starting from our recently introduced technique to self-assemble fibrinogen scaffolds in physiological salt buffers, we here investigated the morphology and secondary structure of our novel fibrinogen nanofibers. Aiming at optimum self-assembly conditions for wound healing scaffolds, we studied the influence of fibrinogen concentration and pH on the protein conformation. Using circular dichroism and Fourier-transform infrared spectroscopy, we observed partial transitions from α-helical structures to β-strands upon fiber formation. Interestingly, a staining with thioflavin T revealed that this conformational transition was not associated with any amyloid formation. Toward novel scaffolds for wound healing, which are stable in aqueous environment, we also introduced cross-linking of fibrinogen scaffolds in formaldehyde vapor. This treatment allowed us to maintain the nanofibrous morphology while the conformation of fibrinogen nanofibers was redeveloped toward a more native state after rehydration. Altogether, self-assembled fibrinogen scaffolds are excellent candidates for novel wound healing systems since their multiscale structures can be well controlled without inducing any pathogenic amyloid transitions.

Determination of the Solution Structure of Antifreeze Glycoproteins Using Two-Dimensional Infrared Spectroscopy

We study the solution structure of antifreeze glycoproteins (AFGPs) with linear and two-dimensional infrared spectroscopy (2D-IR). With 2D-IR, we study the coupling between the amide I and amide II vibrations of AFGPs. The measured nonlinear spectral response constitutes a much more clearly resolved amide I spectrum than the linear absorption spectrum of the amide I vibrations and allows us to identify the different structural elements of AFGPs in solution. We find clear evidence for the presence of polyproline II (PPII) helical structures already at room temperature, and we find that the fraction of PPII structures increases when the temperature is decreased to the biological working temperature of AFGP. We observe that inhibition of the antifreeze activity of AFGP using borate buffer or enhancing the antifreeze activity using sulfate buffer does not lead to significant changes in the protein conformation. This finding indicates that AFGPs bind to ice with their sugar side chains.