Thermodynamic and kinetic determinants of Thermotoga maritima cold shock protein stability: A structural and dynamic analysis

The cold shock protein (CSP) from hyperthermophile Thermotoga maritima (TmCSP) is only marginally stable (ΔG(T_opt) = 0.3 kcal/mol) at 353 K, the optimum environmental temperature (T_opt) for T. maritima. In comparison, homologous CSPs from E. coli (ΔG(T_opt) = 2.2 kcal/mol) and B. subtilis (ΔG(T_opt) = 1.5 kcal/mol) are at least five times more stable at 310 K, the T_opt for the mesophiles. Yet at the room temperature, TmCSP is more stable (ΔG(T_R) = 4.7 kcal/mol) than its homologues (ΔG(T_R) = 3.0 kcal/mol for E. coli CSP and ΔG(T_R) = 2.1 kcal/mol for B. subtilis CSP). This unique observation suggests that kinetic, rather than thermodynamic, barriers toward unfolding might help TmCSP native structure at high temperatures. Consistently, the unfolding rate of TmCSP is considerably slower than its homologues. High temperature (600 K) complete unfolding molecular dynamics (MD) simulations of TmCSP support our hypothesis and reveal an unfolding scheme unique to TmCSP. For all the studied homologues of TmCSP, the unfolding process first starts at the C-terminal region and N-terminal region unfolds in the end. But for TmCSP, both the terminals resist unfolding for consistently longer simulation times and, in the end, unfold simultaneously. In TmCSP, the C-terminal region is better fortified and has better interactions with the N-terminal region due to the charged residues, R2, E47, E49, H61, K63, and E66, being in spatial vicinity. The electrostatic interactions among these residues are unique to TmCSP. Consistently, the room temperature MD simulations show that TmCSP is more rigid at its N- and C-termini as compared to its homologues from E. coli, B. subtilis, and B. caldolyticus.