Revisiting the Environment Specificity of the Protein (De)solubilization Ability of ATP

The intriguingly surplus abundance of adenosine triphosphate (ATP) found in living cells has often been rationalized in terms of ATP acting as a "biological hydrotrope" or "biological aggregation inhibitor" to solubilize proteins under stressed conditions. Here, we scrutinize the universality of such a property of ATP depending on different environmental circumstances; we report instantaneous fibrillation of two globular proteins, namely, bovine serum albumin (BSA) and ovalbumin (Ova), at ambient temperature (25 °C) and at pH 2 in the presence of ATP. This behavior is in stark contrast to the fact that ATP is reported to suppress thermal unfolding of these same two proteins. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and circular dichroism (CD) measurements confirm the aggregates as amyloid-like fibrils. Estimation of protein hydration using terahertz-Fourier transform infrared spectroscopy (THz-FTIR) in the 1.5-16.7 THz (50-550 cm^-1) frequency window confirms the key role of the triphosphate (TPP) moiety of ATP, as it binds directly to the solvent-exposed charged residues of the unfolded proteins, which seeds the aggregation process. Protein residue specificity is found to dictate the aggregation kinetics, as lysine-rich BSA shows faster kinetics compared to that of Ova, which contains a relatively smaller number of lysine residues; however, its abundance of arginine residues is not markedly different to account for the observed large change in the fibrillation rate. This result receives further support from the fact that in lysozyme, where the number of arginine residues are greater than lysine residues, a nonfibrillar crystalline-like aggregate is observed in the presence of ATP. Our study thus embeds a note of caution to reckon ATP as a universal biological hydrotrope, as this ability could get reversed, and ATP can as well act as a "biological aggregation mediator" depending upon its immediate environments.