Quaternary structure dependence of kinetic hole burning and conformational substates interconversion in hemoglobin

Using a sol-gel encapsulation technique, we have prepared samples of CO saturated human adult hemoglobin locked in the R or T quaternary conformation. We report time-resolved spectra of these samples in the Soret region following flash photolysis, in the time interval ranging from 250 ns to 200 ms and in the temperature interval of 100-170 K. A suitable analysis of the measured difference spectra enables us to obtain the spectral contribution of deoxyHb and HbCO molecules as a function of time and/or of the fraction N(t) of deoxyHb molecules. In our experimental time window geminate CO rebinding to hemoglobin in the T quaternary conformation is about 2 orders of magnitude slower than to hemoglobin in the R conformation: this suggests that the barrier distribution for the CO rebinding, g(H), depends strongly on the protein quaternary structure. In our temperature interval, spectral shifts due to kinetic hole burning (KHB) are present: for HbCO the KHB effect is large in the R conformation and small in the T conformation. For deoxyHb the opposite is true. We attribute the observed behavior to the effect of interconversion between the relevant substates. This effect is stronger for HbCO molecules in the T conformation and for deoxyHb molecules in the R conformation; it confirms the quaternary structure dependence of the hemoglobin energy landscape and suggests enhanced dynamics of ligation intermediate species such as T-state HbCO or R-state deoxyHb.