Low temperature X-ray investigation of structural distributions in myoglobin

The results of X-ray structure analysis of metmyoglobin at 300 K, 185 K, 165 K, 115 K and 80 K are reported. The lattice vectors a and b decrease linearly with temperature while c shows non-linearity above 180 K, indicating some type of phase transition. Cooling does change the myoglobin structure but only within the structural distribution as determined by individual (chi 2)-values at room temperature. Two residues showed significant alternative positions for side-chains at higher temperatures while only one position is occupied at low temperatures. In the case of LEU 61 a jump between different positions of the side-chain reduces the potential barrier for the entrance of the O2 molecule to the heme pocket. The mean square displacements, (chi 2), of the individual residues decrease linearly with temperature in most cases, indicating a parabolic envelope for the potential responsible for motions. A separation of rotational and translational disorder of the entire molecule is discussed. Comparison with Mössbauer spectroscopy indicates that protein dynamics on a time scale faster than 10(-7) s is not simply a harmonic process. Extrapolation of the structural distributions to T = 0 K shows that a large zero point distribution of the myoglobin structure exists, thus proving that there is no absolute energy minimum for one well defined conformation.