Aggregation of intrinsically disordered fibrinogen as the influence of backbone conformation

Fib having intrinsically disordered αC domains is involved in coagulation cascade and thrombosis. Fib molecules forms prefibrillar oligomers at 30%, and associate in 40 and 50% TFE to proceed α to β transition, suggesting the formation of an intermolecular β-structure. AFM images confirmed the nature of Fib aggregates at 40 and 50% TFE to be prefibrillar and fibrillar respectively. These aggregates possess high thioflavin T fluorescence with a shifted Congo red absorbance. Kinetics of Fib aggregation data at 50% TFE supports nucleation-dependent polymerization mechanism. At 60 and 70% TFE, no aggregation was observed. The inhibition of protein aggregation appears due to weakening of the hydrophobic interactions that were initially stabilizing the intermolecular β-sheet structure in the protein aggregation. The loss of hydrophobic contacts seems to favor the formation of intramolecular hydrogen bonds over intermolecular hydrogen bonds leading to helix formation. To conclude, protein aggregation is accompanied by the formation of β-sheet conformation, and induction of non-native helical segments in the protein inhibits aggregation. The discrepancy of the secondary structures on aggregation is proposed to stem from the disparity in the nature of the hydrogen bonds and packing of hydrophobic residues of the side chains in the β-sheet and α-helix conformation.