The Engrailed Homeodomain protein has the highest refolding and unfolding rate constants directly observed to date. Temperature jump relaxation measurements gave a refolding rate constant of 37,500 s−1 in water at 25°C, rising to 51,000 s−1 around 42°C. The unfolding rate constant was 1,100 s−1 in water at 25°C and 205,000 s−1 at 63°C. The unfolding half-life is extrapolated to be ≈7.5 ns at 100°C, which allows real-time molecular dynamics unfolding simulations to be tested on this system at a realistic temperature. Preliminary simulations did indeed conform to unfolding on this time scale. Further, similar transition states were observed in simulations at 100°C and 225°C, suggesting that high-temperature simulations provide results applicable to lower temperatures.
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