Dynamics of heme iron in crystals of metmyoglobin and deoxymyoglobin.

Abstract

The 57Fe v-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 andjump frequency or diffusion constant) can be derived as a function oftemperature. Thus, for deoxy Mb at 288 K, the mean square displacement for the fast fluctuation is about 6 x 10-2 A and for the diffusive motion is 1.6 x 10-2 A2; the diffusion constant is 4 X 10-10 cm2/sec. The diffusive process is associated with an activation energy ofabout 0.75 kcal/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.There has been a rapidly growing interest in recent years in the study of the dynamics of structural fluctuations and conformational changes in proteins (1). These studies, which extend our knowledge beyond the time-averaged models of protein structure obtained by x-ray diffraction from protein crystals, are important in understanding aspects of the biological function of proteins. A variety of techniques, appropriate to different time scales, have been used in experimental investigations of the dynamics of proteins (1). The analysis of the x-ray diffraction temperature factors from crystals of some globular proteins (2, 3) has provided valuable information concerning the mean square displacement (msd), (x2), of atoms in different parts of the protein.The dynamics of active sites in proteins are of particular interest. In this respect, Mossbauer resonance absorption spectroscopy has been especially useful in the study ofthe dynamics of iron atoms in iron proteins. From the recoilless fraction f, estimates of the msd of the iron atoms can be obtained. Information on the dependence of (x2) on temperature has been obtained in this way for oxy Mb in frozen solution (4) and for crystals of MetMb (5, 6) and deoxy Mb (7). Rayleigh-Mossbauer scattering also has been used to investigate the dynamics of Mb (8,9). Recent investigations of the 57Fe y-ray absorption spectrum over an extended range of scanning velocities in crystals of the proteins ferritin (10-12), MetMb (11,12), and HbCO (13) have revealed striking new phenomena highly pertinent to the dynamics of iron in these proteins.In this paper we present the results ofa detailed investigation and comparison of these phenomena i...

The dynamic transition in proteins may have a simple explanation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Much of this work has been done on myoglobin using Mö ssbauer spectroscopy 3,6,8 neutron scattering 10,11 or X-ray crystallography 2 of hydrated crystals, powders, or frozen solutions, but similar results have been found in X-ray crystallographic studies of ribonuclease A 12 and in Mö ssbauer 14 and neutron scattering 15 studies of membrane proteins. Some protein functions have been observed to cease with the loss of equilibrium anharmonic dynamics as the protein is cooled through the dynamic transition.…”

“…These include Mö ssbauer spectroscopy of the Fe ion in myoglobin, X-ray scattering measurements of the temperature factor in protein crystals, Rayleigh scattering of Mö ssbauer radiation, and neutron scattering to probe the global dynamics of a relatively limited number of proteins. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Depending on the technique and possibly the protein or the nature of the preparation, the sharpness and temperature of this transition may vary somewhat, but has been generally observed between about 180 and 230 K. It is taken to be an intrinsic property of proteins and, as indicated above, has been associated with the onset of protein function. [14][15][16][17][18][19][20] Recent neutron scattering measurements on a thermophilic glutamate dehydrogenase enzyme in solution, under conditions in which enzyme activity is both possible and measurable, failed to show any relationship between an observed dynamical transition at around 220 K and the onset of activity 13 (Fig.…”

The denaturation and degradation of stable enzymes at high temperatures

“…It is implicit in the concept of induced fit [57,58] that an enzyme must flex over a time scale in keeping with its catalytic-centre activity, and that changes between conformational substates allow catalytic activity. Enzyme activity has been demonstrated at temperatures below the glass transition [59], where mobility does not occur [60][61][62][63], but catalytic-centre activities are very low, and the significance of this activity is not yet clear.…”

Structure and dynamics of the water around myoglobin

“…Neutron experiments show a change in mean square displacements of atoms as extracted from B-factors near 200 K. This has been associated with a glasslike transition in the system. A wide range of experiments has been used to probe this behavior (Parak et al, 1982;Bauminger et al, 1983;Parak & Knapp, 1984;Parak, 1986;Doster et al, 1989). Certain experiments have been interpreted as indicating that this low-temperature glass behavior of protein solutions is induced by the solvent (Doster et al, 1986;Ansari et al, 1987).…”