Time-domain observation of conformational fluctuation of protein by time-resolved hole-burning spectroscopy

Recent progress in structural biology has disclosed that protein function relies largely on its conformational flexibility. A protein molecule assumes a large number of slightly different conformations called "conformational substates (CSs)". At room temperature in an aqueous solution, transitions among many CSs take place and induce large conformational fluctuation, which is considered to be indispensable for its function. Time-resolved transient hole-burning (TRTHB) spectroscopy is becoming one of the most powerful tools to do direct time-domain measurement of the conformational fluctuation dynamics of a protein. In this technique, transition process among CSs is detected through the temporal variation of the "hole-burning" spectrum formed in the absorption spectrum by laser irradiation. Conformational fluctuation dynamics of protein is observed as a function of time in a very wide temporal range from ns up to ms. This article reviews some recent applications of TRTHB method to the observation of conformational fluctuation of myoglobin and cytochrome c.