Conformational dynamics of the transcriptional regulator CooA protein studied by subpicosecond mid-infrared vibrational spectroscopy

CooA, which is a transcriptional regulator heme protein allosterically trig gered by CO, is studied by femtosecond visible-pump mid-IR-probe spectrosco py. Transient bleaching upon excitation of the heme in the Soret band is de tected at similar to 1979 cm(-1), which is the absorption region of the CO bound to the heme, The bleach signal shows a nonexponential. decay with tim e constants of 56 and 290 ps, caused by the rebinding of the CO to the heme . About 98% of dissociated CO recombines geminately. The geminate recombina tion rate in CooA is significantly faster than those in myoglobin and hemog lobin. The angle of the bound CO with respect to the porphyrin plane is cal culated to be about 78 degrees on the basis of the anisotropy measurements. A shift of the bleached mid-IR spectrum of the bound CO is detected and ha s a characteristic time of 160 ps. It is suggested that the spectral shift is caused by a difference in the frequency of the bound CO in different pro tein conformations, particularly in an active conformation and in an interm ediate one, which is on the way toward an inactive conformation. Thus, the biologically relevant conformation change in CooA was traced. Possible assi gnment of the observed conformation change is discussed.