TIME-DEPENDENCE AND TEMPERATURE-DEPENDENCE OF LARGE-SCALE CONFORMATIONAL TRANSITIONS IN MYOGLOBIN

Structure, dynamics, and function of proteins are strongly interrelated. We study the structure-function relationship in carbon-monoxy-myoglobin (MbCO) using two techniques: (i) flash photolysis with rebinding monitored in the CO stretch bands over wide ranges in time (almost-equal-to 3 mu-s to 1 s) and temperature (60 to 260 K); (ii) pressure-jump experiments with protein relaxations monitored in the CO stretch bands by FTIR spectroscopy for times between 10 s and 30 ks and temperatures between 155 and 220 K. The three CO stretch bands correspond to three major conformational substates, A0, A1, and A3, with different structures and rebinding kinetics. The flash-photolysis experiments show that the absorbance change of A0 is nonmonotonic in time during rebinding above about 220 K owing to interconversion of A0 with A1 and A3. The P-jump experiments establish that MbCO experiences large-scale motions which are nonexponential in time, non-Arrhenius in temperature, and strongly dependent on solvent viscosity. One of the motions observed in the P-jump experiments corresponds to the large-scale structural transition, A0 --> A1 + A3, observed during rebinding. These results, together with earlier data, begin to give a detailed picture of the conformational energy landscape of MbCO: it contains many conformational substates which are hierarchically arranged in at least three tiers. Motions in each of the three tiers affect the MbCO rebinding kinetics.