Protein folding and unfolding in microseconds to nanoseconds by experimentand simulation

The Engrailed Homeodomain protein has the highest refolding and unfolding r ate constants directly observed to date. Temperature jump relaxation measur ements gave a refolding rate constant of 37,500 s(-1) in water at 25 degree sC, rising to 51,000 s(-1) around 42 degreesC. The unfolding rate constant was 1,100 s(-1) in water at 25 degreesC and 205,000 s(-1) at 63 degreesC. T he unfolding half-life is extrapolated to be approximate to7.5 ns at 100 de greesC, which allows real-time molecular dynamics unfolding simulations to be tested an this system at a realistic temperature. Preliminary simulation s did indeed conform to unfolding on this time scale. Further, similar tran sition states were observed in simulations at 100 degreesC and 225 degreesC , suggesting that high-temperature simulations provide results applicable t o lower temperatures.