Confirmation of the formation of salt bridges in the denatured state of CutA1 protein using molecular dynamics simulations

It remains unclear how the abundant charged residues in proteins from hyperthermophiles contribute to the stabilization of proteins. Previously, based on molecular dynamics (MD) simulations, we proposed that these charged residues decrease the entropic effect by forming salt bridges in the denatured state under physiological conditions (Yutani et al., Sci. Rep. 8, 7613 (2018)). Because the quality of MD results is strongly dependent on the force fields used, in this study we performed the MD simulations using a different force field (AMBER99SB) along with the one we used before (Gromos43a1), at the same temperatures examined previously as well as at higher temperatures. In these experiments, we used the same ionic mutant (Ec0VV6) of CutA1 from Escherichia coli as in the previous study. In MD simulations at 300 K, Lys87 and Arg88 in the loop region of Ec0VV6 formed salt bridges with different favorable pairs in different force fields. Furthermore, the helical content and radius of gyration differed slightly between two force fields. However, at a higher temperature (600 K), the average numbers of salt bridges for the six substituted residues of Ec0VV6 were 0.87 per residue for Gromos43a1 and 0.88 for AMBER99SB in 400-ns MD simulation, indicating that the values were similar despite the use of different force fields. These observations suggest that the charged residues in Ec0VV6 can form a considerable number of salt bridges, even in the denatured state with drastic fluctuation at 600 K. These results corroborate our previous proposal.