Structural Effects of pH and Deacylation on Surfactant Protein C in an Organic Solvent Mixture: A Constant-pH MD Study

The pulmonary surfactant protein C (SP-C) is a small highly hydrophobic protein that adopts a mainly helical structure while associated with the membrane but misfolds into a β-rich metastable structure upon deacylation, membrane dissociation, and exposure to the neutral pH of the aqueous alveolar subphase, eventually leading to the formation of amyloid aggregates associated with pulmonary alveolar proteinosis. The present constant-pH MD study of the acylated and deacylated isoforms of SP-C in a chloroform/methanol/water mixture, often used to mimic the membrane environment, shows that the loss of the acyl groups has a structural destabilizing effect and that the increase of pH promotes intraprotein contacts which contribute to the loss of helical structure in solution. These contacts result from the poor solvation of charged groups by the solvent mixture, which exhibits a limited membrane-mimetic character. Although a single SP-C molecule was used in the simulations, we propose that analogous intermolecular interactions may play a role in the early stages of the protein misfolding and aggregation in this mixture.