A water network within a protein: temperature-dependent water ligation in H64V-metmyoglobin and relaxation to deoxymyoglobin

Abstract.

The sperm whale myoglobin mutant H64V, where the distal histidine is mutated to valine, is known to be five coordinated in the ferric state at room temperature and physiological pH. A change of the ligation in this H64V-Mbmet has been observed by optical absorption spectroscopy as a function of temperature from 20 K to 300 K. Above the dynamical transition at about 180 K one observes the temperature-dependent equilibrium between five- and six-ligated heme. Below the dynamical transition the equilibrium is frozen-in at about 50% of six-coordinate molecules. The water ligation of the iron occurs at temperatures where protein-specific motions are present, as monitored by Mössbauer spectroscopy. The X-ray structures of H64V-Mbmet at 300 K and 110 K are reported with a resolution of 1.5 Å and 1.3 Å, respectively. The measurements at high resolutions are possible owing to crystallization in the space group P2₁, whereas all mutant myoglobins studies up to now have been carried out with crystals in the space group P6. The overall structure at both temperatures is very close to the native myoglobin. The binding of water at the sixth coordination site at lower temperatures is possible owing to a stabilizing water network extending from the protein surface to the active centre. The reduction of the H64V-Mbmet by electrons obtained by X-ray irradiation of the water-glycerol solvent at 85 K produces an intermediate low-spin state of the water-ligated molecules where Fe(II) retains the six-fold coordination. Mössbauer spectroscopy shows that the relaxation of the metastable low-spin state to high-spin H64V-Mbdeoxy with dissociation of the Fe(II)-H₂O bond starts at about 115 K and is completed at about 170 K. Differences in the dynamics properties of the native and mutant myoglobin and the connection to the dynamical transition around 180 K are discussed.