Dynamics of Heme Iron in Crystals of Metmyoglobin and Deoxymyoglobin

The ⁵⁷Fe γ -ray resonance absorption spectra have been measured in crystals of metmyoglobin and deoxymyoglobin over a wide range of temperatures. Above a critical temperature common to both proteins (220 K), the dynamics of heme iron display a dramatic change, in that two kinds of thermal fluctuations come into play--a fast fluctuation associated with a steep decrease of the total resonance absorption with increasing temperature, and a slower fluctuation of characteristic time 10^-8 sec, associated with bounded diffusive motion. By using both discrete jump and continuous diffusion models, the latter based on the Brownian motion of an overdamped harmonic oscillator, the essential parameters of the iron motion (mean square displacement and jump frequency or diffusion constant) can be derived as a function of temperature. Thus, for deoxy Mb at 288 K, the mean square displacement for the fast fluctuation is about 6× 10^-2 angstrom ² and for the diffusive motion is 1.6× 10^-2 angstrom ²; the diffusion constant is 4× 10^-10 cm²/sec. The diffusive process is associated with an activation energy of about 0.75kcal/mol. Although the same general kinds of phenomena are observed in crystals of MetMb and deoxy Mb, significant differences in behavior are found, which suggest that the main dynamical phenomenon observed reflects internal large-scale motions of the protein.